Forces -Chain Reaction
Teacher background
A force is an external influence that can change the motion, direction or shape of objects. Examples of forces include pushes, pulls, friction, gravity and magnetism. A force can be applied to an object but is not a property of the object itself. All living and non-living things can apply and be affected by forces. A force can cause an object to start moving, stop moving or change its direction. A force applied to an object might cause it to change its motion by transferring energy to the object. It is the energy (movement or kinetic energy) that enables the object to move. Stationary objects also have forces acting on them. Consider a book on the floor; it experiences a downward gravitational pull force but this is balanced by an equal upward push force from the floor, resulting in the book remaining stationary.
Force has two aspects: magnitude and direction. The magnitude of the force refers to the size or amount of force exerted, for example, if it is a strong or a weak push.
More than one force can act on an object at any one time, for example, our standing body is pulled down to the ground and the ground pushes back. We are able to be stationary because even though there are two forces acting on us, these forces are of equal magnitude in opposite directions. The forces are therefore said to be balanced, enabling us to experience constant motion rather than sinking into the ground or rising into the air. An object moving at a steady speed in a straight line is also experiencing balanced forces: its motion is constant, it is not changing.
An object’s motion changes when the forces acting on it are not balanced. Unbalanced forces can make stationary objects move. Forces can make moving objects move faster or more slowly, come to a stop or change direction. Forces can also change the shape of objects. If you push a stationary ball it starts to move along the ground. If it hits a wall it might stop, change shape or change the direction in which it is moving. If the push is large enough, the ball might do all of these things.
Different-sized forces have different effects on different objects. For a given object, a larger force will produce a bigger effect than a smaller force. For example, a big push will make a swing move a lot while a small push will only make the swing move a little. The effect of a force on an object also depends on the object’s mass, which is the amount of matter in the object. For a given force, an object with a smaller mass will experience a greater effect than one with a larger mass. For example, a small push on a light wooden block will make it move a long distance, while the same small push on a heavy wooden block will not make it move as far.
Forces can act through direct contact, such as physical pushes and pulls, friction, and air or water resistance. Some forces act at a distance, such as gravity and magnetism.
A force is an external influence that can change the motion, direction or shape of objects. Examples of forces include pushes, pulls, friction, gravity and magnetism. A force can be applied to an object but is not a property of the object itself. All living and non-living things can apply and be affected by forces. A force can cause an object to start moving, stop moving or change its direction. A force applied to an object might cause it to change its motion by transferring energy to the object. It is the energy (movement or kinetic energy) that enables the object to move. Stationary objects also have forces acting on them. Consider a book on the floor; it experiences a downward gravitational pull force but this is balanced by an equal upward push force from the floor, resulting in the book remaining stationary.
Force has two aspects: magnitude and direction. The magnitude of the force refers to the size or amount of force exerted, for example, if it is a strong or a weak push.
More than one force can act on an object at any one time, for example, our standing body is pulled down to the ground and the ground pushes back. We are able to be stationary because even though there are two forces acting on us, these forces are of equal magnitude in opposite directions. The forces are therefore said to be balanced, enabling us to experience constant motion rather than sinking into the ground or rising into the air. An object moving at a steady speed in a straight line is also experiencing balanced forces: its motion is constant, it is not changing.
An object’s motion changes when the forces acting on it are not balanced. Unbalanced forces can make stationary objects move. Forces can make moving objects move faster or more slowly, come to a stop or change direction. Forces can also change the shape of objects. If you push a stationary ball it starts to move along the ground. If it hits a wall it might stop, change shape or change the direction in which it is moving. If the push is large enough, the ball might do all of these things.
Different-sized forces have different effects on different objects. For a given object, a larger force will produce a bigger effect than a smaller force. For example, a big push will make a swing move a lot while a small push will only make the swing move a little. The effect of a force on an object also depends on the object’s mass, which is the amount of matter in the object. For a given force, an object with a smaller mass will experience a greater effect than one with a larger mass. For example, a small push on a light wooden block will make it move a long distance, while the same small push on a heavy wooden block will not make it move as far.
Forces can act through direct contact, such as physical pushes and pulls, friction, and air or water resistance. Some forces act at a distance, such as gravity and magnetism.
Scientists and engineers try to find ways to both inc ease and decrease friction. This is because there are situations where friction is helpful and other situations where friction is a problem for us. Think about trying to walk on wet kitchen tiles in bare feet (more friction would help) and then trying to slide down a slippery dip with wet trousers (less friction would help).
Everyday applications of friction include:
Everyday applications of friction include:
- allowing us to move: the soles of our shoes grip the ground and help us to push off when walking;
- slowing things down: the brakes on a bicycle decrease the speed of the bicycle and heat the wheel while friction between the tyre and the road stops the bicycle from simply sliding along;
- keeping things in place: tuning pegs on guitars and violins stay in place and keep
the instrument in tune, nails hold materials together and objects on a gentle slope do not slide; - transforming movement energy into heat energy: it can be used to create fire, for example, by rubbing sticks together.
Friction can have some disadvantages. Frictional forces cause surfaces to wear: the soles of shoes wear down, tread on tyres wears down, moving parts in machinery wear down and need to be replaced. Friction generates heat and this can be a problem if things get too hot.
Ways to reduce friction include: - adding a lubricant: a liquid, such as water, oil or detergent makes surfaces slippery
- reducing the force pushing the two objects together: a lighter object is easier to move across a surface than a heavy object.
Experiment
- Move students to an outdoor grassed or soft-mat area. Divide the class into two even groups and explain that each team will collectively pull one end of a long rope towards them but still keep it over a central line marked in the ground. Discuss ‘balance’ and explain how each end of the rope is being pulled but is still balanced.
- 3 Explain that the class will now play a game of ‘Tug of war’ and will see which team can pull the other team across the central line marked on the ground.
- 4 As a class, discuss what things made it hard to pull the other team across the line, such as slippery surface, no grip between hands and the rope.
- Ask the team that didn’t win to put disposable gloves on their hands and repeat the ‘Tug of war’ game with the other team (who have bare hands). Ask students to describe the difference between bare hands and wearing gloves when pulling the rope. Encourage students to provide reasons for their responses.
- 6 Invite the rest of the class to put disposable gloves on their hands and ask everyone to rub their gloved hands together. Ask students to describe what it feels like and why they think it feels like that. (The material that the glove is made of enables us to feel more grip when we rub our hands together.)
- 7 Explain that students are going to apply some detergent to the gloves on their hands and compare how it feels to rubbing just the gloves together. Ask students to apply a small amount of detergent to their gloved hands and ask questions, such as:
- What does the detergent feel like when you rub your gloved hands together?
- Is it easier to rub your gloved hands together with or without the detergent?
- Why do you think it feels different?
- 8 Introduce and discuss the term ‘friction’, as something that acts between two surfaces in contact producing grip. Ask students if they think the gloves without detergent had more or less friction than the gloves with detergent. Record students’ responses in the class science journal.
- 9 Introduce and discuss the term ‘force’. Explain that forces can affect objects in different ways, including the way they move. Explain to students that forces are usually thought of as pushes and pulls and can also include forms such as friction, gravity and magnetism.
- 10 Explain that students will be working in collaborative learning teams to pull a heavy object across different surfaces to investigate the friction force between the surfaces. Ask students to predict if there will be a large or small amount of friction between the heavy object and each surface and record their ideas in their science journal.
- How were the surfaces different?
- What did it feel like when you pulled the heavy object across each surface?
- Was there a difference in the size of the pull needed to pull the heavy object across each surface? Why do you think that?
- How did friction affect the movement of the heavy object?
- Did your team findings match the predictions you made?
- Ask students to create a drawing in their science journal which represents the force used to pull the heavy object across each surface and the opposing friction force. Encourage students to use different-sized arrows to represent a large or small pull and an opposing arrow to represent a large or small amount of friction.
Lesson 4
LI: I am learning that the Earth’s gravitational force (gravity) ‘pulls’ things down to the ground.
SC: I understand that the effects of gravity affect everything and that with out gravity there would be no us. The larger something is (Mass) the greater it's gravity.
SC: I understand that the effects of gravity affect everything and that with out gravity there would be no us. The larger something is (Mass) the greater it's gravity.
When we drop things, we see that the Earth’s gravitational force (gravity) ‘pulls’ them down to the ground. We know that magnets can ‘push’ and ‘pull’ other magnets and can pull things made of iron (or steel) towards them. Both magnetism and gravity are forces that act at a distance rather than through direct contact. Though we cannot see the force, we can see the effect it has on objects.
Gravity
Isaac Newton is claimed to have said, ‘What goes up, must come down’ when he saw an apple fall from the tree he was sitting beneath. He is one of the scientists who have developed our understanding of forces, including gravity.
Every object exerts a gravitational force on other objects but it can be hard to detect unless at least one of the objects has a large mass. ‘Mass’ is a measure of the amount of matter an object has while ‘weight’ is the measure of gravitational pull that acts on an object.
The weight of an object is related to the mass of the object and the magnitude of the gravitational force acting on the object. The weight of an object will change if the gravitational force acting on it changes, but its mass will not. For example, the Moon is not as massive
as the Earth so its gravitational force is not as strong. Because of this, objects will not be attracted as strongly to the centre of the Moon as they would be to the centre of the Earth, and their weight on the Moon is less than their weight on Earth.
The Earth has such a large mass that the gravitational attraction between it and most things is very noticeable; when we jump into the air, the Earth’s gravitational force pulls us back towards the Earth’s centre very quickly. We can also feel the pull of the Earth’s gravity when we try to lift things; the more mass something has, the greater the pull of gravity and the greater the lifting force we need to use. Gravity acts on an object regardless of whether or not the object is moving. It does not require the object to be surrounded by air or water or anything else and can therefore act in the vacuum of space.
Gravity
Isaac Newton is claimed to have said, ‘What goes up, must come down’ when he saw an apple fall from the tree he was sitting beneath. He is one of the scientists who have developed our understanding of forces, including gravity.
Every object exerts a gravitational force on other objects but it can be hard to detect unless at least one of the objects has a large mass. ‘Mass’ is a measure of the amount of matter an object has while ‘weight’ is the measure of gravitational pull that acts on an object.
The weight of an object is related to the mass of the object and the magnitude of the gravitational force acting on the object. The weight of an object will change if the gravitational force acting on it changes, but its mass will not. For example, the Moon is not as massive
as the Earth so its gravitational force is not as strong. Because of this, objects will not be attracted as strongly to the centre of the Moon as they would be to the centre of the Earth, and their weight on the Moon is less than their weight on Earth.
The Earth has such a large mass that the gravitational attraction between it and most things is very noticeable; when we jump into the air, the Earth’s gravitational force pulls us back towards the Earth’s centre very quickly. We can also feel the pull of the Earth’s gravity when we try to lift things; the more mass something has, the greater the pull of gravity and the greater the lifting force we need to use. Gravity acts on an object regardless of whether or not the object is moving. It does not require the object to be surrounded by air or water or anything else and can therefore act in the vacuum of space.
- Invite students to play the game ‘Going up’ as a class or in small groups. Explain
that the aim is for students to keep a balloon moving by hitting it up in the air and not letting it fall to the ground. Ask students to predict what might happen to the balloon if they don’t keep it moving during the game. - 3 Discuss what is happening to the balloon in the game. Encourage students to think of what happened to the balloon each time someone hit it and what happened when someone didn’t hit it. Record students’ responses in the class science journal.
- 4 Introduce a globe to the class and discuss the different countries that people live in around the world. Ask one student to show the class where Australia is located on the globe.
- 5 Ask students if they have friends or family who live in other countries. As a class, discuss some of these countries and where they are located on the globe. Ask students if they think people experience gravity in those countries. Encourage them to provide reasons for their responses.
- 6 Ask students to draw a picture of the world in their science journals. Ask them to draw four people in different places around the globe with an air-filled balloon in their hand. Explain that students will draw an arrow to show where the balloon will go if each person around the globe dropped the balloon.
Note: It is important that students create the entire drawing so you can elicit their
understanding of gravity. - As a class, discuss gravity and the different ways people experience it in their lives.
Introduce the term ‘gravity’ as a force that pulls things towards the centre of the Earth. Ask questions, such as:- How do we know that gravity exists?
- What effects of gravity can we see or experience?
- What does the balloon activity tell us about gravity?
- What might happen if there was no gravity?
- How do we know that gravity exists?
- 8 Invite students to review their picture of the world in their science journal. Ask students to make any changes to the direction of the arrows in their picture to represent their understanding of the direction of gravitational force acting on each balloon in the picture.
Air resistance as a force
LI: That an object falls towards the earth due to gravity and air resistance acts upon the object to slow or stop it falling
SC: I can explain how materials of varying shapes and sizes can affect the strength of the air resistance force.
When you drop the toy, the strings that are attached to the parachute pull down and this open the bag to full size, which creates a large surface area and more wind resistance. More wind resistance slows down the descent of the toy.
You can explain the results of this experiment with the concept of resistance. Wind resistance, also called drag, is simply a force that acts on a solid object. Car designers often factor in wind resistance when designing a car to help it have greater fuel efficiency and accelerate to high speeds more easily. In this experiment, your goal was to create more wind resistance to slow the speed of the object. The largest parachute created more resistance and slowed the descent of the toy the most.
The experiment shows that the size of the parachute makes a difference in the speed of descent, but what if you tried different materials for the parachute? Repeat the experiment with a parachute made from construction paper, plastic grocery bags or other items. Do you think the results will vary? Come up with a new hypothesis each time you try new materials and see if your guesses are correct.
You can explain the results of this experiment with the concept of resistance. Wind resistance, also called drag, is simply a force that acts on a solid object. Car designers often factor in wind resistance when designing a car to help it have greater fuel efficiency and accelerate to high speeds more easily. In this experiment, your goal was to create more wind resistance to slow the speed of the object. The largest parachute created more resistance and slowed the descent of the toy the most.
The experiment shows that the size of the parachute makes a difference in the speed of descent, but what if you tried different materials for the parachute? Repeat the experiment with a parachute made from construction paper, plastic grocery bags or other items. Do you think the results will vary? Come up with a new hypothesis each time you try new materials and see if your guesses are correct.
Lesson 5
When something changes, energy is involved. Energy is abstract but you can often detect it through the effect it has on your body: you can see patterns of light, you can feel the warmth created by heat energy, though you cannot see or feel magnetic energy. Energy from the Sun drives the growth of plants and the development of rainstorms, while energy from chemical reactions gives life to animals and is important to modern industry.
Some important characteristics of energy include:
Some important characteristics of energy include:
- Energy exists in different forms, such as light, sound, heat, electricity and movement.
- Energy can be transformed (changed) from one form to another, for example, kicking a ball transforms chemical energy in our bodies to movement energy in the ball.
- Energy can be transferred from one location to another, for example, electrical energy moves along wires from a power station to our houses.
- Energy can be changed into other forms but it cannot be created or destroyed.
- Energy can be stored in many ways. Batteries and fossil fuels are stores of chemical energy.
When forces are applied to an object, energy is transferred or transformed. When a football player kicks a ball, movement energy in his foot is transferred to the ball. When hands are rubbed together, friction transforms movement energy into heat energy. When you turn on a switch, electrical energy from the power station is transformed into light energy in your room.
Forces act on stationary and moving objects. The forces acting on stationary objects are balanced. They cancel each other out so we are often not aware of them. When forces are acting on a moving object, the effect depends on the size of the force and the mass of the object.
Force-arrow diagrams
Force-arrow diagrams are used to represent the direction and the size of forces acting in a particular situation. The length of a force-arrow represents magnitude and the direction of the arrow shows the direction in which the force is acting. When drawing force- arrow diagrams, longer arrows in the direction of the force are used to represent a larger force while a shorter arrow in the direction of the force is used to represent a smaller force. Asking students to label the force-arrows will assist with their representations and explanations of forces and motion.
- Explain that students will be working in collaborative learning teams to create a real-life scenario and use their understanding of forces and motion to explain the forces acting in the scenario, such as pushing a trolley or pulling a box. Explain that students will use role-play and narrative to present their scenario to the class. Discuss the purpose and features of role-plays and narratives.
Why do we use a role-play?
We use a role-play as a physical representation of a system, process or situation.
What does a role-play include?
A role-play might include speech, gestures, actions and props - Review the class science journal and discuss the different ways that students have represented different-sized forces acting on objects in each lesson. Discuss the limitations of having a range of representations, for example, not everyone has the same representation therefore not everyone will understand what’s being represented. Introduce the scientific convention for epresenting different-sized forces: large forces are represented with a long arrow, smaller forces are represented with a shorter arrow.
- Explain that students will create a force-arrow diagram of their scenario. Discuss the purpose and features of a force-arrow diagram.
- Why do we use a force-arrow diagram?
We use a force-arrow diagram to show push and pull forces.
What does a force-arrow diagram include?
A force-arrow diagram uses arrows to show the direction of forces. A pull is shown by an arrow pointing away from the object. A push is shown by an arrow pointing towards the object.
Lesson 6
- Introduce the matchbox and elastic band to the students. Explain that students will be working in collaborative learning teams to devise ways to move the matchbox using the elastic band.
- Ask students to discuss how they could use the elastic band to change the size of the force on the matchbox, for example, by pulling the elastic band back further or less.
- Ask students what things (variables) might affect the movement of the matchbox, such as the size of the push from the elastic band, the surface of the table, the surface of the matchbox and the weight of the matchbox.
Record students’ responses on self-adhesive notes. - Introduce the enlarged copy of the ‘Forces investigation planner’ (Resource sheet 1). Model how to develop a question for investigation. For example, we might choose to investigate ‘What happens to the distance the matchbox moves when you change the size of the force acting on it?’.
- Introduce the term ‘variables’ as things that can be changed, measured or kept the same in an investigation. Explain that when a variable is kept the same it is said to be ‘controlled’. Ask students why it is important to keep some things the same when you are measuring changes (to make the test fair and so we know what caused the observed changes). For example:
- change: how far the elastic band is pulled back (the size of the force);
- measure/observe: the distance the matchbox moves;
- keep the same: the matchbox, the number of paperclips in the matchbox, slope of the table, the surface of the table.
Discuss ways to keep the investigation fair. Ask questions, such as:
What would happen if each team had matchboxes of different weight and used different-sized forces to move it? (We have changed two variables so we don’t know which one made the difference to the distance the matchbox moved.) What if each team tested their matchbox on a different surface? (If we change the surface that we test the matchbox on and the size of the force used to move the matchbox then we don’t know which one made the difference to the distance the matchbox moved.)
- change: how far the elastic band is pulled back (the size of the force);
- Why do we use a graph?
We use a graph to organise information so we can look for patterns. We use different
types of graphs, such as picture, column, or line graphs, for different purposes.
What does a graph include?
A graph includes a title, axes with labels on them and the units of measurement.
Play the first part of this.
Hooke's Law states that the force need to stretch a rubber band is proportionally related tot eh distance that it is stretched.
Hooke's Law states that the force need to stretch a rubber band is proportionally related tot eh distance that it is stretched.
Assessment Task: create a theme park ride in Minecraft. Present your understanding of the forces involved with your ride.
Success Criteria
- Identify contact forces involved in your ride.
- Identify non-contact forces involved in your ride.
- Describe the effect of contact forces acting on an object.
- Describe the effects of the non-contact forces acting on an object.
- Use scientific terms learnt throughout the term