Lesson 4: How forces work

Activity 1: How forces work

Time required:

• 30 minutes
  

What you need:

• A pen or pencil
• A piece of paper

What to do:

Remember this video from the previous lesson? Even though things seem quite simple, we can learn many important things about how forces work by looking at the action carefully. Watch the video and then try and answer the questions below.

 

1. At the start, the ball is not moving. It is stationary on the ground. What forces are acting on the ball? Are these forces the same size? Do they act in the same direction?
   
2. As the boy kicks the ball, what happens to its motion? What forces are acting on the ball now?
3. Why do you think the ball moves both towards the goal AND up?
4. What kind of force do you think makes the ball change direction when it hits the net of the goal? Think about what happens to the net ropes.
5. As the ball rolls away from the goal, why does it eventually stop?

Here is a summary of what we have learnt about forces that we can observe in the video.

1. The interaction between objects can happen because of their direct contact with each another. These are contact forces.

At the start, the ball was at rest on the ground. Because the ball touched the ground and the ground touched the ball, we know that this was a contact interaction. Understanding force as the result of an interaction between objects, we can say that the ball exerted a force on the ground and that the ground exerted a force on the ball.

When the boy kicked the ball, his foot touched the ball and the ball also touched his foot. Through the contact interaction, the boy’s foot exerted an applied force on the ball and the ball exerted a force on his foot.

2. The interaction between objects can be caused by invisible surrounding fields interacting with each other. This is called a field interaction.

At the end of the video clip, we see the ball dropping down to the ground, without contact with another object. This is the result of the interaction between the gravitational fields around two objects, namely the Earth and the ball. These are non-contact forces because the objects interact without touching each other.

Every object that has mass also has a surrounding gravitational field. The Earth’s gravitational field is much bigger than that of the ball. This is because the Earth has much more mass than the ball and so exerts a bigger force on the ball than the ball on the Earth.

3. Forces work as interacting pairs

As force is the result of an interaction between two objects, it follows that the objects will each exert a force on the other, in opposite directions. For example, the ball lying on the ground pushes down on the ground, and the ground pushes up on the ball.

4. Force is a vector quantity. This means it has size and direction

Force is measured in newtons (N). When we describe a force, we must mention both its magnitude (size) and its direction. For example:

• The ball lying at rest exerts a force of a certain magnitude downwards.
• The boy’s foot on the ball exerts a force of a certain magnitude forward and to the right.

When we describe a force quantitatively – in other words, with numbers – the number tells us its size, and the sign - positive or negative – tells us its direction relative to a predefined frame of reference. For example, if we define up as the positive vertical direction, we know that a force with a negative value acts downwards.

5. We cannot see forces themselves, only their effects

We can see the effect of a force by looking at an object’s shape or motion. A force can cause an object to remain as it is or cause a change in its shape or motion. In the video clip we saw:

• force causing a change in an object’s shape when the ball hit the net and made the net bulge out
• force affecting the speed at which an object moves, when the boy kicked the stationary ball to make it move, or the ball rolling slower and slower along the ground until it eventually stopped
• force affecting the direction in which an object moves, such as when the ball hit the net and rolled away from the net.

6. A change in an object’s shape or motion is caused by unbalanced forces acting on the object. When the forces acting on an object are balanced, its shape or motion will remain unchanged.

The ball fell to the ground because the gravitational force of the Earth pulling the ball downwards was bigger than the gravitational force of the ball on the Earth. That means that in this interacting force pair, the force acting downwards and the one acting upwards were unbalanced, and so the ball’s motion changed.

When the ball lay on the ground, it was acted on by the force of gravity. Yet it lay still; its motion or shape did not change. This tells us that a force equal in size but opposite in direction was also acting on the object. We say two such counteracting forces acting on an object balance each other.

7. The effect of a force is the sum of all the forces acting on an object.

Forces are additive, like all vector quantities. If the two forces in an interacting pair have equal magnitudes but opposite signs, the resultant force will be zero. If the two forces have unequal magnitudes, the sum will be a non-zero value, which means the net effect will be in the direction of the force with the bigger magnitude.

Watch: What is force? - Part 1

This video gives an excellent introduction to what forces are.

Watch: What is Force? - Part 2

If you have an internet connection, watch this video for a great summary of what we have learnt so far.