Lesson 2: Contact Forces

Activity 1: Investigate contact forces

Time required:

25 minutes

What you will need:

• Two different objects, each with a flat surface and of different masses, e.g. two books of different masses; you should be able to hold the objects in your hand
• a flat table
• a chair
• an elastic band
• a piece of paper
• a piece of rope or string
  

What to do:

1. Hold the lighter of the objects in your hand. What do you feel? What forces do you think are involved?
2. Now hold the heavier object and then both objects in your hand. What do you feel each time?
3. Place the lighter object on a flat surface. What happens? How would you get the object to slide across the surface?
4. Now push against the object very gently. Increase the amount of force until it starts to slide across the surface. How hard do you have to push to make the object slide? When you stop pushing, does it keep on moving, or eventually stop? Why do you think this is? What would you have to do to keep the object moving?
5. Repeat step 4 with the other, heavier, object. What is the same? What is different? Why is it different?
6. Hold the piece of paper above the ground and let it go. What happens?
7. Now crumple the piece of paper into a tight ball, hold it the same height above the ground as before and let it go. What happens now? Did the paper fall faster or slower than before? Why does the paper fall at a different speed when in the shape of a ball? What force do you think is involved?
8. Loop the elastic band around your index finger. Hook the index finger of your other hand through the free end and pull on the elastic band. Keep stretching it slowly then stop pulling and hold it in place. Now slowly release it. What interactions do you observe? What forces are involved?
9. Now loop the free end around your paper ball and pull to stretch the elastic with the paper ball in it. What do you think will happen if you release the elastic with the paper ball in it? Test this to see if you are right? What forces are involved?
10. Tie the string or piece of rope to a table or chair leg. Pull on the rope. What do you see happen? What happens if you keep pulling harder and harder on the rope or string?

What did you find

When you held the lighter object in your hand, you could feel it pressing against your hand. This is because the Earth's gravity was pulling the object down. This is the force you felt - the force of the object pressing against your hand because of gravity pulling it towards the Earth.

But the object was stationary. It did not fall to the ground. Therefore, there was an equal but opposite force pushing it up to balance out (or counteract) the force downwards due to gravity. This force was your hand pushing up on the object. This counteracting force pushing up on the object is called the normal force. The normal force is always present when an object rests on a surface and always acts perpendicular (at \( 90^\circ \)or a right angle) to the surface.

This also shows us that forces always work in interacting pairs. The object pushes down on your hand and your hand pushes up on the object.

When you held the heavier object in your hand, it felt heavier. This is because, the heavier something is, the more strongly gravity pulls that object towards the Earth. Therefore, both objects together, would have pushed down on your hand with even more force and felt even heavier.

However, to keep the objects stationary, the counteracting upward normal force was also bigger. Your hand pushed up with more force. It was equal and opposite to the force of the objects pushing down on your hand due to gravity. Notice how we use longer arrows to indicate bigger forces.

Remember that forces always work in pairs. The object pushed down on your hand and your hand pushed up on the objects. Because the objects were stationary, these forces balanced and there was no overall force acting on the object to move it.

When you placed the object on the flat surface, both gravity (pulling the object down) and the normal force (pushing the object up) acted on the object. Again, they balanced each other, and so the object did not move.

To make the object slide across the surface, you have to push it from the side. You have to apply a force to it. This is called an applied force. When you do this, the object experiences an unbalanced force in the horizontal direction. However, to make the object slide, you have to apply a certain amount of force. You have to apply more force than the force of friction pushing in the opposite direction. As soon as you do, the unbalanced force makes the object slide.

Notice how we use a longer arrow for the applied force to indicate that it is greater than the force of friction. When the force arrows are different lengths, we can see we have an unbalanced force.

Note also that because the unbalanced force was in the sideways (horizontal) direction, the object moved in this direction. The vertical forces were still balanced so the object did not move up or down.

Friction is a force that opposes motion and acts in the opposite direction to the motion. Friction always exists when two surfaces are in contact with each other, but is most notable when one surface moves over another and when one of the surfaces is rough.

When you stopped pushing the object, it slowed down and stopped. This is because, the force of friction was still acting in the opposite direction to the motion but you were no longer applying a force in the other direction. Once again, there was an unbalanced force, but this time in the other direction.

To keep the block moving, you would have to keep applying a force at least as great as the force of friction pushing in the opposite direction.

Once again, the object did not move on its own. You had to apply a sideways force to it that was greater than the force of friction acting in the opposite direction. But this time, because the object was heavier, you had to apply a greater force.

We know that the heavier an object is (the more mass it has) the greater the force it will apply to a surface because of gravity and, therefore, the greater the normal force acting up on it will be. Therefore, the heavier an object is, the greater the forces between it and the surface. This means that the force of friction will be greater.

Therefore, when we have similar objects (like two books) on the same surface, the heavier object will experience a greater force of friction and so a greater horizontal force will need to be applied to overcome friction (create an unbalanced applied force) to move it across the surface.

You might have also noticed that, when you stop pushing, the heavier object stopped moving more quickly. Again, this is because the force of friction acting in the opposite direction to the motion is greater for the heavier object.

When you let go of the piece of paper, it floated slowly down to the ground. This is what you expect. We know that all objects are pulled towards the Earth by gravity. No surprises here!

When we crumple that same piece of paper into a tight ball and drop it, it falls much more quickly? This is because, anything that falls to the ground has to fall through the air. If you have ever ridden a bicycle or stuck your hand out of a moving car, you will know that the air can apply quite a lot of force. This force is called air resistance. The sheet of paper had a big surface that had to move through the air. It experienced lots of air resistance. The crumpled ball of paper was much smaller with a smaller surface that had to get through the air. Therefore it experienced far less air resistance.

In both cases, the mass of the paper was the same, therefore, the force due to gravity was the same. But the greater air resistance acting upwards on the sheet of paper, meant that the overall force pulling it down was less than the overall force pulling the ball of paper down and so it fell more slowly.

An object that can stretch and return to its original length exerts an elastic or spring force. You observed this when you stretched and released the elastic band. As your finger pulled on the elastic band, it pulled back in the opposite direction. The more you stretched it, the greater the force with which it pulled back and so the more force you needed to apply to stretch it further or just hold it in position.

As you reduced the amount of force you applied, and the spring force of the elastic band became greater than your applied force, the band started to contract again and pull your finger with it.

When you released the elastic band with the ball of paper in it, the spring force of the elastic band acted on the paper ball to shoot it forward. This is how catapults work. There was a large unbalanced force acting on the paper ball. The greater this unbalanced force, the further the paper ball will fly.

When you pulled on the rope tied to a fixed point, you would have experienced a tension force. A tension force exists in a rope, cable, chain or string pulled tight. This is not an elastic force because the rope or cable does not stretch further than its original length when it is pulled tight.

As you keep pulling harder, this tension force gets bigger. As it grows, it will either become greater than the friction force being experienced by the object to which it is attached and that object will start to move, or the rope or string will snap.