1. Machines 2. Friction

2. Friction is a force that resists,
or works against, motion

3. Vocabulary:
friction
a force that works against motion when one object rubs against another object

4. Every motion that takes place on Earth is affected by friction
Without friction, life would be very different

5. You could not walk without friction
to keep your feet from sliding on the ground

6. Concrete, grass, and dirt are rough surfaces
Rough surfaces provide more friction than smooth surfaces

7. If you have ever tried to walk on ice,
you know that your feet slip and slide

8. Smooth surfaces such as ice provide very little friction

9. Without friction, you could not pick things up or hold them
Everything you tried to hold would slip through your hands and fall

10. Without friction cars could not move
Their tires would not hold on to the road

11. Mud does not provide enough friction for the tires to hold
When a car gets stuck in mud, the wheels turn around but the car does not move
Mud does not provide enough friction for the tires to hold

12. Have you ever tried to push a heavy box?

13. Friction between the ground and the box makes this difficult
You can make the box easier to push by taking some things out of the box

14. This makes the box lighter
The heavier something is,
the more friction slows it down

15. Try rubbing your hands together
Friction makes heat
Try rubbing your hands together

16. You will feel them getting warmer

17. When you rub two sticks together, they get warmer and warmer
When they get hot enough,
they start to burn