1. Force
S8P3. Students will investigate relationship between force, mass and the motion of objects

2. Force
Force – a push or a pull exerted on an object in order to change the motion of the object; force has size and direction
Measured in Newton’s (N)

3. Unseen Force Sometimes things exert a force and you can’t see it!
Magnets on a refrigerator
Static electricity
Gravity

4. Determining Net Force
Net Force – combination of all of the forces acting on an object
If the forces are acting in the same direction ….you add them.
Example: You are asked to move a piano. You and your friend are exerting force in the same direction N + 20 N = 45 N

5. Determining Net Force
If the forces are in the opposite direction you subtract the smaller force from the larger force
Example: 2 dogs playing tug of war. Large dog 12 N of force. Small dog 10 N of force. What is the net force?
12 N – 10 N = 2 N

6. Balanced and Unbalanced Forces
If you know the net force on an object, you can determine the effect of the net force on the object’s motion. Why? The net force tells you whether the forces on the object are balanced or unbalanced.

7. Balanced Forces
When the forces on an object produce a Net force of 0 N, the forces are balanced. Balanced forces will not cause a change in the motion of a moving object or a nonmoving object to start moving.

8. Unbalanced Force
Unbalanced forces produce a change in motion, such as a change in speed or direction.
Unbalance forces are necessary to change the motion of moving objects.

9. Class Work
Answer questions 2 – 9 on page 127

10. ??????????????
While playing ball, your friend throws the ball out of your reach. Rather than running for the ball, you walk after it. You know that the ball will stop. But do you know why?
Friction

11. Friction
Friction – a force that opposes motion between 2 surfaces that are in contact.
Friction occurs because the surface of any object is rough. Even surfaces that feel smooth are covered with microscopic hills and valleys. When 2 surfaces are in contact, the hills and valleys of 1 surface stick to the hills and valleys of the other surface. This contact causes friction.

12. Friction
The amount of friction between two surfaces depends on many factors. Two factors include the force pushing the surfaces together and the roughness of the surfaces

13. Kinetic Friction The friction between moving surfaces.
The amount of Kinetic friction depends on how the surfaces move. Surfaces can slide past each other or they can roll over another surface. Usually the sliding friction is greater than the force of rolling friction…..therefore, it is easier to move objects on wheels than to slide objects along the floor
Firgure 3 pg 130

14. How can friction be helpful?
Stops your car
You would fall down when walking without friction
Using a pencil

15. How can friction be harmful?
Moving engine parts causes them to wear down
Causes holes in your socks
Causes holes in your blue jeans

16. How can you reduce friction?
Oil/lubricant (you must keep oil in the engine of your car, you put oil on the chain of your bicycle)
Make the surfaces smoother (use sandpaper and sand the surface smoother)

17. How can you increase friction?
Make the surface rougher (putting sand and rock on the roads when they are icy)
Wearing textured batting gloves when hitting a baseball/softball

18. Class Work
Complete questions 2 – 10 on page 133