1. 3.3 Motion & Force

2. Objectives Explain how forces and motion are related.
Compare and contrast static friction and sliding friction.
Describe the effects of air resistance on falling objects.

3. Force Push or pull that one object exerts on another
Vector with size (strength of force) & direction
SI Units = Newtons (N)
Can cause the motion of an object to change
Example: hitting a tennis ball
shooting pool/billiards

4. Balanced Forces Forces equal in size but opposite in direction
doesn’t change velocity

5. Net Force When 2 or more forces act on an object at the same time
Sum of all forces acting on an object

6. Unbalanced Forces
When forces combine to produce a net force that is not zero
Forces that aren’t equal  The object moves
Changes the velocity of an object

8. Friction
The force between two objects, in contact, that opposes the motion of either object
Unbalanced force
Why does a ball stop rolling?
Why do we have to keep applying gas to the engines in our cars to keep moving?
Better yet, why do we put oil in a car’s engine?
Why do runners wear running shoes?
Why do parachutes work?

9. Friction Friction depends on the surfaces of the objects in contact
Smooth things tend to have less frictional force  they are more slippery
Ice, oil on concrete, my old shoes, my road bike tires
Rough things tend to have more frictional force  less slippery
Asphalt, my new running shoes, my mountain bike tires

10. Frictional Force
Increases when the force pushing surfaces together increases
Static Friction = frictional force that prevents 2 surfaces from sliding past each other

11. Rolling friction = similar to sliding friction
Sliding friction = force that acts in opposite direction to the motion of a surface sliding past another surface
Rolling friction = similar to sliding friction
why it is easier to move something on wheels

12. Friction Air Resistance is a form of friction
Caused by the interaction between an object and the air molecules it comes into contact with
The bigger the object the more air resistance
The faster the object the more air resistance

13. Air Resistance
Causes objects to fall at different acceleration and speed
Acts in opposite direction to velocity
Air resistance not mass causes objects to fall at different rates

14. v v

15. v v v

17. Terminal Velocity When an object falls with constant velocity
Upward air resistance becomes large enough to balance downward force of gravity