1. Gravity and Friction
Ch. 2 Section 1

2. Force A push or pull of an object
Some forces can only be applied when touched
Other forces are applied even when objects do not touch!

3. TOUCHING! Force Contact Force
Contact Force- a push or pull on one object by another that is touching it
TOUCHING!
Force

4. NOT TOUCHING! Noncontact Force
Noncontact Force- a force that one object can apply to another object without touching it
NOT TOUCHING!

5. Forces Forces have both magnitude and direction
What is this like?
Arrows can be used to represent this
(just as we did for velocity and acceleration)

6. Newtons The SI unit for force is a newton (N)
You apply a force of about 1N when lifting a stick of butter
You apply a force of 20N when lifting a 2L bottle of water

7. An attractive force that exists between all objects that have mass
Gravity
An attractive force that exists between all objects that have mass
Objects fall to the ground because Earth exerts an attractive force on them
You also exert an attractive force on objects!

8. The amount of matter in an object
Mass
The amount of matter in an object
Often measured in kilograms (kg)
The mass of an object -the amount of matter the object has- stays the same no matter where the object is.

9. The Law of Universal Gravitation
All objects are attracted to each other by gravitational forces
The strength of the force depends on the mass of each object and the distance between them
Sir Isaac Newton in the late 1600s

10. Gravitational Force and Mass
Notice the force arrows are the same size between the objects!

11. Gravitational Force and Distance
Notice the force arrows are the same size between the objects!

12. Effect of Distance on Gravity
The gravitational force between objects decreases as the distance between the objects increase

13. The gravitational force exerted on an object
Weight
The gravitational force exerted on an object
Earth is more massive than anything on it, so everything is pulled toward it

14. Weight
Near Earth’s surface an object’s weight is the gravitational force exerted on the object by Earth
Because weight is a force it is measured in Newtons

15. Does an astronaut weight the same on Earth as on the moon?
NO!

16. Does an astronaut have the same mass on the Earth as on the moon?
YES!

17. A force that resists the motion of two surfaces that are touching
Friction
A force that resists the motion of two surfaces that are touching

18. Surfaces are Uneven!
Atoms cling together at many points of contact when slid against each other creating friction

19. Friction
The direction of the friction force is always in a direction opposing motion

21. Friction
To push a heavy box, you must apply enough force to overcome friction
So you have to be winning the net force battle

22. Static Friction vs. Sliding Friction
The initial force to get a stationary object to move
Example: If you push on a crate, it takes more force to get it going than it takes to keep it sliding.
The force of friction of a sliding object
Example: The force of friction against the tires of a sliding car

23. Static Friction vs. Sliding Friction

24. Fluid Friction
Friction between a surface and a fluid (water, air, etc.)
Fluid friction in air is air resistance
SURFACE AREA MATTERS!

25. Reducing Friction Soap on your hands creates a lubricant
How can you reduce air resistance (friction) on a falling object?