1. Friction and Gravity

2. What is Friction?
Friction is the resistance to the sliding, rolling, or flowing motion of an object due to its contact with another object. 
Without friction, an object would continue to move at constant speed forever.

3. The strength of the force of friction depends on the type of surfaces and how hard the surfaces push together.
Rough surfaces produce greater friction than smooth surfaces.

4. Three Types of Friction
Static Friction
Sliding Friction
Fluid Friction

5. Static Friction
When the force on an object and the friction are balanced.
An object is not in motion when there is static friction

6. Sliding Friction
A resistance to movement that is created when two things rub together.

7. Fluid Friction
The force that tries to slow objects down when they move through a liquid or a gas. It's also known as "drag", or "air resistance".
All gases and liquids are fluids.
An airplane and a swimmer both experience fluid friction.

8. Ways to Reduce Friction
Smooth the surface
Put ball bearings in wheels
Replace sliding with rolling
Add oil or another type of lubricant

9. Gravity
The attractive force between all objects that have mass

10. Universal Gravitation
The law of universal gravitation states that the force of gravity acts between all objects in the universe.

11. Gravity and Free Fall An object is in free fall when the only
force acting on the object is gravity.
Free-falling objects do not encounter
air resistance.
In free fall, the force of gravity is an
unbalanced force.

12. Calculating Free Fall
All objects in free fall accelerate at the same rate – 9.8 m/s² - regardless of their mass.
in one second = 9.8 m/s²
in two seconds = 19.6 m/s²
in three seconds = 29.4 m/s²
in four seconds = ________
The velocity continues to increase as the object falls.

13. Gravity and Air Resistance
Most objects falling through air experience a type of fluid friction called air resistance.
Air resistance is not the same for all objects.
The greater the surface area of the object the greater the air resistance.
However, since the elephant has more mass, it has more downward force of gravity and falls faster.

14. Weight The gravitational force exerted on an object.
Mass: amount of matter in an object.

15. Contact vs. Noncontact Forces
Contact Force: a push or pull by touching
Ex: Friction
Noncontact Force: a force exerted on an object without touching it
Ex: Gravity