1. Friction, Gravity, & Elastic Forces
Chapter 10, Section 2
Friction, Gravity, & Elastic Forces

2. Friction
A force that two surfaces exert on each other when they rub against each other
Smooth surfaces produce less friction than rough surfaces
Without friction, a moving object will not stop until it strikes another object
Table Talk: Share with a partner when you might have experienced friction.

3. The strength of friction depends on 2 things:
Causes of Friction
The strength of friction depends on 2 things:
Types of surfaces
How hard the surfaces push together
Try It! Rub both of your hands together, first very lightly, then harder. Voila! Friction!

4. Types of Friction
Static Friction = the friction that acts on objects that are not moving
Sliding Friction = occurs when 2 solid surfaces slide over each other
Rolling Friction = when a object rolls across a surface
Fluid Friction = when a solid object moves through a fluid

5. Gravity A force that pulls objects toward each other
Not just on Earth, it’s also the force that keeps all planets orbiting the sun
The Universal Law of Gravitation states that the force of gravity acts between all objects in the universe
Thank you, Isaac Newton! 

6. Mass and Distance
The force of gravity between objects increases with greater mass and decreases with greater distance
Table Talk: Now that you know this….why does the sun exert such a large gravitational force on the planets?
Cuz I’m so BIG!

7. Gravity & Weight
Weight = the gravitational force exerted on a person or object at the surface of the planet
It varies with the strength of the gravitational force
Weight = mass x acceleration
(due to gravity)
Weight is measured in Newtons (N)
Mass is measured in kilograms (kg)
Acceleration due to gravity = 9.8 m/s2
Try It! How much would a 50 kg person weigh?
50kg X 9.8 m/s2 = 490 N
Table Talk: Would you weigh more or less on the moon? Would your mass change?

8. Gravity & Motion
Free Fall = when the only force acting on an object is gravity
Acceleration is in the downward direction
ALL objects free fall accelerate at the same rate regardless of their masses
9.8 m/s2
Table Talk: If this is true, why would the ball hit the ground faster than the feather when they are dropped from the same height at the same time?

9. Air Resistance A type of fluid friction
Acts on objects falling through the air
Objects with greater surface area experience more air resistance as they fall
Try It! Take two pieces of paper, exactly the same size. Leave one flat and crumple one into a ball. Drop them from the same height at the same time. What happens?
In looking at this picture, what else could affect how quickly the papers fall?

10. Elastic Forces
Matter is elastic if it returns to its original shape after squeezed or stretched. Compression = An elastic force that squeezes or pushes matter together. Tension = An elastic force that stretches or pulls matter.
Table Talk: Can you think of some other things that may involve compression and tension forces?
The upward force of tension balances the shoe’s weight
See BrainPop “Gravity” end