1. Section 2 – pg 380 Friction, Gravity, and Elastic Forces
Chapter 10
Section 2 – pg 380
Friction, Gravity, and Elastic Forces

2. Pg 381
Friction
Friction: a force that two surfaces exert on each other when they rub against each other
Acts in a direction opposite to the motion of objects

3. The Causes of Friction Pg 381
Smooth surfaces produce less friction than rough surfaces
Even the smoothest objects have irregular, bumpy surfaces
When the irregularities of one surface come into contact with those of another surface, friction occurs
The strength of the force of
friction depends on the types of
surfaces involved and on how
hard the surfaces push together
Iron under microscope

4. Pg 382
Static Friction
Static Friction: the friction that acts on objects that are not moving
B/c of static friction, you must use extra force to start the motion of stationary objects

5. Pg 382
Sliding Friction
Sliding Friction occurs when two solid surfaces slide over each other
When an object is pulled at a constant velocity across a level surface, the pulling force equals the sliding force
Bike brakes work using sliding friction: the rubber pads push against the metal rim of the wheel, causing the wheels to slow and eventually stop

6. Pg 382
Rolling Friction
When an object rolls across a surface, rolling friction occurs
Rolling friction is less than sliding friction for similar materials
Engineers use ball bearings to reduce the friction between wheels and the rest of the product. These ball bearings are small, smooth steel balls that reduce friction by rolling between moving parts

7. Pg 382
Fluid Friction
Fluid Friction occurs when a solid object moves through a fluid
Fluid friction is usually less than sliding friction
This is why many machine parts need to remain oiled

8. Gravity Pg 384 Gravity: a force that pulls objects towards each other
Gravity acts everywhere in the universe
Reason moon orbits earth and planets orbit the sun
Universal Law of Gravitation states that the force of gravity acts between all objects in the universe
All objects in the universe are attracted to each other
If the objects are small then the attraction force is also small

9. Pg
Mass and Distance
The force of gravity between objects increases with greater mass and decreases with greater distance
Mass: the measure of the amount of matter in an object
The more mass an object has, the greater its gravitational force
The farther apart two objects are, the lesser the gravitational force between them

10. Pg 385
Gravity and Weight
Weight: the gravitational force exerted on a person or object at the surface of a planet
Objects that have a greater mass will have a greater weight
Weight = Mass x Acceleration due to gravity
Weight is measured in Newtons and mass in kilograms
Unlike mass, weight varies with the strength of the gravitational force

12. Pg 386
Gravity and Motion
Gravity and air resistance are the two forces responsible for how things fall

13. Pg 386
Free Fall
Free Fall: when the only force acting on an object is gravity
An object in free fall is accelerating
In free fall, the force of gravity alone causes an object to accelerate in the downward direction
Near the surface of the Earth, the acceleration due to gravity is 9.8 m/s2
After 1 sec, falling at 9.8 m/s
After 2 secs, falling 19.6 m/s
After 3 secs, falling m/s

14. Pg 387
Air Resistance
In a vacuum, where there is no air, all objects fall with exactly the same rate of acceleration
A type of fluid friction called air resistance acts on objects falling through airs
A upward force exerted on falling
objects
Objects with greater surface area
experience more air resistance as they
fall
A leaf falls more slowly than an acorn
because the leaf has a larger surface area

15. Projectile Motion Projectile: an object that is thrown Pg 387
A projectile that is thrown horizontally will land on the
ground at the same time as an
object dropped

16. Pg 388
Elastic Forces
Matter is considered elastic if it returns to its original shape after it is squeezed or stretched
Compression and tension are two types of elastic forces
Compression: an elastic force that
squeezes or pushes matter together
Tension: an elastic force that
stretches or pulls matter

17. Chapter 10 Section 2 Homework – pg 388

18. 1A. What factors affect the friction force between two surfaces?

19. 1B. What are the four types of friction?

20. 1C. What types of friction occur when you ride a bike through a puddle?

21. 2A. What is the law of universal gravitation?

22. 2B. How do mass and distance affect the force of gravity between objects?

23. 2C. How would your weight change on the surface of an Earth-sized planet whose mass was greater than Earth’s? Why?

24. 3A. Why does an object accelerate when it falls toward Earth’s surface?

25. 3B. How does the mass of an object affect its acceleration during free fall?

26. 3C. What force changes when a sky diver’s parachute opens
3C. What force changes when a sky diver’s parachute opens? What force stays the same?

27. 4A. What are two types of elastic forces?

28. 4B. Describe the forces on a rope that support a tire swing when a child sits on a tire.