UNIT 3.3 – FRICTION

AND GRAVITY

FRICTION

Friction - the force that two surfaces exert on

each other when they rub against each other.

The amount of friction is determined by two

factors:

1. How hard the surfaces push together

2. The type (roughness) of the two surfaces

4 Types of Friction

1. Static Friction- friction between 2 things not

moving past each other

2. Sliding Friction- friction between 2 things

that are moving

3. Rolling Friction- friction between a rolling

object and the object it rolls on

4. Fluid Friction – friction that occurs when a

solid object moves through a fluid (liquids and

gases are fluids)

Copyright 2009 Pearson-

Motion, Forces, and Energy

Gravity

Gravity – a force that pulls objects toward each other Anything that has mass can cause a

gravitational force

Law of Universal Gravitation – the force of gravity acts between all objects in the universe.

Factors Affecting Gravity

There are two factors that affect gravitational attraction between two objects:

1. Mass – amount of matter an object has

2. Distance – how far the two objects are from each other

Weight vs. Mass

Mass – how much matter an object has.

Weight – the force of gravity on a person

Weight = Mass x Gravity

Free Fall

Free Fall – when

the only force acting

on an object is

gravity

During free fall,

gravity is an

unbalanced force!

Free Fall

During free fall, objects

accelerate (or increase

velocity)

Acceleration due to gravity

is 9.8 m/s2

A ball starts at 0 m/s

After 1 second, its velocity is

9.8 m/s

After 2 seconds, its speed is

19.6 m/s (9.8 m/s + 9.8 m/s )

Air Resistance

Theoretically, all

objects are supposed

to fall at the same rate

(due to free fall).

Air Resistance – fluid

friction experienced by

objects falling

The leaf experiences

more air resistance

Projectile Motion

Projectile – an object that is thrown

Projectile Motion occurs when you throw a ball in any direction except vertically (up or down).

As a ball is dropped vertically, the ball in projectile motion is thrown at the same time. They will land on the ground at the same time as well.

This is because the force of gravity acts on both balls the same way, in a downward direction.

Friction Gravity

Effect on motion Opposes motion Pulls objects toward one

another

Depends on

Types of surfaces

involved, how hard the

surfaces push together

Mass and distance

Measured in Newtons Newtons

- Friction and Gravity

Comparison Chart