Chapter 12

Forces & Motion

Forces “a push or a pull” A force can start an

object in motion or change the motion of an object.

A force gives energy to an object

Representing forces We often use arrows. Larger the arrow, the

greater the force Combined balanced

forces equal zero Normal force = the

force acting perpendicular on an object in contact with another object.

Balanced Forces Forces opposite in

direction and equal in size.

Net result: no change in motion.

No change in motion!

Balanced forces will keep an object moving at a constant velocity

ENGINEFRICTION

Friction:

One of the most common forces. Always acts in a direction opposite

the direction of motion. There are four main types of

friction.

Static Friction The force that must one must overcome to

get an object to move. Examples: Trying to move a heavy object Tires on pavement when a car is moving Walking

Sliding Friction Two surfaces

sliding over each other.

Caused by two factors:

1. Weight of moving object

2. Types of surfaces in contact

Rolling Friction An object rolling

over a surface Not as strong as

sliding Can be reduced

by using wheels and ball bearings.

Fluid Friction When an object is

moving through a fluid (either gases or liquids)

ex. a fish swimming under water, air resistance

Is friction ever helpful?

YES. Examples:

1. Stopping a car

2. Driving on ice

3. Walking

GRAViTYThe pull of

one body of mass on another.

How fast do objects fall on earth?

Galileo 16th century Dropped two metal

spheres of different sizes off the Leaning Tower of Pisa

Which hit first? They hit at the same

time.

Acceleration due to gravity All objects fall at the

same rate, regardless of their mass or weight

The acceleration due to gravity on earth is about 9.8 m/sec2.

This value will change with elevation and location on earth.

Terminal Velocity After a period of

free fall, the force of gravity will be canceled by the force of air friction and objects will travel at a constant velocity

Projectile motion

Any object thrown in the air becomes a projectile.

The object will move forward due to inertia, and downward due to gravity.

Always a curved path.

Aristotle 384 B.C. – 322 B.C. Greek scientist and

philosopher First to work with

force. Incorrectly stated that

a force is required to keep an object moving at constant speed

Sir Isaac Newton 1642-1727 British

Physicist Considered the

“Founder of modern physics”

Newton’s First Law of motion An object at rest will remain at rest and

an object in motion will remain in motion unless acted upon by an outside force.

Often referred to as the Law of Inertia.

(the property of matter that resists any change in motion)

Newton’s Second Law of Motion The force of a moving object is directly

proportional to the object’s mass and acceleration.

The most important of Newton’s laws.

How do we label force???

F = ma Mass is measured in kg Acceleration is measured in m/sec2

If we multiply these two units :

1 kg m/sec2 = 1 newton (N)

Weight and Mass

MASS The measure of inertia

of an object. The amount of matter

in an object Stays constant with

changes in location A scalar

WEIGHT A measure of the force

of gravity acting on an object.

Measured in newtons. Changes with changes

in location. A vector

Newton’s Third Law of Motion For every action there is an equal and

opposite reaction, or All forces occur in pairs examples:

Momentum

All moving objects have momentum. It is the product of an object’s mass and

velocity. Which has more momentum? A moving car

or a moving train? Not enough information. A 1500 kg car traveling 50 m/sec? Or a

48,000 kg train traveling 2 m/sec?

Universal Forces

Electromagnetic force: Forces that attract and repel.

Nuclear forces

STRONG Responsible for

holding the nucleus together

Strongest known force

WEAK Very small

attractive force in the nucleus

Newton’s Law of Universal Gravitation Every object in the universe attracts

every other object. Depends on two factors:

1. Distance between the objects

2. Mass of the objects

The Earth, Moon and Tides

Earth’s gravitational pull on the moon combined with the inertia or momentum of the moon keeps the moon in orbit around the earth.

The pull of the moon and sun causes the tides

Satellites

Satellites must be placed at a specific height above the earth’s surface in order to stay in orbit.

If it is too close, or moving too slow, it can spiral into the earth’s atmosphere and burn.

Assignment:

Page 385-386 1-11, 16, 17, 18, 20, 23,27,

31,32,33 1-6 on page 387