Force

A force is a push or a pull.

Pushing on a stalled car is an example.

The force of friction between your feet and the ground is yet another.

Force

Weight is the force of the earth's gravity exerted downward upon your body.

It is true that the more mass you have, the more you weigh.

Weight and mass are proportional but they are not the same thing!

All forces have a direction.Weight, being a force, always points down,

toward the center of the earth.

Mass has no direction.

Force vs. Mass

Mass is related to how much matter there is in an object.

An anvil has a great deal of mass, and here on Earth it weighs quite of

bit, too.

Force vs. Mass

But in space, where there is no gravity,

the astronaut and the anvil are both weightless.

They both have the same amount of mass they did on

earth.

Mass and Weight

Weight is the force of gravity exerted upon a mass.

Weight and mass are related, but are not the same.

An object's weight depends upon two things:

1. its mass

2. the acceleration of gravity in the part of the universe it occupies.

Weight is Proportional to Mass

Here is the equation for weight.

weight = mass x acceleration of gravity

Weight also Depends on Gravity

On the surface of the earth, the acceleration of gravity is 9.8 m/s2.

It is quite a bit less on the moon: 1.6 m/s2.

It is too small to notice in interstellar space.

Consequently, an object from Earth would weigh less on the Moon, and would weigh

almost nothing in space.

Metric Units of Weight

Many people mistakenly use the kilogram as a unit of weight. This is wrong. The

kilogram is actually a unit of mass.

w = mgw = (1 kg)(9.8 m/s2)

w = 9.8 Newtons

1 kilogram actually weighs 9.8 newtons.

Isaac NewtonThe metric unit of force is named after Isaac

Newton (1642-1727), one of the greatest figures in all of science.

His mathematical description of the

world went unchallenged until

the twentieth century.

Newton's First Law of Motion

An object at rest tends to remain at rest and an object in motion tends to move in a straight line at a constant speed, unless

acted upon by an unbalanced force.

Why Things Move

Force is a push or a pull.

An unbalanced force causes an object to

accelerate.

The direction of a force is just as important as its size.

Why Things Move

If the unbalanced force is in the same direction that the object was already moving, then the object speeds up.

Force motion

Why Things Move

If the unbalanced force is in the opposite direction as the object was moving, then it slows down.

motion Force

Why Things Move

If the unbalanced force is applied sideways to the object's motion, then the object will change

direction.

Why Things Move

When an unbalanced force is applied to an object, it will either speed up,

slow down, or change direction.

All of these results are called acceleration.

Why Things Move

When a single force is applied to a motionless object, it will

cause the object's motion

to change.

The single force was unbalanced. The football will now accelerate.

Why Things Move

The direction of a force is important.

Two equal forces, applied to the

same object but in opposite

directions, cancel each other out.

In this case, there is no unbalanced force, and the object has no change in its motion.

Why Things Move

The term net force has the same meaning as unbalanced force.

To find the net force means to sum up the effects of all the forces acting upon an

object.

Only a net force can change the motion of an object.

Why Things MoveWhen you push on a book,

the force of friction resists you.

What is the net force acting on this book?

Why Things Move

Two students try to push a stalled car.

What is the net force acting upon this car?

Why Things Move

The larger the net force, the greater the acceleration becomes.

Why Things Move

If the net force is doubled, that causes the acceleration to double.

We say that the acceleration is directly proportional to the net force.

a Fnet

Why Things Move

In the previous example, we had to hold the mass of the object constant while we changed

the applied force.

What do you predict would happen if we used the same net force on a

larger amount of mass?

Why Things Move

The larger the mass, the smaller the amount of acceleration.

Why Things Move

Note that the objects still speed up, but not as quickly as before.

In fact, if you double the mass, the acceleration of it becomes only half as great.

The acceleration is proportional to the inverse of the mass.

a

1m

Newton's 2nd Law of Motion

Combining both proportionalities into one equation:

Action-Reaction

Forces come in pairs!

Whenever an object exerts a force on another object, the other object exerts the same amount

of force back on the first, but in the opposite direction.

This is known as Newton's 3rd Law of Motion.

It is popularly known as the law of action-reaction.

Newton's 3rd Law

Newton's 3rd Law is always true,even when the objects are different sizes...

Hammer's force drives the nail;

Nail's force decelerates hammer.

Why Things MoveNewton's 3rd Law is always true, even when

one or more of the objects doesn't move.

You push on the wall;

the wall pushes back on you.

Why Things Move

Newton's 3rd Law of Motion, also known as the Law of Force Pairs:

For every force, there is a reaction force equal in magnitude but opposite in direction.

If object A exerts a force on object B, then object B exerts the same amount of force back

on object A.

Newton's 3rd Law

Action-reaction force pairs:

Not-so-obvious Forces

Forces that are often overlooked:

The normal force is the force the table

exerts on the cement block in reaction to the block pressing

down upon it.

Not-so-obvious Forces

More forces that are easily overlooked:

If the force of friction is as large as your push, then there is no

unbalanced force and the block will

not accelerate.

It will move forward at constant speed.

The Free-body DiagramThe five forces acting upon an automobile.

Normal force

Force of gravity(weight of car)

Friction with the air

Rolling friction

Drive force

Explaining Changes in Motion

How those five forces combine (sum up) will determine how much, if any, net force is exerted

on the automobile.

The magnitude of that net force, along with its direction will determine what will happen next to

the car.

In other words, whether it will speed up, slow down, change direction, or just continue to go

ahead at constant speed.

Explaining Changes in Motion

Imagine you are taking a trip in your car.

See if you can use Newton's Laws of Motion to explain what will happen when you...

Stop at a stop light.

Accelerate when the light turns green

Make a left turn

Jam on the brakes when a pedestrian steps in front of your car