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Chapter 4Forces in One Dimension

- Newtons Laws of Motion

Force is the action which has the ability to

change motion.

- Objects need a force applied to them in order to

change their motion.

- A force is also called a push or a pull.

Sir Issac Newton described three Laws that govern

motion.

- Force is measured in Newtons (N) in the metric

system. - 1 N 1 kgm/s2
- The English unit for force is a pound (lb).
- 4.48 N 1 lb
- Forces are vector quantities and can
- be denoted with arrows because you
- can push in specific direction.

Force diagrams Four types of forces can act on

an object

Fn normal force. This is the support force that

objects resting on each other exert.

Fa applied

force This is the force that is being applied to

the object. It always acts like it comes out of

the center of mass

Ff frictional force This is a force that

happens whenever two surfaces touch. This force

will always oppose the applied force.

Weight Fg Gravitational force This is the force

created because of gravity pulling on the mass of

the object.

What if forces are balanced?Newtons First Law

of Motion

- An object at rest stays at rest or an object in

motion stays in motion (constant velocity) unless

acted on by an unbalanced (net) force. - It is also known as the law of inertia
- Inertia an objects ability to resist a change in

motion.

INERTIA

Example The man pushed the cart in order to

make it roll. The bigger the cart the harder

the man must push

What if forces are unbalanced?Newtons Second

Law of Motion

- If an unbalanced (net) force is applied to a mass

it will accelerate. - The mathematical relationship between

acceleration and mass is directly proportional

and inversely proportional to force. A math

formula can be written - F m a
- F force
- m mass
- a acceleration

F m x a

What is mass?

The amount of stuff (atoms) in an object

You try!

Example The man pushed the cart with 10 N of

force to make it accelerate 20 m/s2. What was the

mass of the cart? See if you can

calculate the mass.

F a m

10 N

20 m/s2

0.5

kg

?

What if forces are unbalanced?

Tracker A is pulling on the 10 kg block with a

force of 50 N and tracker B is pulling on the

block with 20 N. How fast is the block

accelerating and in what direction?

F m a

30 N

Remember your vector rules

10 kg

Net Force 50 N 20 N 30 N

?

3 m/s2

Remember what happens if all four forces are

balanced?

The object is standing still or moving at a

constant velocity!

Weight and Gravity

- Gravity is the rate at which all things fall to

the earth. - It is always 9.8 m/s2
- Weight is a measure of how gravity pulls on your

mass to create a downward force. - Your weight can be calculated just like any other

force is calculated. Fw weight - m mass
- g gravity

Notice that gravity is an acceleration value

Fw mg

g 9.8 m/s2

Air resistance is friction for falling objects.

Air resistance is why it seems like things fall

at different rates.

All things fall at this rate no matter what their

size!!!!

Remember the things that effect gravity are

- The size of the mass.
- The bigger the mass the greater the gravitational

pull. - The distance between the masses.
- The farther apart two masses, the less

gravitational force is felt between

them.

Weight vs. Mass

- Your mass is constant because it is a measure of

the amount of atoms in your body. - Your weight will change if the gravity of the

planet changes.

Since Mars is a smaller planet, it will have a

smaller gravity and pull down on a mass with less

force.

Forces come in pairsNewtons Third Law of Motion

- For every action there is an equal and opposite

reaction. - These forces always occur in pairs and are often

called action-reaction forces. - Remember if forces are equal, what is happening

to the object?

EQUAL and OPPOSITE REACTIONS

They are standing still or moving at constant

velocity.

Action The boy threw the ball forward with a

certain amount of force

Reaction The boy rolled backward with the same

amount of force

Momentum An objects tendency to continue with a

certain motion

- Momentum has to do with collisions and what

happens to motion when objects collide. - The Law of conservation of momentum states that

the amount of momentum after a collision must

equal the amount of momentum before the collision

if no outside forces act on the objects.

- You can calculate momentum using this formula
- p momentum
- p mv m mass
- v velocity