Forces & Newton’s Laws

Ch. 4

Forces

• What is a force? – Push or pull one body exerts on another– Units = Newton (N)– Examples:

List all of the forces that you can

encounter/exert in a day

Forces

• Types of forces:

1. Friction- force that opposes motion between two surfaces that are touching. – Air Resistance – Special type of frictional

force. • Occurs when the object is traveling through the air

Forces

2. Applied Force – A force that is applied to an object by another object.

Example: You, wind, water, etc

3. Gravitational Force – The force of gravity on an object.

Forces

• Balanced forces: forces exerted on an object of equal magnitude (size) but in opposite directions.

Unbalanced Forces: unequal pulling of forces on an object.

Forces

• Example:

In a tug of war, the net force causes an acceleration in the same direction.

Are the forces balanced or

unbalanced in the picture?

ForcesIf forces are balanced

(all forces cancel)

Acceleration = 0 m/s2

Fnet on object = 0 N

Object is doing1 of 2 things…

Forces

Object is doing1 of 2 things

At restv = 0 m/s

In motion“constant” velocity

staysat

rest

staysin

motion

Forces

• Inertia– The tendency of an object to resist any

change in motion. – The velocity of an object will remain the same

unless a force acts on it.

Example:

Forces

• Inertia is dependent on mass.

• Would you expect a bowling ball and a tennis ball to have the same inertia?

**Think about resistance to change motion**

• More mass greater inertia!!

Forces

• Newton’s 1st Law Law of Inertia

An object at rest will stay at rest.

An object in motion will stay in motion.

Unless acted on by an outside force.

Forces

• Examples:

• A rolling basketball would continue to roll forever if an outside force did not act on it.

• A rock will never move unless an outside force moves it.

Newton’s 2nd LawIf forces are unbalanced

(they do not cancel)

The mass acceleratesin the direction of the larger force.

Newton’s 2nd Law

The relationship betweenF and a

Fnet = ma

N kg m/s2

“net” means total

Newton’s 2nd Law

• Newton’s 2nd Law Law of Force

• If a larger force exists, then there must be an acceleration in the direction of that force.

Newton’s 2nd Law

• Helpful Hints:

Fm a

N

kg ms2

Newton’s 2nd Law

Example:

Newton’s 3rd LawEqual and Opposite

• For every action there is an equal and opposite reaction

• Example:– Imagine a rocket is being

launched from the earth. Hot gases are pushed out from the bottom of the rocket as the rocket is thrust upward. The force of the gases pushing against the surface of the earth is equal and opposite to the force with which the rocket moves upward

Momentum

• A property a moving object has because of its mass and velocity

• Momentum depends on both mass and velocity

Momentum

• Momentum can be calculated using the following equation

–p= mv

• p= momentum v= velocity(m/s)

• m= mass (kg)

• Unit for momentum is kg·m/s

Momentum

• Objects that are not moving (at rest) have ZERO momentum

• Example:– A bicycle and a car are traveling at the same

velocity. Which has a greater momentum?

Which has a greater momentum?

• WHY?

• The car would be have a greater momentum, because the car has a greater mass

Law of Conservation of Momentum

• The amount of momentum a group of objects does not change unless and outside force acts on them.

• Example: