# Newton's Laws

# Momentum and energy

## Newton's Laws

- The Law of Inertia
- F = ma
- For every force there is an equal and opposite force

## Newton's First Law

Every object continues in a state of rest or of uniform speed in a straight line unless acted upon by a nonzero force.

You may have heard this as

An object in (constant) motion stays in (constant) motion, and object at rest stays at rest.

## Newton's First Law

## newton's first law

- When you slam on the brakes in your car, your body keeps going forward (one good reason to wear your seat belt).
- When you are rear-ended in your car, your head stays where it is, giving you whiplash if you don't have a headrest (this is sometimes tricky -- why does your head seem like it is going backwards? Is it
*really*"going backwards"?) - The Voyager I spacecraft, launched in 1977, is traveling away from Earth at 38,600mi/hr, but it hasn't had propellant since 1978.
- Flip a coin while on an airplane -- the coin doesn't fly backwards at 500mi/hr -- it lands in your hand.

## newton's second law

*directly proportional*to the net force on an object, and

*inversely proportional*to the mass of the object.

## newton's second law

##
finally! Why objects fall at the same rate

## Checkpoint

*equal forces of gravity*act on both the hammer and the feather in a vacuum?

## Falling with Air Resistance

## Newton's Third Law

*cannot*act on its own:

*forces always come in pairs*in an interaction.

- Support yourself on a wall. Your hand pushes on the wall, and the wall pushes back on your hand (and you can feel it)

- A sledge-hammer strikes a stake: the hammer pushes on the stake, and the stake pushes back on the hammer (and the hammer stops)
- You pull on a rope attached to a sled. The sled pulls back on you (and you feel it).
- A boxer punches a wall (wall pushes back) / a boxer punches a piece of paper (paper pushes back, but overall push isn't as much--neither is the punch!)

## newton's third law

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

If we say that there is an "action force" and a "reaction force," then we can say:

To every action there is always an opposed equal reaction.

(by the way, it doesn't matter which force is the action and which force is the reaction)

## newton's third law

**ACTION: Object A exerts a force on object B.**

**REACTION: Object B exerts a force on object A.**

- Hammer pushes down on nail : nail pushes up on hammer.
- Foot kicks ball forward : ball pushes backwards on foot.
- Book pushes down on table : table pushes up on book.
- Earth pulls down on human through gravity : ???

## newton's third law

*equal*and opposite, why does the bullet travel forward at hundreds of miles per hour, while the gun recoils backward at a much slower speed? It's all about acceleration:

## Defining the System

## defining the system

## momentum

*momentum*than the car. By "momentum," we mean "inertia in motion." Specifically,

## Momentum and Impulse

*Forces*. The greater the force, the greater the acceleration.

*But*, there is another aspect: the amount of time the force acts. If a force acts for a longer time, there is more of an acceleration, and more of a change in momentum.

## momentum and impulse

## the impulse–momentum relationship

**Case 1: Increasing Momentum**

**Case 2: Decreasing Momentum Over a Long Time**

**Case 3: What about Bouncing?**

## conservation of momentum

*p*, and the cannon itself travels in the opposite direction with a momentum -

*p*. Momentum is a vector quality, so it can cancel!

## The Law of Conservation of momentum

In the absence of an external force, the momentum of a system remains unchanged.

Look at examples of *elastic* and *inelastic *collisions (**interactive**)

An elastic collision is a collision where there is no lasting deformation or generation of heat. Elastic collisions conserve momentum and energy.

An inelastic collision is a collision with lasting deformation and/or the generation of heat. Completely inelastic collisions result in two bodies fusing as one.

## energy and work

Energy is the property of a system that enables it to do work.

(this definition works, but it is not perfect!)

Work is defined as 'force X distance'

We have to be very careful about what we mean by "distance"

When you push a crate across a floor, you do work, but you don't do work when you carry a book at constant speed across the room.

## energy and work

*hold*a heavy weight above your head?

## work and energy

*on*an object to the work done to hold an object (that would be internal to your muscles).

## potential energy

**PE = mgh (Measured in "Joules")**

## potential energy

*it does not matter how the weight gets to that height!*

## kinetic energy

*kinetic energy*(KE), and it depends on the object's mass and it's speed.

KE =½mv² (also measured in Joules)

If you throw a ball, your hand does work on the ball to give it kinetic energy. The ball can now do work on something when it hits it.

The kinetic energy is equal to the amount of work required to bring it to rest:

Net force X Distance = kinetic energy

## kinetic energy

*squared*in the kinetic energy equation. This means that if an object is

*twice as fast*, it can do

*four times*as much work. If it is

*ten times*as fast, it can do

*100*

*times*as much work!

## conservation of energy

Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount never changes.

## conservation of energy

**hint: v = sqrt(2KE/m)**

## Conservation Of Energy

## power

*work*is the same whether you walk or run!

## power

*how fast*work is done.

*Back to the Future?*

## Sources of Energy

## Check questions for chapters

## 2 and 3

#### physical science chapters 2 and 3

By Chris Gregg