Newton’s Laws

Describing motion Newton’s three laws describe how

things move and how this motion can be changed by other forces/objects

Newton’s laws lead to the formulas that lets us express motion with math

Newton’s 1st Law This is also known as the Law of

Inertia An object at rest will stay at rest An object in motion will stay in motion

– in a straight line An object will remain at rest or in

linear motion unless acted on by a force

A car parked in the parking lot does not move until the engine is started or another car pushes it

A soccer ball keeps rolling across a parking lot in a straight line

A soccer ball slows down as it crosses the field because the force of friction caused by the grass pushes against the ball

Mass & Inertia Mass is a measure of the amount of

matter in an object Ultimately, this is a measure of the

amount of atoms in an object The more mass an object has, the more

inertia is has It is harder to push a big suburban than a

little sports car because the suburban has more mass

It is harder to catch a thrown bowling ball than a softball

Resistance to Change Inertia is a resistance to change in

motion This can be seen with the car

examples, or anything else either in motion or sitting still (motion of zero)

Whether in motion or at rest, inertia is resistance to change

Inertia is also a resistance to changing the direction of motion

Reorder the following based on their inertia (lowest to highest)

A speeding motorcycle

A slow moving semi truck

A five story skyscraper

A speeding bullet from a rifle

A fast pitch softball

A classroom metal chair

The tire from a farm tractor

A golf ball hit with a driving iron

Reorder the following based on their inertia (lowest to highest)

A speeding bullet from a rifle

A golf ball hit with a driving iron

A fast pitch softball

A classroom metal chair

The tire from a farm tractor

A speeding motorcycle

A slow moving semi truck

A five story skyscraper

Weight and mass are not the same Weight depends on gravity

If you weigh 150 lbs on earth, you would weight 25 lbs on the moon and would not weigh anything on the International Space Station

SI Units Grams and kilograms are used to

measure mass A newton is used to measure

weight, just like a pound measures weight

1 kg on earth weighs 9.8 newtons Newton’s 2nd law will help us calculate

this!

Newton’s 2nd Law The acceleration of an object is

directly proportional to the force pushing on it and inversely proportional to the mass of the object

Force A force is a push or pull on an object

A force can be caused by contact The force of your arm pushes the door

open A force can be caused over a distance

The force of the pull from the magnetic north pole pulls the needle in a compass to point towards north

Newton’s 2nd Law Formula F=m*a

F = force, measured in newtons m = mass, measured in kilograms a = acceleration, in m/s2

Using F=m*a to find force (weight) of gravity for a 1 kg block

F = 5 kg * 9.8m/s2

F = 49 N (N is used for newtons)

Acceleration and Force If the force increases, the

acceleration increases If two people push a car, is might

move, but it will move slowly and increase speed slowly

If ten football players push the same car, it will accelerate faster

Acceleration and Mass Acceleration and Mass are inversely

related This means that as mass increases,

the other acceleration decreases Ten football players push a small sports

car, it accelerates at a moderate rate Ten football players push a suburban

full of golfers with their clubs, the suburban accelerates much slower because it has more mass

Find the Force How much force do the football

players need to push the car with to make it accelerate at .2 m/s2 if the car is 780 kg?

Find the Force How much force do the football

players need to push the car with to make it accelerate at .2 m/s2 if the car is 780 kg?

F= .2 m/s2 * 780 kg F = 156 N

Finding the Acceleration If a car engine generates 350 N,

and the car has a mass of 700 kg, what will its acceleration be?

Finding the Acceleration If a car engine generates 350 N,

and the car has a mass of 700 kg, what will its acceleration be?

F = m*a or a = f/m A = 350 N/700 kg A = .5 m/s2

Newton’s 3rd Law Every action has an equal and opposite

reaction Every force has a corresponding force in

the opposite direction Try it – tape the latch on a door so it can not

latch and stay closed, then push as hard as you can on the door

Instead of pushing real hard on the door, you tend to fall through the doorway

The door did not “push back” to keep you from falling

What happens if you try to throw a basketball while on a skateboard?

If you throw the ball in the same direction the skateboard is facing, you will start to roll backwards You pushed the ball when you threw

it, and the ball pushed you backwards!!

What are the two balanced forces pushing on you? The force of gravity is pulling you

down . . . So why don’t you keep going down?

The chair you are sitting in is pushing you up at with the same amount of force as gravity, just in the opposite direction

Find the opposite force A propeller on an airplane pushes

the plane forward, how? What is the opposite force?

Find the opposite force A propeller on an airplane pushes the

plane forward, how? What is the opposite force?

The airplane moves forward because the propeller pushes air backwards

The opposite force is the air pushes the propeller forwards (and the plane since it is attached to the propeller)

Find the opposite force What are the two forces in action

when you use a pull-up bar to do a chin-up?

Find the opposite force What are the two forces in action

when you use a pull-up bar to do a chin-up?

You pull down on the bar The bar pulls up on you