Newton’s Laws Notes

Page 5

First we need to define the word FORCE:

• The cause of motion (what causes objects to move)

• Two types of forces– Pushes– Pulls

Forces are measured in Newtons

• SI unit of force

• Symbol: N

• Measured by using a spring scale

Forces may be balanced or unbalanced

• Balanced forces – all forces acting on an object are equal– There is NO MOTION

• Unbalanced forces – one or more forces acting on an object are stronger than others– There is MOTION

• A NET FORCE

Newton’s Laws

• First Law – Inertia

• Second Law – Acceleration, Force & Mass

• Third Law – Action-Reaction

First Law

• Inertia– An object at rest remains at

rest and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.

First Law• Inertia & Mass

– More MASS = more INERTIA

–Bigger objects are harder to start & stop.

Law 1 Examples

• Chair or cart loaded with books

• Baby in car seat or stroller (car & coin)

• Roller coaster

• Seat belts

Second Law• Acceleration & Mass

–The acceleration of an object depends on the mass of the object and the amount of force applied.

– F = ma – (Force = mass x acceleration)

Second Law

• Acceleration & Mass Review–Acceleration is a change in

velocity [speed or direction]

–Mass is the amount of matter in an object

Second Law

• Acceleration & Force– more force = more acceleration

• Acceleration & Mass–more mass [or inertia] an

object has, the more force it takes to move the object

Examples of law 2

• Coin flick

• Empty chair & Loaded chair

• Big moving van & Mini-Cooper

• Small blocker trying to stop Larger running back

• Bunting vs homerun

Third Law

• Action – Reaction–Whenever one object

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

Third Law

• Action – Reaction–Forces are always produced in

pairs with opposite directions & equal strengths

10 N 10 N

Examples of Law 3

• Roller skating or skateboarding

• Rocket launching

• Balloon

• Newton’s cradle

The truck is in motion. What is the force that causes it to stop?

The push of the stopped car.

The car is at rest. What is the force that causes it to move?

The push of the truck.

What about the ladder on top of the truck?

The ladder is in motion because the truck is in motion.

When the truck stops, the ladder stays in motion.

The truck is stopped by the force of the car, but the ladder is not.

What force stops the ladder?

Gravity.

The truck is in motion, the car is at rest. How do each of these vehicles accelerate?

The truck stops moving. The car starts moving.

Which one will be the hardest to accelerate?

The truck because it has the most mass.

Why does the car move [accelerate] when it is hit by the truck?

The heavy and moving truck has more force than the small, at rest car.

Why does the truck stop moving when it hits the car?

The force of the car pushing back on the truck, plus the force of friction between the massive truck and the road slow down, the stop the truck.

The truck hits the car. An action force stops the truck.

What is the equal and opposite reaction force?

The force that pushes the car forward.

Describe Friction pg 7 • Force that resists motion between two

touching surfaces

• Acts in the opposite direction of the object’s motion

• Produces heat