Newton’s First Law

Building Science Champions

In ThoughtDo you believe this picture shows a

balanced or unbalanced force. Explain your answer in three sentences.

ObjectivesExplain how balanced and unbalanced

forces are related to motion

State Newton’s first law of motion

Define inertia

Key Terms Force

Net force

Unbalanced force

Balanced force

Inertia

What is Force Force is either a push or a pull

When one object pushes another, the first object is exerting force on the second object

Force is not only described as how strong (magnitude), but also by direction

Unbalanced Forces Net force – the overall force on an

object after all the forces are added together

Unbalanced force – can cause an object to start or stop moving, or change direction

8N

8N

5 N 10 N

Balanced Forces Balanced forces – equal forces acting

on one object in opposite directions

Balanced forces on an object will not change the objects acceleration, velocity or motion

The sum of all forces equal 0 N

= 0 N10 N 10 N

5N

5N

Balanced vs. Unbalanced ForcesBalanced forces always equal 0 N and

there is no change in motion

Unbalanced force are always > 0 N and there is a change in motion

InertiaGalileo Galilee, an Italian astronomer, first

suggested that a force is needed to keep an object in motion

Inertia – is the tendency of an object to resist change

If an object is moving it will continue to move

If an object is not moving it will continue to be still

Force is needed to change an objects inertia/motion

Inertia

The bus does not want to move, the force of the tourist is pushing the bus.

Newton’s First Law of MotionAN OBJECT AT REST WILL REMAIN AT

REST AND AN OBJECT MOVING AT A CONSTANT VELOCITY WILL CONTINUE TO MOVE AT A CONSTANT VELOCITY UNLESS ACTED UPON BY AN UNBALANCED FORCE.

IMPORTANT

First Law AppliedNewton’s first law explains why a

passenger in a car continues to move when a car rapidly stops. The seatbelt is the force that holds the passenger in the car because the seatbelt is attached the car. If the passenger is not wearing a seatbelt the windshield may be the force that holds the passenger in the car, if the passenger breaks the windshield the asphalt will be the unbalanced force stopping the passenger.

MassMass is measured in kilograms (kg). A

small car may have a mass of 1,000 kg and a bicycle may have a mass of 10 kg. Which object is easier to start and stop?

Mass has a direct correlation with inertia

The greater the mass the greater the inertia

The smaller the mass the smaller the inertia

* 1 kg = 1,000 g

ReviewForce is a push or a pull

Unbalanced force creates, changes or stops acceleration

Balanced forces do not change acceleration

Inertia is the resistance of an object to change

Coin and cup demo

ReferencesAnderson, M. et all (2012) Physical

Science. McGraw-Hill: Columbus

Frank, D.V et al (2001). Physical Science. Prentice Hall: New Jersey