6.1 Newton’s First Law6.1 Newton’s First Law

pp. 138-140pp. 138-140

NEWTON’S THREE LAWS OF NEWTON’S THREE LAWS OF MOTIONMOTION

In his book, In his book, Principia, Newton Principia, Newton described the three described the three laws of motion.laws of motion.

He did not He did not discover discover all three laws, he all three laws, he explained them in a explained them in a way that many way that many people could people could understandunderstand

NEWTON’S FIRST LAW OF NEWTON’S FIRST LAW OF MOTIONMOTION

This is also known as the This is also known as the Law of InertiaLaw of Inertia It says : It says : An object in motion will An object in motion will

remain in motion at a constant speed remain in motion at a constant speed and in the same direction unless acted and in the same direction unless acted on by an unbalanced force.on by an unbalanced force.

An object at rest will remain at rest An object at rest will remain at rest unless acted upon by an unbalanced unless acted upon by an unbalanced force.force.

6.1 Law of inertia6.1 Law of inertia Newton’s first lawNewton’s first law says that objects says that objects

continue the motion they already have continue the motion they already have unless they are acted on by a net force. unless they are acted on by a net force.

If the net force is zero, an object at rest If the net force is zero, an object at rest will stay at rest. will stay at rest.

If an object is acted upon by If an object is acted upon by unbalanced forcesunbalanced forces, its motion will , its motion will change.change.

NEWTON’S FIRST LAW OF NEWTON’S FIRST LAW OF MOTIONMOTION

In other words…In other words… Unless you apply a force, things tend Unless you apply a force, things tend

to keep on doing what they were to keep on doing what they were doing in the first place.doing in the first place.

Objects at rest (not moving) will not Objects at rest (not moving) will not begin moving unless a push or pull is begin moving unless a push or pull is exerted on them. exerted on them.

6.1 Net force6.1 Net force Newton’s first law is Newton’s first law is

often written in often written in terms of the net terms of the net force: force:

““An object at rest An object at rest will stay at rest and will stay at rest and an object in motion an object in motion will continue in will continue in motion at constant motion at constant velocity velocity UNLESS UNLESS there is a net force.”there is a net force.” According to these

vectors, in what direction is the net force?

6.1 Force changes motion6.1 Force changes motion

Forces can be used to Forces can be used to increase or increase or decrease the speed of an object, or decrease the speed of an object, or to change the direction an object is to change the direction an object is moving.moving.

NEWTON’S FIRST LAW OF NEWTON’S FIRST LAW OF MOTIONMOTION

An object moving An object moving at a certain at a certain velocity will continue to move forever velocity will continue to move forever at that same speed and in the same at that same speed and in the same direction unless an unbalanced force direction unless an unbalanced force acts on it.acts on it.

Inertia is related to Inertia is related to massmass. Mass is a . Mass is a measure of measure of inertiainertia..

NEWTON’S FIRST LAW OF NEWTON’S FIRST LAW OF MOTIONMOTION

An object with a An object with a small mass has less small mass has less inertia than an inertia than an object with a large object with a large mass.mass.

For example…For example… A tractor trailer A tractor trailer

has more mass has more mass than a motorcyclethan a motorcycle

6.1 Law of inertia6.1 Law of inertia InertiaInertia is the is the

property of an property of an object that resists object that resists changes in changes in motion.motion.

Objects with more Objects with more mass have more mass have more inertia and inertia and are are more resistant to more resistant to changes in their changes in their motion.motion.

Which ball has more inertia?

NEWTON’S FIRST LAW OF NEWTON’S FIRST LAW OF MOTIONMOTION

NEWTON’S FIRST LAW OF NEWTON’S FIRST LAW OF MOTIONMOTION

How do ABS How do ABS (automatic brake (automatic brake systems) work in cars?systems) work in cars?

The system is The system is controlled by a controlled by a computer that computer that prevents prevents skidding by skidding by sensing when the sensing when the wheels are about to wheels are about to lock.lock.

ABS BRAKES EXPLAINEDABS BRAKES EXPLAINED

The computer The computer releases and releases and reapplies the reapplies the brakes up to brakes up to 25 25 times a second. times a second. Instead of skidding Instead of skidding out of control,out of control, the the car slows down and car slows down and stops safely.stops safely.

Solving ProblemsSolving Problems

A car drives along the highway at constant A car drives along the highway at constant velocity. Find the car’s weight and the velocity. Find the car’s weight and the friction force if the engine produces a friction force if the engine produces a force of 2,000 newtons between the tires force of 2,000 newtons between the tires and the road and the normal force on the and the road and the normal force on the car is 12,000 N.car is 12,000 N.

Solving ProblemsSolving Problems

1.1. Looking for:Looking for: ……weight of car in newtons, force due to weight of car in newtons, force due to

frictionfriction

2.2. Given: Given: ……ForceForceNN = 12,000N (up); = 12,000N (up); ……ForceForceEE = 2,000N (forward) = 2,000N (forward)

3.3. Relationships: Relationships: Newton’s 1Newton’s 1stst Law: Law:

net force = zero at constant velocity; so net force = zero at constant velocity; so ForceForceNN = Force = ForceWW and Force and ForceEE = Force = ForceFF

FE = 200 N

Solving ProblemsSolving Problems

4.4. SolutionSolution Draw a free body Draw a free body

diagram.diagram. There is no net force There is no net force

upward, so the weight of upward, so the weight of the car is an equal the car is an equal downward force of downward force of 12,000 N. 12,000 N.

The forward engine The forward engine force balances the force balances the friction force so the friction force so the friction force is 2,000 N.friction force is 2,000 N.

FF = 200 N FW

= 1

2,0

00N

FN

= 1

2,0

00N