An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

**Newton's Laws of Motion**

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards.

The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards).

Action-reaction force pairs make it possible for birds to fly.

For every action, there is an equal and opposite reaction.

3rd Law

1. What acceleration will result when a 12 N net force applied to a 3 kg object? A 6 kg object?

2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.

3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?

4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

Check Your Understanding

1) Write the formula

F = m x a

The net force of an object is equal to the product of its mass and acceleration, or

2nd Law

Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

3rd Law

When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N).

One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.

Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases.

The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

3rd Law

Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.

3rd Law

Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.

The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

Newton’s 3rd Law in Nature

There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

3rd Law

According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

3rd Law

In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever! (Or at least to the end of the table top.)

Slide a book across a table and watch it slide to a rest position. The book comes to a rest because of the presence of a force - that force being the force of friction - which brings the book to a rest position.

The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

Other examples of Newton’s Third Law

Sir Isaac Newton

(1643-1727)

AKA The Law of Intertia

Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

2nd Law –

Force equals mass times acceleration.

3rd Law –

For every action there is an equal and opposite reaction.

3 Laws

**Newton's 3 Laws**

1st Law of Motion

1st Law of Motion

**1st Law**

The Tablecloth Trick

Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? It’s a force we sometimes cannot see –

**2nd Law**

How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second?

2nd Law

3)Solve for the unknown

2800 kg-meters/second/second

or 2800 N

2)Fill in given numbers and units

F = 1400 kg x 2 meters per second/second

1kg/m/s/s = 1 N

Remember Gravity = 10m/s/s

12 N = 3 kg x 4 m/s/s

16 N = 3.2 kg x 5 m/s/s

66 kg-m/sec/sec or 66 N

9800 kg-m/sec/sec or 9800 N

When the air resistance slows down a vehicle it is called

drag

Car Motion

A force which moves a vehicle forward is called

thrust

Friction

also resists the motion of a car

(Drag)

(Thrust)

If 2 objects had the same acceleration but one was twice the size of the other, which object has the greater force?

2nd Law

3rd Law

FRICTION

F = m x a

Solution