Newton’s Laws of Motion

Newton’s Three Laws of Motion

• Sir Isaac Newton first presented his three laws of motion in the Principia Mathematica Philosophiae Naturalisin 1686.

• To understand Newton’s laws of motion, you must remember that on Earth we always have GRAVITY pulling down on us and AIR around us.

• Newton’s laws are used in the design of aircraft and spacecraft.

First Law of Motion

Objects in motion

tend to stay in

motion

AND

Objects at rest tend

to stay at rest

UNLESS

Something pushes

or pulls on the

object.

First Law of MotionExample

Think of a block sitting still, or “at rest”.

If the block is to move, it must receive

a push. Otherwise it will stay at rest.

According to Newton’s First Law,

When Will the Block Stop?

Push Push

First Law of Motion

Example

Think of a baseball thrown in space.

Without GRAVITY to pull it down, the ball

would just keep moving until something got

in its way and stopped it.

Aerospace Examples of the First

Law of Motion

The movement of an airplane changes when the pilot changes the throttle setting (or force) of the engine.

More force moves the airplane at a greater speed.

Aerospace Examples of the First

Law of Motion

The movement of a model rocket being launched into the atmosphere

The rocket was at rest (not moving) until the force of the engine pushed it upward.

Second Law of Motion

The acceleration of an object depends on:

– The size of the force

on the object

– The direction of the

force on the object

– The mass of the

object

The relationship:

F = m * a

Force = mass * acceleration

Or

a=F/m

acceleration = Force/mass

Examples of Second Law of Motion

The direction of acceleration is the same as the direction of the external force.

Remember the box?

If you want to move the box to the right, you push toward the right.

If you want to move the box to the left, you push toward the left.

Push Push

Examples of Second Law of Motion

The magnitude (size) of the acceleration is equal to the external force divided by the mass of the object.

The bigger the object (the more mass), the more force is needed to accelerate the object.

Back to the box (but let’s add a car).

If you push the car and the eraser with the same force,

The eraser will accelerate much faster than the car.

50 lb

50 lb

50 lbs

50 lbs

Third Law of Motion

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

You, on roller blades, push against a wall.

Object A: You Object B: Wall

Action: You apply

a force on the wall.Reaction: The wall

applies the same

force back on you.

Examples of the Third Law of

MotionYou, on roller blades, push against a wall.

Your backward movement is the result of the wall pushing back against you.

Object A: You Object B: Wall

Examples of the Third Law of

MotionFlaming hot gases are pushed out the bottom of

a rocket.

Object B: Rocket

Object A:

Flaming hot gasAction: Gas is pushed

downward out of the

rocket

Reaction: The rocket

moves upward

Third Law of Motion

Examples

The upward

movement of

the rocket is

the result of

the gases

pushing

downward from

the rocket.

Third Law of Motion

Key PointsThe third law can be used to explain how a wing produces lift and a jet engine produces thrust.

On a wing, the air is pushed downward because of the wing’s unique shape (an airfoil). In reaction, the air pushes the wing upward.

A jet engine produces hot exhaust

gases which flow out the back of

the engine. In reaction, a

thrusting force is produced in the

opposite direction.

Image Resources

Microsoft, Inc. (2009). Clip art. Retrieved June 16, 2009, from http://office.microsoft.com/en-us/clipart/default.aspx

National Aeronautics and Space Administration (NASA). (n.d.). Newton’s third law of motion. Retrieved June 19, 2009, from http://www.grc.nasa.gov/WWW/K-12/airplane/newton3.html