The Laws of Motion Newton’s Three Laws. What is a Force? It is something we experience every single day. You are exerting a force on your pencil right.

The Laws of MotionNewton’s Three Laws

What is a Force?

It is something we experience every single day.

You are exerting a force on your pencil right now as you copy these notes or perhaps you are pressing down on computer keys

Even if you think to yourself, “well I am not taking notes,” you are still experiencing the force of gravity pulling you down and your chair exerting a force up so…there is no escape! Well there is…but we will get to that another day

Newton’s First Law

If an object does not interact with other objects, it is possible to identify a reference frame in which the object has zero acceleration

What does this mean?

It’s complicated…but we can easily simplify it

“Any intelligent fool can make things bigger, more complex and more violent. It takes a touch of genius –and a lot of courage – to move in the opposite direction” -Albert Einstein

Newton’s First Law (cont.)

In terms of a reference frame, imagine you have an air hockey table inside a train moving 70 mph. A hockey puck on the table, experiencing almost friction, appears to not be moving but we know the train is moving so the puck must also be moving at 70 mph

In reference to the train, the puck is not moving at all…but in reference to the earth it is moving 70 mph

Newton’s First Law (cont.)

If the train were to speed up or slow down, the puck would then begin to move and maintain its 70 mph speed because there is no friction between it and the table

So the first law can be simplified – A body at rest or at constant velocity in the absence of an external force remains at rest or at constant velocity

In other words, when no force acts on an object, the acceleration of the object is zero

Newton’s Second Law

What happens when to an object when a force acts on it?

Imagine exerting a force on a block and moving it along a horizontal, frictionless surface. The block experiences a change in velocity (acceleration) because it went from stationary to moving. The more force you apply, the more the block accelerates

So there is obviously a direct relationship between force and acceleration

Newton’s Second Law (cont.)

What else affects the acceleration?

Would a heavier block accelerate as quickly with the same amount of force it took to move the first one? The answer is, of course, no – it would take more force to move the heavier block at the same acceleration

So there is an inverse relationship between acceleration and the mass of the object

Newton’s Second Law (cont.)

So, when viewed from an inertial reference frame, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to it’s mass

This brings the formula:

Newton’s Second Law (cont.)

Practice Problem

If that first block weighed 10 kg and the force you applied accelerated it by 10 m/s2, how much force would you need to apply to the heavier block that weighs 50 kg in order to achieve the same acceleration?

Newton’s Third Law

The law of Action and Reaction

If two objects interact, the force exerted by object 1 on object 2 is equal to the magnitude and opposite in direction to the force exerted by object 2 on object 1

The sum of the two forces is zero because they equal and opposite which would mean they cancel out

So, if there is an equal and opposite force, how does anything ever move?

Newton’s Third Law (cont.)

Imagine book sitting on a table. It is not moving because the table is exerting a force up equal the amount of force down that the earth is pulling on the book. This force the table is exerting is not the reaction force.

The reaction force is that the book is also pulling on the earth with the same amount of force that the earth is pulling on the book – equal and opposite!

Newton’s Third Law (cont.)

So, the reaction force must act on two different objects and must be of the same type.

In order for the object(s) to move, an external force is needed

Review

First Law: A body at rest tends to stay at rest unless acted on by an external force and an object in motion tends to stay in motion unless acted on by an external force

Second Law: Acceleration is directly proportional to force and inversely proportional to mass F = ma

Third Law: For every action there is an equal and opposite reaction