Isaac Newton's three laws of motion.
Jump to: Second Law | Third Law Isaac Newton GRAVITY STUDENT NOTES
Isaac Newton's First
Law of Motion
The first law deals with forces and changes in velocity.

For just a moment, let us imagine that you can apply only one force to an object. That is, you could choose to push the object to the right or you could choose to push it to the left, but not to the left and right at the same time, and also not up and to the right at the same time, and so on.

Under these conditions the first law says that if an object is not pushed or pulled upon, its velocity will naturally remain constant. This means that if an object is moving along, untouched by a force of any kind, it will continue to move along in a perfectly straight line at a constant speed.

Once moving at a steady speed and in a straight line
It will continue moving at a steady speed
In a straight line.

This also means that if an object is standing still and is not contacted by any forces, it will continue to remain motionless. Actually, a motionless object is just a special case of an object that is maintaining constant velocity. Its velocity is constantly 0 m/s.

Once Standing still the Object
will continue to stay still until acted upon.

Now, what about if there is more than one force on the object? You really can push an object, say, to the left and down at the same time, so, what happens then?

Under these conditions we must realize that a group of forces on an object adds up so that all the forces appear to the object as one force. This one force that is the sum of all the forces is called the NET FORCE
. The word net in this context means total. It is this net force that may change the velocity of the object. Let us look at some examples.

Imagine that two forces act at the same time on an object. One is a very strong force to the left, and the other is a weaker force to the right. These two forces add up to one net force. Since the force to the left is stronger, the net force is to the left. This net force to the left will cause the velocity of the object to change. The object experiences this one net force as if this was the only force pushing it, although, actually, there are two separate forces present.

The net force is the total force. It could be the sum of two forces or more than two forces. If only one force acts upon an object, then this one force would be the net force. If the net force on an object is zero, then the object experiences no velocity change. If the net force on an object is not zero, then the object will show a change in velocity.

Lastly, this net force must be external to the object.

The net force can not come from the object itself. You could not, for example, put on ice skates, stand on a frozen pond, push on your back by reaching around with your arms, and expect to get going. Although if someone else came up from behind and gave a you a shove, then your velocity would change.

Newton's first law of motion contains the same information as Galileo's explanation of inertia.
Galileo stated. INERTIA is a property of matter.

It is that property of matter, which opposes changes in velocity. Simply stated, an object will not change its velocity spontaneously.   That is, if something is moving along at a constant speed in a straight line, it will continue to move along at the same constant speed in the same straight line. It will not, all on its own, speed up, slow down, or change direction. Something else must push on the object to speed it up, slow it down, or change its direction. What does this sound like? Newtons First Law

Review: 1st Law of Motion

An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

This First Law of Motion is often called
"the law of inertia".

Inertia = the resistance an object has to a change in its state of motion.

Isaac Newton's Second

Law of Motion

Newton's second law of motion explains how an object will change velocity if it is pushed or pulled upon.

Firstly, this law states that if you do place a force on an object,
it will accelerate, i.e., change its velocity, and it will change its velocity in the direction of the force.

It Accelerates in the direction
that you push it.

Secondly, this acceleration is directly proportional to the force. For example, if you are pushing on an object, causing it to accelerate, and then you push, say, three times harder, the acceleration will be three times greater.

If you push twice as hard,
It will accelerate twice as much.

Thirdly, this acceleration is inversely proportional to the mass of the object. For example, if you are pushing equally on two objects, and one of the objects has five times more mass than the other, it will accelerate at one fifth the acceleration of the other.

If it gains twice that mass
it will accelerate half as much.

Review: 2nd Law of Motion

Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

The Second Law of Motion can be expressed
as a mathematical equation:

Using what we know lets say that on the way to work my vehicle breaks down. My truck weighs 1000 Kg. As I am pushing my truck I am able to make it roll at 0.05 m/s2. Since you know Newton's Second Law of Motion you can calculate how much force (aka NEWTONS) I am pushing (applying to) on my truck.

FORCE = 1000 x .05

Isaac Newton's Third
Law of Motion
The third law states that for every force there is an equal and opposite force. For example, if you push on a wall, it will push back on you as hard as you are pushing on it.

If you push on the object (mass)
it pushes back on you.

Review: 3rd Law of Motion

For every action there is an equal and opposite re-action.

The terms "action" and "reaction" refer to the action force and reaction force. In addition, the term "equal" means equal in magnitude and the term "opposite" means opposite directions.

Action-reaction force must occur in pairs!

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