Newton's First Law

Definition: Objects will remain moving with a constant velocity unless acted upon by a net force.


This is saying that, unless there is a force that acts upon an object, the object will keep moving until there is something that stops the object. This also applies to objects that are staying still. They will stay still until there is something that makes the object move. The main reason that objects stop is because of friction. Friction is a net force that has effect on almost every moving object, and makes the object slow down and eventually come to a stop. Friction also applies to objects that are staying still. A force that acts on an object, to make an object move, will have to overpower the static friction. And if a ball is moving across the floor with a constant speed, the force will have to be greater than the kinetic force, to make the object accelerate.

So, if F is a net force and A is acceleration, if F=0 then A=0. This is because an object will not accelerate if there is no net force. It will either choose to remain at rest or remain at a constant velocity and with both of these, there is no acceleration. An object, if it has a choice, will choose to keep a constant velocity or remain still because of Newton's first law and the lack of a net force. Although you might be wondering why a ball doesn't keep rolling forever at a constant speed because of Newton's first law, it is because of friction. It actually does have a force acting upon it.


For Example:
A block is being pulled by 50,000N by a string parallel to the horizontal. If the block is moving at a constant velocity. What is the amount of friction force applied to the block?

The answer is -50,000N. If you use Newton's 1st law you know that the forces must be equal because the object is not accelerating. If the object is not accelerating then there is no net force. (50,000N - 50,000N = 0N)


Newton's Second Law

Equation= Fnet=m x a (Net force= mass x acceleration)

The main idea of Newton's Second law consists of acceleration, mass, and net forces. With the second law equation you can determine an object's acceleration relative to Earth or another object. To have acceleration you must have a net force. A net force is a force that does not have an equal force in the opposite direction to balance it out. An easy way to find the net force in an object is to draw a force diagram for the object's forces. If there is not the same amount of force in opposite directions, then the object will have a net force, which means acceleration. If there is only one force acting on an object in the horizontal direction, that is its net force. If there are two, and they are pointing in the same direction, you add them and that is your net force. If there are two and they are facing opposite directions, you subtract them and that's your net force. If you can find the net force, and you know the mass, this is an easy equation to find your system's acceleration, or vice-versa.


For Example:
When 500N force is acting on a 50kg box
F=ma
500N=50kg x a
a=10ms^-2
When a 50kg box has 6ms^-2 acceleration
F= 50kg x 6ms^-2
F= 300N
When 100N is causing 10ms^-2 acceleration of an object
100N= m x 10ms^-2
m= 10kg


Newton's Third Law

This is Newton’s 3rd Law in his own words:

“For every action, there is an equal and opposite re-action.”
This is one of the simpler laws that Sir Isaac Newton made. It means that for any force, there is an equal force of the same type pushing back on the object. It is called an action-reaction pair. This confuses a lot of people at first. For example, if a huge truck smashes into a stationary car, one would think that the truck hit the car with more force than the car hit the truck with. This is because the car wasn’t moving. However, at the instant that they collided, they both hit each other with the same amount of force, according to Newton’s 3rd Law. A confused student would argue that the truck hit the car with more force, because the car would go sliding away with the impact, and the truck wouldn’t move that much. However, with these situations, we must not pay any attention to the after-effects. The actual reason why the stationary car was effected more by the collision was because of inertia. Inertia is the resistance of change. A larger mass object has more inertia. And because the truck has more inertia, it will want to keep moving at its original speed. Even though the car pushed back on the truck the same amount, the truck will push the car farther forward then the car pushed the truck back.


external image newton_third_law_2.gif
external image newton_third_law_2.gif

As you can see in this example that no matter if you are pulling against a wall or a very strong cave man, the force on each side of the spring is the same.


You can easily find an action-reaction pair by remembering these steps:
  • The pair always uses the same type of force (ex: normal force, tension force, etc.)
  • Call the first object ‘x’. The force will be on x, by the other object, which we’ll call ‘y’.
  • The other force will be on y, and by x.

Try to find the action-reaction force in this situation.
Q. A box is sitting on a shelf.
A. The Box applies a normal force on the shelf. The shelf applies a normal force on the box.

How about this one?
Q. A rocket lifts off earth and enters the atmosphere.
A. The fuel applies a force on the space ship. The space shuttle puts a normal force on the fuel.