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Intro to Forces and Newton's Laws

Important Force terms

Force: A push or pull on an object resulting from an interaction with another object.

Contact forces arise from direct contact between objects. In other words, objects must actually be touching

Field forces require no contact (example: magnetism and gravity). Field forces do not require objects be touching one another.

Net Force: In this case, net means "all". When we talk about a Net Force, we are talking about the result of adding all of the forces acting on an object together.

Unbalanced Force This is the result of all of the forces not canceling each other out. Simply consider a textbook sitting on a desk. Clearly, as the book is at rest on the desk, it is easy to realize that the weight (force) of the object is balanced out by the desk pushing (force) on the book.

Force due to Gravity The gravitational field of the earth pulls on all objects with mass. It is this "pulling force" that results in weight. Thus, weight is the same thing as the Force due to Gravity.

Normal Force A normal force is simply a force that points out of a surface. From math, the definition of normal is 90 degrees (a right angle). Thus, 90 degrees to a surface points directly out of a surface.

Newton’s Laws of Motion

First law: The law of laziness. An object will tend to do what it is already doing.

Consider sitting on your couch watching tv when you are told to go clean your room. Chances are, you’ll continue to sit on the couch and not clean your room.
Ever walked out the door and realized you forgot your phone or wallet, only deciding you’ve gone too far to go back? It takes too much effort to turn around!

IN PHYSICS TERMS
1) An object at rest remains at rest unless acted upon by an unbalanced (net) force
An unbalanced (net) force is required in order to take an object at rest and put it into motion. Consider an object sitting at rest on a desk. Does it move on its own? Of course not. You need to apply an unbalanced force in order to begin to move it.


2) An object in motion will remain in motion unless acted upon by an unbalanced force
An object will continue at a constant speed in straight line motion (constant velocity) unless an unbalanced force causes it to change speed or direction.

This seems counter-intuitive, because in our every day life, we must keep appplying a force to maintain motion. Why is this?
When you are driving at a constant velocity (in a straight line) there is a need to balance out the air resistance that would otherwise slow you down. Thus, we are actually opposing another force that is present. This is why the engine must keep doing work. The sum of forces on the car is indeed 0 (All of the forces balance.)
Remember that trip we took to the moon? Once we left the Earth's atmosphere, we turned off the rocket thrusters! This is because in space there was no air resistance. Once we were set in motion, we did not require the force of the thrusters to keep propelling us towards the moon!


The first law is also sometimes referred to as the law of inertia. Inertia can often be interchanged with mass. In other words, if an object has mass it has inertia. It can be thought of as something’s resistance to changing motion.
The more inertia (mass) something has, the harder it is to change its motion. (More force would be required). As long as something has mass, it has inertia. Inertia will never be zero!!!!!



Second Law: An unbalanced (non-zero net) force will cause a mass to accelerate. The acceleration is equal to the net force divided by the mass. (The unbalanced force and acceleration MUST be in the same direction.)
There is a direct relationship between the net force applied to an object and the resulting acceleration. If there are no net forces, there will be no acceleration. The acceleration is always in the direction of the net force.
Equation seen as: Fnet=ma OR a=Fnetm.

It is important to realize the indication of the relationship above. In actuality, we can apply a force, or change our mass, but we cannot "apply an acceleration". To find this relationship, we could push on objects with different forces, and observe how the acceleration changes with each force. This is similar to the linear relationships we talked about earlier on the site.

Third Law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

For every action there is an equal and opposite reaction. Consider holding a 10 lb weight in your hand. You can feel the force of the weight that you have to oppose in order to hold it still. Not only does the weight exert 10 lb of force on you, but you exert 10 lbs back.
The size, mass, weight, etc of the objects does NOT MATTER. If object A exerts a force on object B, then B exerts the same amount of force back on A.
When a baseball bat is in contact with a baseball, the magnitude of the force on each object is EXACTLY the same. The difference in motion observed is because they have a different mass. Thus the acceleration is different according to the 2nd law.

Don't start a fight, but if you do...
You have a bad day, so when you get home you give your younger brother a smack upside the head. He cries and goes to your parents about the issue, claiming you hit him first, and you hit him harder!
Ha! This situation is laughable. According to the 3rd law, and the fundamentals of physics, not only did your brother hit you at the SAME TIME he hit you EXACTLY AS HARD!!!! What a sucker.
Important Force terms
Force: A push or pull on an object resulting from an interaction with another object. 
Contact forces arise from direct contact between objects. In other words, objects must actually be touching 
Field forces require no contact (example: magnetism and gravity). Field forces do not require objects be touching one another.
Net Force In this case, net means "all". When we talk about a Net Force, we are talking about the result of adding all of the forces acting on an object together.
Unbalanced Force This is the result of all of the forces not canceling each other out.  Simply consider a textbook sitting on a desk. Clearly, as the book is at rest on the desk, it is easy to realize that the weight (force) of the object is balanced out by the desk pushing (force) on the book.
Force due to Gravity The gravitational field of the earth pulls on all objects with mass.  It is this "pulling force" that results in weight.  Thus, weight is the same thing as the Force due to Gravity.
Normal Force A normal force is simply a force that points out of a surface. From math, the definition of normal is 90 degrees (a right angle). Thus, 90 degrees to a surface points directly out of a surface.
Newton’s Laws of Motion
First law: The law of laziness. An object will tend to do what it is already doing.  
Consider sitting on your couch watching tv when you are told to go clean your room.  Chances are, you’ll continue to sit on the couch and not clean your room.  
Ever walked out the door and realized you forgot your phone or wallet, only deciding you’ve gone too far to go back?  It takes too much effort to turn around!
IN PHYSICS TERMS
1) An object at rest remains at rest unless acted upon by an unbalanced (net) force
An unbalanced (net) force is required in order to take an object at rest and put it into motion. Consider an object sitting at rest on a desk.  Does it move on its own? Of course not.  You need to apply an unbalanced force in order to begin to move it.
 
2) An object in motion will remain in motion unless acted upon by an unbalanced force
An object will continue at a constant speed in straight line motion (constant velocity) unless an unbalanced force causes it to change speed or direction. 
This seems counter-intuitive, because in our every day life, we must keep appplying a force to maintain motion. Why is this?
When you are driving at a constant velocity (in a straight line) there is a need to balance out the air resistance that would otherwise slow you down. Thus, we are actually opposing another force that is present. This is why the engine must keep doing work.  The sum of forces on the car is indeed 0 (All of the forces balance.)
Remember that trip we took to the moon? Once we left the Earth's atmosphere, we turned off the rocket thrusters! This is because in space there was no air resistance. Once we were set in motion, we did not require the force of the thrusters to keep propelling us towards the moon!
 
The first law is also sometimes referred to as the law of inertia.  Inertia can often be interchanged with mass.  In other words, if an object has mass it has inertia.  It can be thought of as something’s resistance to changing motion.
The more inertia (mass) something has, the harder it is to change its motion.  (More force would be required).  As long as something has mass, it has inertia.  Inertia will never be zero!!!!!


Second Law: An unbalanced (non-zero net)  force will cause a mass to accelerate.  The acceleration is equal to the net force divided by the mass. (The unbalanced force and acceleration MUST be in the same direction.)
There is a direct relationship between the net force applied to an object and the resulting acceleration.  If there are no net forces, there will be no acceleration.  The acceleration is always in the direction of the net force.
Equation seen as:  Fnet=ma OR a=Fnetm.
 
It is important to realize the indication of the relationship above.  In actuality, we can apply a force, or change our mass, but we cannot "apply an acceleration". To find this relationship, we could push on objects with different forces, and observe how the acceleration changes with each force. This is similar to the linear relationships we talked about earlier on the site.
 
Third Law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.
 
For every action there is an equal and opposite reaction.  Consider holding a 10 lb weight in your hand.  You can feel the force of the weight that you have to oppose in order to hold it still.  Not only does the weight exert 10 lb of force on you, but you exert 10 lbs back.  
The size, mass, weight, etc of the objects does NOT MATTER.  If object A exerts a force on object B, then B exerts the same amount of force back on A.
When a baseball bat is in contact with a baseball, the magnitude of the force on each object is EXACTLY the same.  The difference in motion observed is because they have a different mass.  Thus the acceleration is different according to the 2nd law.
 
Don't start a fight, but if you do...
You have a bad day, so when you get home you give your younger brother a smack upside the head.  He cries and goes to your parents about the issue, claiming you hit him first, and you hit him harder!
Ha!  This situation is laughable.  According to the 3rd law, and the fundamentals of physics, not only did your brother hit you at the SAME TIME he hit you EXACTLY AS HARD!!!!  What a sucker.
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