What is the summary of all 3 of Newton's Laws in physics?

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dbrock1's profile pic

dbrock1 | High School Teacher | (Level 1) Adjunct Educator

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Newton's first law of motion governs objects both in motion and at rest. It is often simply referred to as Inertia. Essentially inertia states that an object at rest remains at rest while an object in motion remains in straight-line motion unless an unbalanced force acts upon the object. Inertia is the reason why someone can quickly yank a tablecloth out from under dishes set on the table. The dishes are objects at rest that remain in position when the tablecloth is removed. Inertia is also the reason that someone riding in a moving car leans right when the car turns left. The person's body has inertia which means it should be moving in a straight line, which is why it appears to lean right when the car turns. The body is simply moving in the same direction it was moving before the turn.

Newton's second law of motion is often summed up with the equation, F = ma. Essentially it relates the three variables of force, mass and acceleration. When a non-zero force (aka unbalanced force) acts on an object, the object will accelerate and its acceleration is dependent upon the object's mass. For example, suppose there are two objects, one with a mass of 100 kg and the second with a mass of 500 kg. (object #1 is less massive than object #2) If the same magnitude of force is applied to both objects, which one moves more? The smaller object will accelerate 5 times faster than the larger object. 

Newton's third law of motion refers to a pair of forces often called action/reaction forces. Essentially it states that every force acting on an object also has a reaction force exerted by that object. For example when a force is applied on an object (whether or not it causes motion), the object exerts a reaction force. Imagine two students, each standing on a skateboard facing each other. Student #1 exerts a pushing force on student #2. What is the result? There are 4 options:

a) student #1 moves

b) student #2 moves

c) both students move

d) neither student moves

The answer is that both students move. When Student #1 exerts a force on Student #2, the reaction force is that Student #2 exerts a force back on Student #1. Additionally Newton's third law states that the action/reaction forces are equal in magnitude and opposite in direction. So assuming the mass of the students are equal, the two students will move in opposite directions an equal distance. (Of course there are other factors involved that are ignored including the forces of friction on the skateboards.) 

Hope this helps! 

martiniquez's profile pic

martiniquez | Student, Graduate | eNotes Newbie

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First Law

An object at rest tends to stay at rest. (With no outside forces this object will not move).

An object in motion tends to stay in motion with the same direction and speed. (With no outside forces this object will never stop).

Second Law

By exerting the same force on two objects with different mass will result in different acceleration. (The more force, the more acceleration).

Formula: F=ma (force = mass x acceleration)

Third Law

For every action there is an equal and opposite reaction. (If you push an object, the object pushes back in the opposite direction equally hard)

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