Newton’s First, Second and Third law


Newton’s First law (N’sFL)

N’sFL= a resultant force is needed to make something start moving’ speed up or slow down

~>if resultant force of a stationary object is zero it will remain stationary
~>if resultant force of a moving object is zero it will continue to move at same velocity (same speed and direction)
~>if moving at a constant velocity resistive and driving forces are equal
~>velocity only changes due to non-zero resultant force

~>a non-zero resultant force produces acceleration or deceleration in direction of the force
->’acceleration’ is either: speeding up :slowing down
: starting : changing direction
: stopping
~> on a free body diagrm this means arrows are equal

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Newton’s Second Law (N’sSL)

N’sSL= larger resultant force on an object the more the object accelerates

->force and acceleration are directly proportional

->mass and acceleration are inversely proportional

(heavier objects accelerate less for a fixed resultant force)


Acceleration is change in velocity over time

From rest to typical speed is approximately ~10seconds

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Newton’s Third Law (N’sTL)

N’sTL= When two objects interact, the forces they exert on each other are equal and opposite

Acceleration is force divided by mass

Although forces are always equal things move differently due to the mass of different objects.

Equilibrium situation of N’sTL is pushing against a wall.
->pushing a wall has a normal contact force acting back on him
->applying a force to the wall causes the wall to ‘push’ back with an equal source.

A book on the ground is in equilibrium as its weight is equal to the normal contact force but THIS IS NOT NEWTONS THIRD LAW as the forces are different types.

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Inertia is the tendency for motion to remain unchanged

~> until acted upon by a resultant force objects at rest stay at rest and moving objects stay constant (NEWTONS FIRST LAW) The tendency to stay in this state of motion is called inertia

~>an object’s inertial mass measures how difficult it is to change velocity

~> Inertial mass is found using NEWTON’S SECOND LAW F=ma
~>rearranging is m= F÷a Inertial mass is ratio of force over acceleration.

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