Newton's Second Law of Motion

Newton's second law of motion states that the acceleration experienced by a body is directly proportional to the unbalanced force acting on it. If a body is subject to an unbalanced forceit experiences an acceleration a given byIn fact the constant of proportionality is whereis the mass of the body and we can more conveniently writeThe examples below show how to calculate the acceleration of a body subject to different forces.


The net force is 2N to the right.

to the right.


The net force is 4N to the right.

to the right.


The net force is 3N to the left.

to the right.

It is not necessarily always the case that bodies must speed up. Bodies also slow down. In this case a deceleration or negative acceleration is produced and still there must be an unbalanced force, in this case opposed to the direction of motion. When a driver takes his foot off the car accelerator pedal, the forward force due to the engine decreases but friction remains the same, so the forces opposing motion are greater than those making the car move and the car decelerates. Another common example is when something is thrown up in the air. The force of gravity always acts down, so the body may be travelling up but decelerating constantly at 9.8 m/s 2 , or acceleration at -9.8 m/s 2 . This acceleration is always in the downwards direction whether the body is moving upwards or downwards.

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