Displacement in the direction opposite to the Force


 
 
Concept Explanation
 

Displacement in the direction opposite to the Force

Displacement in the direction opposite to the force: In certain cases, the displacement of an object may be in the direction opposite to the direction of the force acting on it. If the force acting on an object and its displacement are in opposite directions, the work done by the force on the object is

W = - F x s, where F is the force and s is the displacement of the object.

When Displacement is opposite the force

   Work done =  (Force × displacement)= Fcos  theta

                                (Here theta is an obtuse angle)

Examples :

1. When brakes are applied to a moving vehicle, the work done by the braking force is negative. This is because the braking force and the displacement act in opposite directions.

2. When a spring is compressed than the force applied by the spring and the displacement will be in opposite to each other, so work done by spring will be negative.

Sample Questions
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Question : 1

Negative value of work done indicates that

Right Option : C
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Question : 2

The work done by the weight of a 1 kg mass while it moves up through 1 m is

Right Option : B
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Question : 3

Which of the following are correct :

(a)  When brakes are applied to a moving vehicle, the work done by the braking force is negative. This is because the braking force and the displacement act in opposite directions.

(b)  When brakes are applied to a moving vehicle, the work done by the braking force is positive. This is because the braking force and the displacement act in same directions.

(c)  When brakes are applied to a moving vehicle, the work done by the braking force is zero. This is because the braking force and the displacement act in same directions.

Right Option : A
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