when u kick a ball applying 10 newton and ball displaces through 10 m.then net work done?

a.100joule  b.0   c.can not be said. d.other

The answer is C since we do not know that the force is applied in which direction and what is the direction of the displacement.

 

Note that the work done if F.d !!!!

 

cheers

 

The work done cannot be determined because the force is not applied continuously but only for a moment by kicking it.  If the contact of "kick" can be known as "dt" (say for 0.1 sec or so), then impulse force = change of momentum

or F x dt  = m x dv; here dv = V-0 = V (since ball was at rest);

This impulse force is used to impart kinetic energy (or change in KE if it is moving) = mxVxV/2, where V was velocity at the time the contact of kick ends.

 

In the given problem, we have been given that it moves a distance of S (it is clear that  work done will be equal to F x dS, ie the distance dS travelled by the ball when it is in contact with the "kick force") and not over the total distance S travelled by the ball.  On the other hand if we lift the ball up against gravity with a constant force F (obviously F > mg, where m is the mass of the ball) over a distance S (and then the force F is removed), then work done = F x S.  Part of the work done is used in changing the potential energy of the ball = mgS and the other part = FS - mgS will be converted into kinetic energy = (m x UxU/2) and the ball will continue to move upward for another height H, such that mxUxU/2 = mxgxH , as here work is done on the ball by the force of gravity.

we do not know the direction of the displacement of the ball.so F.d cannot be determined.Agreed.

But in the question , the work done by net forces is asked. And according to the w.e theorem the net work done (ie the work done by all forces) is the chenge in kinetic energy.

So,

Wnet=0

 

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