A10.0-kg object is initially moving with a velocity of 20.0 m/s to the north and is acted on by a constant net force. after the object moves 30.0 m to the north, its velocity is 12.0 m/s north. what is the constant net force acting on the object?

The direction of motion of the object did not change, so we can find the force that acted on the object by using the work-energy theorem, which states that the work done by the force is equal to the variation of kinetic energy of the object:


where
F is the force
d is the distance through which the force has been applied (30.0 m)
is the object mass
is the initial velocity of the object
is its final velocity

Re-arranging the formula, we find the magnitude of the force:

And the negative sign means the force is in the opposite direction of the motion of the object (in fact, the object is decelerating)

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