kinetic energy?Here’s How to Crack It
To stop an object means to change its kinetic energy to zero. So, if the initial kinetic
energy is 30 J, then the change in kinetic energy has to be 0 − 30 = −30 J. By the
work–energy theorem, Wtotal = ΔK, the total amount of work that would be required
is −30 J.
Kinematics vs.
Work–Kinetic Energy
For objects moving in
a straight line with a
constant force, you can
use the work–kinetic
energy theorem or Big
Five #5 and Fnet = ma for
problems where time is
not involved.- An object has two forces acting on it, one performing 50 J of
work and the other (friction) performing −20 J. What is the final
kinetic energy of this object?
Here’s How to Crack It
The total work done is (40 J) + (−20 J) = 20 J. So, by the work–energy theorem,
Wtotal = ΔK, we have 20 J = ΔK. Since ΔK = Kf − Ki, we find that Kf = Ki + ΔK =
10 J + 20 J = 30 J.
- A pool cue striking a stationary billiard ball (mass = 0.25 kg)
gives the ball a speed of 2 m/s. If the force of the cue on the ball
was 25 N, over what distance did this force act?