A40 kg box is placed on a 2 m tall shelf. a. what is the gravitational potential energy of the box? b. if the box falls off the shelf, what is its kinetic energy as it strikes the ground? c. what is the velocity of the box immediately before it strikes the ground?

a) P.E= mgh

where m=40kg g=9.8m/s^2 h=2m

P.E=40×10×2

=800N

b)K.E=P.E

i.e, 1/2mv^2=mgh

1/2v^2=gh

where,g=9.8 h=2 v=?

1/2v^2=9.8×2

v^2=19.6×2

v^2=39.2

v=6.26m/s

therefore, K.E=1/2mv^

1/2×40×6.26^2

1/2×1567.5

K.E=783.8N

c) K.E=1/2mv^2

where, K.E=783.8N m=40kg v=?

783.8=1/2×40×v^2

783.8=20v^2

v^2=783.8/20

v^2=39.19

v=√39.19

v=6.26m/s^2

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.

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