Physics- Work?

Question

Dear Yahoo Answers,
Please Help with this confusing question..Which of the following situations does NOT involve any work being done?


(a) A 10 kg crate is lifted 2 metres
(b) A car is pushed along a road
(c) A spacecraft travels through the solar system without being affected by air resistance or gravity
(d) A skateboard rolls to a stop
(e) A book sits on a desk

Any contribution will be appreciated.

Answer 1

Work can be defined in several ways:
1) Change in total energy
2) Distance worked against friction
3) Force in the direction of distance traversed

So we look at each situation:
(a) shows increased potential energy... work done
(b) Distance traversed due to a force... work done
(c) assuming no thrust (big assumption) no work is done
(d) Friction changes kinetic energy... work done
(e) no force and no change in position or energy, no work done

Answer 2

Work and energy are both measured in Joules. Work was not done if the object's energy (potential or kinetic) did not change.

c. The spacecraft continues moving with its original velocity. No work was done to change it.

e. The book's velocity is zero and remains zero. No work is done to change it.

Answer 3

work = force X distance and force = mass X acceleration. So to have work you have to have one of the following:
1-change in distance
2-change in mass
3-acceleration

Answer 4

all involve work except for e(for sure), and possibly c

Answer 5

c)

best answer please!