Question

In which scenario is an animal doing work? Check all that apply.

A cat lifts up her kitten by its neck.
A dog rubs his back against a large tree.
A bird flies a worm from the ground to its nest in a tree.
A goat butts its head against the barn wall.
A horse pulls a wagon along a road.

Answer 1

Work is done when a force causes displacement in the direction of the force. So the cases where an animal does work are:

- A (cat lifts her kitten) — kitten is displaced upward by the cat’s upward force.
- C (bird flies a worm to its nest) — the worm is moved upward against gravity by the bird.
- E (horse pulls a wagon) — the wagon is displaced along the road in the direction of the pull.

Not work:
- B (dog rubs against a tree) — the tree doesn’t move, so no work is done on the tree.
- D (goat butts the barn wall) — the wall doesn’t move, so no work is done on the wall.