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 carries a worm to its nest.
a goat butts its head against the barn wall.
a horse pulls a wagon along a road.

A cat lifts up her kitten by its neck.

A bird carries a worm to its nest.

A horse pulls a wagon along a road.

Explanation:

Work is defined as the product between the force applied to an object and the distance the object has been moved:

therefore, work is non-zero only if the object has been moved by a distance different from zero. This is exactly what happens in the three following scenarios:

A cat lifts up her kitten by its neck.

A bird carries a worm to its nest.

A horse pulls a wagon along a road.

In all these cases, an object (the kitten, the nest and the wagon) are moved through a certain distance, so work is done. On the contrary, in the other two examples:

A dog rubs his back against a large tree

A goat butts its head against the barn wall.

No object is moved, so d=0 in the formula and no work is done.

true tan speed = ang vel x radius

increase masses, decrease separation