Talk:Work (physics)/Archives/2008/September

Ladders

When you're climbing a ladder, your applying a force downwards but your displacement is upwards, to be considered work by its definition, does the force have to be applied in the same direction of the displacement? If that is the case does that mean that climbing a ladder actually produces no work?--TEAKAY-C II R (talk) 22:11, 3 September 2008 (UTC)

The ladder applies an upward force on you (reaction force), causing you to move upwards. Thus the ladder has done work on you.93.96.80.31 (talk) 22:23, 22 September 2008 (UTC)

The ladder does not do work, because the steps do not move up (and how could it, there is no fuel or electricity supply). The muscles that stretch the legs apply a force downward and upward; the downward force is resisted by the step, and only the upward force does work, because it lifts the body.--Patrick (talk) 23:41, 22 September 2008 (UTC)

TEAKAY-C II R asked does the force have to be applied in the same direction of the displacement? The answer is no. If the force and the displacement are in the same direction, positive work is done; if the force and the displacement are in opposite directions, negative work is done; if the force and the displacement are perpendicular, no work is done.

For example, when a ball is thrown vertically upwards, the weight of the ball acts in the opposite direction to the direction of motion, negative work is done, and the kinetic energy of the ball is progressively reduced until it reaches zero. On the way down, the weight of the ball acts in the same direction as the motion; positive work is done and the kinetic energy of the ball progressively increases. The Work-Energy Theorem specifies that the change in kinetic energy of the ball between two points in space is equal to the work done on the ball between those two points.

The trajectory of a ball is a simple example of the Work-Energy Theorem. A person climbing a ladder is not such a good example because it is more complex. As the person climbs the ladder the positive work done by his/her leg muscles is closely matched by the negative work done by his/her weight. The end result is that the total work done on the person is zero and the person’s kinetic energy is the same when he/she reaches the top of the ladder as when he/she was at the bottom.

Note that the rungs of the ladder have zero displacement and so they do no work on the person. It is the extending of the leg muscles that provides the displacement and does the work.

The work done by a person of weight W in climbing a ladder through a height h is Wh. The work done by the person’s weight (ie gravity) is –Wh. Dolphin51 (talk) 03:53, 23 September 2008 (UTC)