Explain why is there no work in the following: 1. carrying a 15-kg backpack as you walk a horizontal distance of 50 m
2. pushing against a stationary wall for a long time
3.carrying a box inside a tricycle

Explanation:

Well, this is one of those physics questions that relies heavily upon technicalities.

Something to remember is work is determined by force*distance.

1. When you are carrying a 15kg backpack, the weight or the backpack itself is quite indifferent. This is because the backpack is moving relative to you, so the backpack isn't doing diddly squat itself. The only force being exerted is yourself against the ground and vice versa because of gravity and Newton's Law, but when you have a horizontal distance (not vertical), the direction of your motion is 90 degrees to the exerted force. When you calculate this out using the formula and cos(90), you'll find the answer is technically 0.

If it were vertically, this'd be different.

2. Again, wording is important. Work is Force times DISTANCE. Key word in this situation, DISTANCE! When you push against a stationary wall (stationary as in, the wall "can do this all day" whereas you definitely can't), you're exerting a force against an object that really, really doesn't want to be moved. Walls in of themselves are MADE to not be moved, that's why they are walls. So, any force you exert is going to get multiplied by a distance of 0, and as we should know from basic times table charts, anything multiplied by 0 will ALWAYS be 0.

So, again, there's technically no WORK done, according to physics. Because again, work is a product of force and distance. So push harder, ya lazy good for nothin weakling!!

3. You can carry anything you want inside a vehicle, it doesn't mean there's any force being done. YOU'RE not doing squat, the vehicle itself is. In a tricycle, if the box is on the tricycle and not affecting you in any way, then there is no force being done despite an obviously moving distance.

By physics definition, there's EFFORT, but no WORK.

Hope this helps a little :)

~Troy

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