Physics-Conceptual Help...?

Question

Okay, so my class is working on Newton's third law. Along with this, our teacher gave us an example of a boy who stands on a scale. The boy weighs 1000 N and he is holding a dumbell in his hands of 50 N. We need to find out why the scale reads 1050 N instead of just 1000. He wanted us to explain why the force of the boy's hand holding up the dumbell at 50 N wouldn't cancel out the dumbell's weight, and none of us knew the answer. Please help me understand!

Answer 1

Equal and opposite forces. The boy is exerting a 50-N force on the dumbbell, so he must also be exerting an equal, opposite force somewhere else and this is the basic reason why it is showing a downward movement

basically the law is expained with a suitable example

If two particles interact, the force exerted by the first particle on the second particle (called the action force) is equal in magnitude and opposite in direction to the force exerted by the second particle on the first particle (called the reaction force).
To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. -- Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone. If a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone, as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other.

hope this will help u out

Answer 2

If I remember high school physics correctly, Newton's 3rd law states that there is an equal and opposite reaction to every action. This is a type of equilibrium. If one action or force overcomes the opposite force, the object starts to move.

The force of the boy's hand holding up the dumbell counteracts gravity so the dumbell is held stationary in his hand. Otherwise, it would drop to the ground. By the same reasoning, the force the boy exerts on the ground by his weight is counteracted by the force the ground exerts back. So he is standing still. Otherwise, he too would be in free fall. Since the boy and the dumbell are stationary, everything is in equilibrium and so the total weight of the boy and the dumbell is 1000 N + 50 N.

Answer 3

Look at Newton's 3rd law in terms of action-reaction pairs. If object A pushes object B, then B pushes A back. This example w/the scale, however, might not be the best demonstration of the law.

In this example, it might be helpful to look at the issue in terms of what objects supply the support. If the dumbbell pushes the boy down w/50 N, then the object supporting the boy will then be pushed on 50 N harder (much like if another person were to additionally push down against the scale). Here's an impromptu free-body diagram that labels the force acting on the boy:

scale pushes up
BOY
gravity pulls down
dumbbell pushes down

The forces pushing up (the scale) will equal the forces pushing down (the boy's weight + the dumbbell). Hope this helps!

Answer 4

The dumbbell isn't moving, so there is no acceleration force present. It pushes down on the hand with 50 N of force, and the hand supporting it pushes up with 50 N of force. The body transmits the force on the hand, along with the force arising from the boy's mass, to his feet, so the scale indicates the sum of these, or 1050 newtons. You can use this stunt to weigh a piece of baggage which is too large to fit directly on a scale.

Answer 5

I don't think this was a good example to use in class to explain the 3rd law, which states that for every action there is an equal but opposite reaction. In this case, the boy is holding UP a dumbell weighing 50N, which means he is exerting an UPWARD force of 50N, which means that there is now an equal and OPPOSITE force, or downwards, of 50N, which is added to his own weight of 1000N. But for physics initiates, this can be confusing.

Answer 6

the dumbell IS exerting 50N downward

the fact that the boy exerts 50N upward doesn't "cancel" that force, it just means the net force on the dumbell is zero and therefore from the 2nd law it doesn't accelerate.

use the method of reductio ad absurdum to prove the question idiotic. start by replacing the boy with another dumbell so that you have one dumbell on top of the other. simplify it to just a vertical line of particles....let me know if you need help finishing this.

the conceptual problem is that we're taught that there are no stupid questions. the sharp student would have immediately told the teacher his question is stupid.

Answer 7

Equal and opposite forces. The boy is exerting a 50-N force on the dumbbell, so he must also be exerting an equal, opposite force somewhere; this happens to be downward against the scale.

Answer 8

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Answer 9

The boy is only supporting the dumbbell at a certain level to prevent it from falling, not cancelling its mass!