Does a Swinging Object Affect the Weight Reading on a Scale?

[cagdassalur]

firstly i want to say this is my first post and english isn't my first language. so I am very sorry about mistakes.(spelling etc.)

think a man, he's about 80kg. he is standing on machine that calculates your weight(from now on i call it mtcyw) later he get a thing which is 5kg and tied up a rope. he gets on the mtcyw and mtcyw shows 85kg. this is simple.. now, what if man swinging the thing with rope around his head while on mtcyw ? mtcyw shows under 85kg? or shows 85kg.
i have no idea with this question and teacher asked me the prove with details and not just the answer, so if you help me i will be so happy.

there is a sketch i made with my super awesome drawing skills;
http://img20.imageshack.us/i/55035215.jpg/"

 

[Doc Al]

Does he swing it in a horizontal or a vertical circle?

Hint: What forces act on the man? In what direction do they act? Consider how the tension in the rope might affect the scale reading.

 

[cagdassalur]

hight of the thing is never changing.

 

[Doc Al]

What vertical component of force does the rope exert on the man?

 

[cagdassalur]

Yes it effects. Everything effects, like in real life.

 

[HallsofIvy]

Doc Al did not ask if the swinging weight affects anything. He asked what is the vertical component of force exerted by the rope on the man- a specific number.

 

[cagdassalur]

sorry, i understand now. i don't know what is the value of the force, i don't want an spesific answer anyway. all i want to learn is it be the same(weight), or less.

 

[Doc Al]

Do this:

Draw a force diagram for the 5kg mass showing all forces acting on it. (Only two forces act on it.) What direction is it accelerating? What's the net vertical force on it?

Draw a force diagram for the man, showing all forces acting on him. See what you can conclude.

 

[cagdassalur]

sorry again, as i said my eng is very bad. i don't understand you want the diagram to better understand my question?, or you saying drawing a diagram helps me to solve my problem?

 

[Doc Al]

or you saying drawing a diagram helps me to solve my problem?

That's what I'm saying. For you to solve the problem, you need to draw a diagram (or two). Whenever you are dealing with forces, it's usually a good idea to draw yourself a diagram.

(I believe I understand the problem very clearly. You want to know how the scale reading will change if he starts swinging that mass in a horizontal circle. )

 

[cagdassalur]

i think when swinging the rope we have to give more pressure to arm then just holding the hope. and nothing is just raises something must be down to raise other thing. they teached us that. when I am spending more energy the things force on the groun must be down. so, it will be 80>total weight>85 right?

 

[Doc Al]

so, it will be 80>total weight>85 right?

No, not right. You need to examine the forces acting on the mass. For example, say the swinging rope makes an angle of 45 degrees with the vertical (or you can just call the angle θ). Draw a diagram of the forces acting on the mass. Is the mass accelerating? What's the net vertical force on the mass?

 

[cagdassalur]

i think about it and drawing a diagram really helped me and i found that the thing doesn't have a vertical force that effects on arm. only horizental force. so it will be 80. am i right?

 

[Doc Al]

i think about it and drawing a diagram really helped me and i found that the thing doesn't have a vertical force that effects on arm. only horizental force. so it will be 80. am i right?

No. The man exerts a force on the rope that has both horizontal and vertical components. After all, the swinging mass has no vertical acceleration, thus something must be holding it up!

 

[cagdassalur]

thus something must be holding it up!

ok. but how can the rope has vertical force on the man? its horizontal.

... ____O...
...o /...
.../.|...<---- like this
.../...|...
.../.\...

 

[Doc Al]

ok. but how can the rope has vertical force on the man? its horizontal.

Ah, but it cannot be perfectly horizontal. It must be angled downward, since it has to support the weight of the mass. The faster you swing the mass, the more horizontal the rope becomes, but it never is completely horizontal.

 

[cagdassalur]

yes. i understand but when i say its be 80>x>85 you said its wrong?

 

[Doc Al]

yes. i understand but when i say its be 80>x>85 you said its wrong?

Yes, that's an incorrect answer.

 

[cagdassalur]

but it can't be stay same right?

 

[Doc Al]

but it can't be stay same right?

What vertical force does the man exert on the rope?

 

[cagdassalur]

there is no vertical force i think. but I am probably wrong.

 

[Doc Al]

there is no vertical force i think.

Then what's holding up the mass? It's not falling, right?

 

[cagdassalur]

force that opposite way to arm holding the mass i don't know what's this word in english. it may be centrifugal.

 

[Doc Al]

force that opposite way to arm holding the mass i don't know what's this word in english. it may be centrifugal.

No, I'm not talking about centrifugal force.

Answer this: What forces act on the mass. In what direction is the mass accelerating?

 

[cagdassalur]

there is a centrifugal force. and there is anti centrifugal that rope has on mass and gravity force.

 

[Doc Al]

there is a centrifugal force. and there is anti centrifugal that rope has on mass and gravity force.

Forget about centrifugal force--stick to an inertial frame.

The only forces acting on the mass are: the rope tension (which acts at an angle) and the weight (which acts down).

Since it's going in a horizontal circle, its acceleration is towards the center (centripetal) and horizontal.

So what must be the net vertical force on the mass?

 

[cagdassalur]

0?





.

 

[Doc Al]

Exactly!

Which means that the rope must pull up with a vertical force equal to the weight of the mass.

 

[cagdassalur]

ok. i get that well but what will be the total weight?

 

[Doc Al]

ok. i get that well but what will be the total weight?

You tell me!

If the man must pull upward on the rope with a vertical component of force equal to the weight of the mass, what must the scale read? Is this really any different from the case where he's just holding the mass without swinging it (as far as the scale reading goes)?

 

[cagdassalur]

oh! finally i understood. it will be the same cos man has to pull that weight. thanks very much you have very patience. you could just tell me the answer but you didn't. if you did i just take answer to the teacher but i learned now.