What are the forces on the center of mass?

In summary: But you're not given anything about the forces acting on the center of mass.I think the key is to think about what would happen if the external forces were zero. Then you could solve for the acceleration of the center of mass.Thanks again for the help!
  • #1
eprparadox
138
2

Homework Statement


[/B]
A light rope passes over a light frictionless pulley attached to the ceiling. An object with a large mass is tied to one end and an object with a smaller mass is tied to the other end. Starting from rest the heavier object moves downward and the lighter object moves upward with the same magnitude acceleration. Which of the following statements is true for the system consisting of the two masses?

A. The center of mass remains at rest.
B. The net external force is zero.
C. The velocity of the center of mass is a constant.
D. The acceleration of the center of mass is g, downward.
E. None of the above statements are true.

Homework Equations

The Attempt at a Solution



The answer is E for this question, but I'm stuck on justifying choice B.

In particular, I want to be able to draw out the forces on this system of two masses. I don't know how to account for the tension in the rope on both sides and gravity.

I think I have trouble in general when talking about "system" like this and drawing out the forces. Any thoughts on how to draw out this force diagram?
 
Physics news on Phys.org
  • #2
I'm not sure what exactly your problem is. The tension in the string has to be calculated, but as the pulley is frictionless the tension must be the same on both sides. That said, you don't really need to consider the tension to rule out option B.

Can you see a simple argument to rule out option B? Why do you think the external force might be zero? What would happen to the system if it were?
 
  • #3
Hey @PeroK thanks for the response.

If the external forces are zero, then the center of mass can't accelerate. But intuitively, this doesn't make sense because there is an external force due to the gravitational acceleration downward.

But then that has me asking what other forces are acting on the center of mass that are preventing it from just simply falling with acceleration of ## g ## downward?

Thanks again
 
  • #4
approach all problems one by one and see if they are true
I am no expert so correct me if i am wrong
A)Hint: masses are moving with same acceleration
B)Hint:Draw diagram and use formulas
C)Hint:same as A
D)Hint:same as A

As an example i will tell you this
If we throw a particle in parabolic path and it breaks into 2 or more pieces the center of mass will continue to travel along the same path.
 
  • #5
eprparadox said:
Hey @PeroK thanks for the response.

If the external forces are zero, then the center of mass can't accelerate. But intuitively, this doesn't make sense because there is an external force due to the gravitational acceleration downward.

But then that has me asking what other forces are acting on the center of mass that are preventing it from just simply falling with acceleration of ## g ## downward?

Thanks again

Okay, if you approach the problem that way, then you need to look at the other forces (tension).

But, if you look at the problem differently ...

Hint: you don't need to calculate the acceleration of the centre of mass; you just need to know that it is accelerating.
 
  • #6
You might start by locating the center of mass and following it in time and space.
 
  • Like
Likes PeroK
  • #7
Suyash Singh said:
As an example i will tell you this
If we throw a particle in parabolic path and it breaks into 2 or more pieces the center of mass will continue to travel along the same path.

Which isn't very helpful.
 
  • #8
You know the driving force and the total mass being accelerated.
 

Related to What are the forces on the center of mass?

What are the forces on the center of mass?

The forces on the center of mass refer to the overall net force acting on an object's center of mass, which is the point at which the object's mass is evenly distributed.

How do you calculate the forces on the center of mass?

To calculate the forces on the center of mass, you must first determine the individual forces acting on each part of the object. Then, you can use the equations for determining the center of mass to find the overall net force.

What factors affect the forces on the center of mass?

The forces on the center of mass can be affected by a variety of factors, such as the mass and distribution of the object, as well as any external forces acting on the object.

Why is it important to consider the forces on the center of mass?

Understanding the forces on the center of mass is important in order to accurately predict the motion of an object. It can also help determine the stability and balance of an object.

How can the forces on the center of mass be used in real-world applications?

The forces on the center of mass can be used in various real-world applications, such as in designing structures, analyzing the motion of objects, and predicting the stability of vehicles and other systems.

Similar threads

  • Introductory Physics Homework Help
Replies
3
Views
349
  • Introductory Physics Homework Help
Replies
3
Views
906
  • Introductory Physics Homework Help
Replies
17
Views
824
  • Introductory Physics Homework Help
Replies
22
Views
3K
  • Introductory Physics Homework Help
Replies
17
Views
1K
  • Introductory Physics Homework Help
Replies
30
Views
2K
  • Introductory Physics Homework Help
Replies
31
Views
1K
Replies
10
Views
1K
  • Introductory Physics Homework Help
Replies
4
Views
1K
  • Introductory Physics Homework Help
2
Replies
38
Views
6K
Back
Top