Simple question about torque / gravity / force

In summary, the problem involves a bar AB of no mass and 1m length that is balanced on a support P at its midpoint. A 0.6kg weight is hung on one end and a 1kg weight is hung 0.3m away from the midpoint on the other side. The question asks for the upward force from the support P. The solution involves understanding the concept of torque and how it affects the balance of the bar. The answer is 16N, which is the sum of the two weights on either side of the bar. The position of the weights does not affect the normal force as long as the bar remains balanced.
  • #1
slain4ever
63
0

Homework Statement


a bar AB of no mass and 1m length is balanced on a support P at its midpoint. 0.6kg is hung on one end and 1kg is hung 0.3m away from the mid point on the other side. What is the upward force from support P

Homework Equations



torque = perpendicular force * radius

The Attempt at a Solution



i thought it would be simple. i figured out the force on one side was 3N and the other side was the same. So 3N + 3N both acting down is 6 N since the bar isn't moving down it must be being pushed from the support with a force of 6N. However the answer in my book is 16N i don't understand how it can possibly be this.
Please show me where i went wrong.
Thank you.
 
Physics news on Phys.org
  • #2
If I place a 1kg box on a table, what is the normal force exerted by the table on the box? Does this force depend on how the mass is distributed within the box? Going back to your problem, would it matter if both weights were placed at the midpoint of the bar?
 
  • #3
so your saying i just add up the two forces 10 + 6=16N ? That's the correct answer but why wouldn't i need to use force * radius?

I think it does matter if they are placed away from the midpoint
 
  • #4
Actually i think i may understand this. Because the torque on each side is equal the bar remains horizontal, so the force of each is directly down and counteracting the rotation caused by the other mass.
 
  • #5
Well the problem as you've stated it is just asking for the upward force from the support which is touching the bar at the midpoint. Which is just the weight of the two masses correct? There is no calculation of torque needed to solve this problem. Now you're correct about the net torque on the bar depending on where the masses are placed since the equation for torque involves the radius from the pivot. So if the torque on each side wasn't balanced the bar would rotate. But as it is, they are balanced in this problem and the net torque on the bar is zero. This would also be true if the two masses were placed at the midpoint of the bar where the support P is, since the force due to their combined weight would be acting on the pivot, in which case they would not contribute to the torque on the bar and the net torque would again be zero. Either way as long as the bar doesn't fall off the pivot, which could correspond to a number of combinations of where the two masses are placed, then the normal force is just equal to the weight of the two masses; regardless of their position.
 
  • #6
thank you for your prompt help
 

1. What is torque?

Torque is a measure of the force that causes an object to rotate around an axis. It is calculated by multiplying the force applied by the distance from the axis of rotation.

2. How does gravity affect torque?

Gravity can affect torque by exerting a force on an object, causing it to rotate around an axis. This is seen in objects such as pendulums or seesaws, where gravity provides the force that causes the rotation.

3. What is the relationship between force and torque?

Force and torque are closely related, as torque is essentially a rotational force. The greater the force applied to an object, the greater the torque and the more likely it is to rotate.

4. Can torque be negative?

Yes, torque can be negative. This occurs when the direction of the force applied is opposite to the direction of rotation, causing the object to rotate in the opposite direction.

5. How is torque calculated?

Torque is calculated by multiplying the force applied by the distance from the axis of rotation. This can be represented by the equation T = F x r, where T is torque, F is force, and r is the distance from the axis of rotation.

Similar threads

Replies
6
Views
684
  • Introductory Physics Homework Help
Replies
6
Views
712
  • Introductory Physics Homework Help
Replies
14
Views
1K
  • Introductory Physics Homework Help
Replies
5
Views
1K
  • Introductory Physics Homework Help
Replies
28
Views
2K
  • Introductory Physics Homework Help
Replies
30
Views
1K
  • Introductory Physics Homework Help
Replies
22
Views
2K
  • Introductory Physics Homework Help
Replies
6
Views
1K
  • Introductory Physics Homework Help
Replies
4
Views
1K
  • Introductory Physics Homework Help
Replies
8
Views
2K
Back
Top