Centre of Mass: Hammer, Ruler, Balance

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In summary, the conversation discusses the concept of center of mass and its role in determining the stability of a mobile with a hanging hammer. The center of mass is the weighted average location of the mass in any context and does not have to be located where there is any mass. The pivot point is the point where the center of mass must be aligned in order for the mobile to remain stable. The restoring force created by the weight of the hammer helps to bring it back to equilibrium when it is displaced. Adding weight to one side of the ruler can cause the center of mass to shift and potentially lead to the mobile collapsing.
  • #1
tanyh123
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http://resources.scienceworld.ca/pdf/forces/Balance_Hammer.jpg

How do I explain this in terms of centre of mass?

And what's the definition of centre of mass in this context?
 
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  • #2
tanyh123 said:
How do I explain this in terms of centre of mass?
Where's the center of mass of the hammer?

And what's the definition of centre of mass in this context?
The same as it is in any context. Think of it as the (weighted) average location of the mass.
 
  • #3
Doc Al said:
Where's the center of mass of the hammer?

The centre of mass of the hammer is near the hammer head, so the centre of mass of the whole mobile should be found there as well?

Does the centre of mass of the mobile have to be vertically aligned with the table and a part of the ruler which is resting on the table or does it have to be under the table only?


Doc Al said:
The same as it is in any context. Think of it as the (weighted) average location of the mass.

But the centre of gravity of a donut is found around the centre of the donut, which is actually empty space. How do I explain that then?
 
  • #4
The center of mass is below the pivot point therefore the system is inherently stable.
 
  • #5
CWatters said:
The center of mass is below the pivot point therefore the system is inherently stable.

Can you explain to me why does positioning the centre of mass of the mobile below the pivot point, which I'm assuming is the edge of the table, makes the mobile stable and balanced?
 
  • #6
tanyh123 said:
The centre of mass of the hammer is near the hammer head, so the centre of mass of the whole mobile should be found there as well?
Right.

Does the centre of mass of the mobile have to be vertically aligned with the table and a part of the ruler which is resting on the table or does it have to be under the table only?
The center of mass must be under the part of the ruler resting on the table.

But the centre of gravity of a donut is found around the centre of the donut, which is actually empty space. How do I explain that then?
Explain what? The center of mass does not have to be located where there is any mass. Think of it as a mathematical point that is useful for describing the behavior of a system, not as a part of a body.
 
  • #7
Which is the pivot point, the part of the ruler resting on the table of the table itself?
 
  • #8
The center of mass is EXACTLY below the pivot point (eg like a stationary pendulum).

The question is where is the pivot point? In the example photo it can move. It can be anywhere between the edge of the table or the right hand end of the ruler. This actually helps increase stability...

If the hammer is disturbed by pushing/swinging it slightly to the left the pivot point will probably move to the right hand end of the ruler. This increases the restoring force on the hammer.

If the hammer is disturbed by pushing it slightly to the right the pivot point will probably move to the left eg the edge of the table. This also increases the restoring force on the hammer.
 
  • #9
CWatters said:
The center of mass is EXACTLY below the pivot point (eg like a stationary pendulum).

The question is where is the pivot point? In the example photo it can move. It can be anywhere between the edge of the table or the right hand end of the ruler. This actually helps increase stability...

If the hammer is disturbed by pushing/swinging it slightly to the left the pivot point will probably move to the right hand end of the ruler. This increases the restoring force on the hammer.

If the hammer is disturbed by pushing it slightly to the right the pivot point will probably move to the left eg the edge of the table. This also increases the restoring force on the hammer.

What do you mean by restoring force? And if the hammer is disturbed to an extent where the centre of mass is no longer under the pivot point, the mobile will collapse, won't it?
 
  • #10
tanyh123 said:
What do you mean by restoring force?
If you displace the hammer from its equilibrium position, its weight will create a torque that tends to bring it back to equilibrium. Imagine you had a ball on the end of a string, hanging down from your hand. If you move the ball aside, it will naturally tend to swing back. Same idea.
And if the hammer is disturbed to an extent where the centre of mass is no longer under the pivot point, the mobile will collapse, won't it?
No. When released from that point, it will tend to return to its equilibrium position.
 
  • #11
What is required for it to fall?
 
  • #12
tanyh123 said:
What is required for it to fall?

If you add weight to the left hand side of the rule the center of mass will move to the left. Keep adding weight and the rule will take up an angle to the horizontal (down on the left). The pivot point will move to the edge of the table and eventually the rule might reach an angle steep enough for the rule to slip off the edge of the table. That's if the head of the hammer doesn't hit the underside of the table first.
 

1. What is the centre of mass of an object?

The centre of mass of an object is the point where its entire mass is considered to be concentrated. This point is also known as the balance point, as it is the point at which an object will be in equilibrium when suspended.

2. How is the centre of mass of an object determined?

The centre of mass of an object can be determined by finding the point where all the individual mass elements of the object balance each other out. This can be done using mathematical equations or by physically balancing the object on a pivot point.

3. What is the significance of the centre of mass?

The centre of mass is an important concept in physics as it helps in understanding the stability and equilibrium of an object. It is also used in calculating the motion and forces acting on an object.

4. How does the centre of mass affect the balance of an object?

The centre of mass plays a crucial role in determining the balance of an object. If the centre of mass is located within the base of support, the object will be stable and will not topple over. However, if the centre of mass is outside the base of support, the object will be unstable and may fall over.

5. Can the centre of mass change?

Yes, the centre of mass of an object can change depending on its shape, size, and distribution of mass. For example, if an object is cut in half, the centre of mass will shift towards the heavier half. It can also change when external forces act on the object, causing its shape or position to change.

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