TBR PHYSICS ch 5 passage 1 and TPR PHYSICS pg 358 #1

[crazy person]

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Hey guys !


Can someone please help me out with these question. I'm getting confused on Simple Harmonic Motion on a Pendulum.

I thought that for a pendulum, mechanical energy is always conserved, therefore the Kinetic Energy will be conserved. How can TBR is saying that for this passage, the pendulum's Kinetic Energy is not conserved?

As I'm referring to TPR for content, they say that mechanical energy is always conserved for simple harmonic motion so I don't get TBR 's response on that note.

Furthermore, on page 358 of TPR, this passage entails that 5 balls are suspended from two wires that connect to a frame, and are free to move individually or as a group. So for question #1 : it says that Kinetic energy is conserved as the balls swing left to right. So, here is another example why TBR could be wrong?

But as the balls are swinging, how can there be momentum ? I thought you could only have momentum when no other external forces are considered. TPR is saying that there is momentum as the balls are swinging, but gravity (force) is acting on it, and is acting as an external force, so therefore, there is no momentum. Right?

Am I missing something or am I wrong?

Thank you guys !

 

[DrknoSDN]

Can you write the question out?

Also, momentum is mass * velocity so there is momentum if there is movement.

 

[dudewheresmymd]

haha yeah i didn't answer this b/c wasn't sure what OP was asking

 

[crazy person]

1) Which of the following statements is true for the mass of an oscillating pendulum?

a) the mass angular momentum and kinetic energy are conserved
b) the mass momentum and angular momentum are conserved
c) the mass momentum and kinetic energy are conserved
d) the mass momentum and angular momentum are not conserved.


The answer is D.

I thought it would be C since mechanical energy is always conserved in an simple harmonic motion. It also says the same thing in the TPR book so this where I was getting confused and think maybe TBR is wrong.

2) In a given passage, you have 5 balls suspended from two wires. The balls are suspended so that they are touching each other and free to move individually or as a group.

Which of the following is true ?

a) Kinetic energy is not conserved and the collisions are inelastic.
b) Kinetic energy is not conserved and the collisions are elastic
c) Kinetic energy is conserved and the collisions are inelastic
d) Kinetic energy is conserved and the collisions are elastic

The answer is D

But as the balls are swinging, how can there be momentum ? I thought you could only have momentum when no other external forces are considered. TPR is saying that there is momentum as the balls are swinging, but gravity (force) is acting on it, and is acting as an external force, so therefore, there is no momentum. Right?

 

[DrknoSDN]

Short explanation:
#1, Kinetic energy is conserved (converted) to potential energy in a pendulum but mass momentum is not so C is incorrect because of the mass momentum part . There is greater momentum when the kinetic energy is greatest (bottom of the swing). Angular momentum is basically degrees (or radians) per unit time. The time required to move a specific angle changes based on where the pendulum is so the answer is D.

#2, yes kinetic energy is conserved (still). And yes for in-compressible metal balls the collisions would be elastic.

Momentum is always present if you have a mass and that mass is moving. If a ball is stationary in a gravitation field it will have zero momentum (Mass * zeroVelocity = zero). If you allow the ball to fall it will gain momentum due to the gravitational force acting on it.


Your understanding of kinetic energy and potential energy seems correct. Momentum is simply mass * velocity.
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