Physics Check Answers
Hey Basil ,
I was wondering if someone can check if my answers are correct for this practice review:
12. If a disk's angular velocity is in the positive direction as the angular speed increases at a constant rate, which of the following describes a plot of its angular acceleration versus time?
Plot has zero slope or plot has constant negative slope?
A: I chose zero slope.
14. You ride a rotating merry-go-round near the center. If you walk outward toward the rim, what happens to your angular speed?
Angular speed increases or remains the same?
A: I chose angular speed increases.
15. You ride a rotating merry-go-round near the center. If you walk outward toward the rim, what happens to your linear speed?
Increases or remains the same?
A: I chose increases.
16. A cockroach rides the rim of a disk rotating like a merry-go-round. If the angular speed of the disk increases, what happens to the radial acceleration of the cockroach?
Increases or remains the same?
A: I chose increases.
17. A cockroach rides the rim of a disk rotating like a merry-go-round. If the cockroach has a positive tangential acceleration, what happens to its radial acceleration?
Increases or remains the same?
A: I chose increases.
26. You push tangentially on the rim of a playground merry-go-round. Then you gradually change the direction of your push until you are pushing directly toward the center of the merry-go-round. What has happened to the magnitude of your torque on the merry-go-round?
Decreased or remains unchanged?
A: I chose decreased.
27. What relates the torque on a playground merry-go-round to the resulting angular acceleration?
The merry go rounds mass or rotational inertia?
A: I chose rotational inertia.
28. A force causes an object to rotate. If you increase the distance between the pivot point and the point where the force is applied, what happens to the angular acceleration?
Increases or remains the same?
A: I chose increases.
30. Which gives the power of a torque?
the integral of the torque with respect to time or the derivative of the work with respect to time
A: I chose integral of the torque with respect to time.
Sorry its so much, I just wanna know if I have the correct answers or not because I need to use this to study for the test thats coming soon </3.
Thanks so much for any help, I really appreciate it.
14 Comments • Newest first
@ballsface has a point ... havent done those problems for a while
I hate these multiple choice questions, the wording is so confusing. I prefer using the formulas & do the actual problems
[quote=Allison118]Thanks for the help everyone. But, I just wanna make sure, would number 30 be the integral of torque with respect to time? Cause some1 has been telling me its the derivative of work with respect to time[/quote]
No, number 30 would be the second option. Power is the derivative of work with respect to time.
For translational (linear) motion, Work = Force * distance.
Power = derivative of work = Force * velocity.
For rotational motion, Work = Torque x distance.
Power = derivative of work = Torque x angular velocity.
For 28, I don't believe either of the answers are correct.
From F=ma
and a = (alpha)*r, where (alpha) is angular acceleration.
Substitute that in and we get
F = m*(alpha)*r
Rearranging gives
F/m = (alpha)*r
If we assume a constant applied force and constant mass, F/m = constant. Let the constant equal c
c = (alpha)*r
(alpha) = c / r
Therefore angular acceleration and distance to the pivot point have an inversely proportional relationship.
That means by increasing the distance between the pivot point and the point where the force is applied, angular acceleration would actually decrease.
[quote=momozzz]Oh you answered that before l edited it. "This one" is referring to 17, not 14. Rereread it and realized l was right about thinking poorly in the first place.[/quote]
Thanks for the help everyone. But, I just wanna make sure, would number 30 be the integral of torque with respect to time? Cause some1 has been telling me its the derivative of work with respect to time
@momozzz
I think you are right ... it kinda makes sense, the tangential velocity will increase with tangential acceleration, since radial acceleration is v^2/r, it should also increase.
[quote=cchpm]The tangential velocity does increase, but not the angular velocity.
Think about this, angular velocity is in radians/second. No matter if you are near the center or at the outer edge, it will take the same time to complete a circle.[/quote]
Oh you answered that before l edited it. "This one" is referring to 17, not 14. Rereread it and realized l was right about thinking poorly in the first place.
[quote=momozzz]Yeah see l'm stupid l0l but l'm still set on this one increasing unless the question restricts the scenario to it remaining positive for the entire curve.[/quote]
The tangential velocity does increase, but not the angular velocity.
Think about this, angular velocity is in radians/second. No matter if you are near the center or at the outer edge, it will take the same time to complete a circle.
[quote=cchpm]12 - correct
[b]14 - angular speed remains the same - no matter where you are, the angular velocity remains constant[/b]
17 - I think the radial acceleration might remain the same.
[/quote]
Yeah see l'm stupid l0l but l'm still set on 17 increasing unless the question restricts the scenario to it remaining positive for the entire curve.
12 - correct
14 - angular speed remains the same - no matter where you are, the angular velocity remains constant
15 - correct
16 - correct
17 - edit : correct
26 - correct
27 - correct
28 - correct
30 - correct
Oh haha, thanks
Like one that l'm not certain about but my gut is like 95 to 5 on it being right.
nvm l just reread it and l'm a dumbass today apparently. you so smarts
Are they really all correct? lol
Look at you, oh so smart
It looks right.
It is correct.