Power Problem

[silverice]

---

Members don't see this ad.

A 10,000 kg locomotive speeds toward Lois Lane at 10m/s, and will hit her in 10 s if Superman doesn't try to stop it. If superman applies a constant force, what average power does he need in order to stop the train?

A. 2.5x10^4W
B. 5.0x10^4W
C. 2.5x10^5W
D. 5.0x10^5W

I used W=.5mv^2=.5(10,000kg)(10m/s)^2=500,000J
Then p=W/t=5000,000J/10s=5x10^5

But the answer is A.

Please help, thanks.

 

[peacefulheart]

A 10,000 kg locomotive speeds toward Lois Lane at 10m/s, and will hit her in 10 s if Superman doesn't try to stop it. If superman applies a constant force, what average power does he need in order to stop the train?

A. 2.5x10^4W
B. 5.0x10^4W
C. 2.5x10^5W
D. 5.0x10^5W

I used W=.5mv^2=.5(10,000kg)(10m/s)^2=500,000J
Then p=W/t=5000,000J/10s=5x10^5

But the answer is A.

Please help, thanks.

1. I did the question before. I had the similar problem.

2. we should not use 10s when applying the formula of p=w/t . we assumed that it will
take superman 10s to stop the train. However that is not the case.



3. a. Fd=1/2mv^2, d=10s*10m/s=100m,
we have F of 5,000N

b. F=ma,
we have a=1/2

c.V=V0-at , V0=10m/s, V=0

we have t=20s instead of 10 s.

thanks

 

[silverice]

1. I did the question before. I had the similar problem.

2. we should not use 10s when applying the formula of p=w/t . we assumed that it will
take superman 10s to stop the train. However that is not the case.



3. a. Fd=1/2mv^2, d=10s*10m/s=100m,
we have F of 5,000N

b. F=ma,
we have a=1/2

c.V=V0-at , V0=10m/s, V=0

we have t=20s instead of 10 s.

thanks

Yeah, I realized that while I was discussing it with a friend. But this question caused another confusion. The question below can use P=Fv=mgv to solve.

A 5,000 kg rocket moves straight up at 30m/s near the earth's surface. what is the minimum power of the rocket's engines?
Answer P=mgv=5,000 kg x 10m/s^2 x 30 m/s
Explanation: In order to keep the rocket moving at a constant velocity, the rocket's engines must supply enough force to counter gravity.

One obvious difference between this and the previous problem is that, the object in the previous problem is being decelerated at a given distance. But isn't the rocket in the second problem isn't being decelerated by gravity?

I think I don't fully understand this concept that's why i'm having problem. Please help, thanks a bunch.

 

[Deadlifts]

Yeah, I realized that while I was discussing it with a friend. But this question caused another confusion. The question below can use P=Fv=mgv to solve.

A 5,000 kg rocket moves straight up at 30m/s near the earth's surface. what is the minimum power of the rocket's engines?
Answer P=mgv=5,000 kg x 10m/s^2 x 30 m/s
Explanation: In order to keep the rocket moving at a constant velocity, the rocket's engines must supply enough force to counter gravity.

One obvious difference between this and the previous problem is that, the object in the previous problem is being decelerated at a given distance. But isn't the rocket in the second problem isn't being decelerated by gravity?

I think I don't fully understand this concept that's why i'm having problem. Please help, thanks a bunch.

Minimum means you're probably gonna set an equation equal to zero.
Constant velocity so there's no net acceleration.
Pnet=Procket-Pgravity. 0=Procket-Pgravity. Procket=mgv.