Which power equation to use???

[Ineedhopenow]

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1) Power = Work/time = delta KE/ time

2) Instataneous power = Force*velocity

Problem: A bullet (mass 0.5 kg) is launched from a gun, horizontally. It takes 2 sec for the bullet to reach 10 m/s. Find the power?

I used the instataneous power equation to solve the problem, because I thought you needed to use instataneous power when something changes abruptly, like a gunshot. I was wrong...you actually need to use the first power equation.

How do you know when to use the first power equation or the second?

 

[Jepstein30]

1) Power = Work/time = delta KE/ time

2) Instataneous power = Force*velocity

Problem: A bullet (mass 0.5 kg) is launched from a gun, horizontally. It takes 2 sec for the bullet to reach 10 m/s. Find the power?

I used the instataneous power equation to solve the problem, because I thought you needed to use instataneous power when something changes abruptly, like a gunshot. I was wrong...you actually need to use the first power equation.

How do you know when to use the first power equation or the second?

i usually use the first equation.. would only use the second if it explicitly asked for instantaneous power (which I doubt it would ever).

 

[milski]

The problem talks about the velocity reaching certain value 10 m/s. That means that the velocity has been changing during these two seconds. If it's changing, applying an equation which has velocity as part of it is probably going to lead to complications, to say the least.

 

[dechristine]

I agree with Jepstein30, usually if the test-maker wants you to find the instantaneous power, he will use the word "instantaneous" or a synonym. Instead, this question was asking for the amount of power needed to accelerate a bullet to 10 m/s in 2 seconds. 2 seconds isn't instantaneous 😉

 

[milski]

The bullet is actually decelerating to 10 m/s. 10 m/s about 20 mph which is not very impressive speed for a bullet. That is not directly related to the problem but it's certainly better to know what's going on.

 

[Ineedhopenow]

Okay, thanks everyone. I'll wait until I see instantaneous before utilizing that equation.

 

[oohhii]

.

 

[milski]

If you use P=Fv (the way you want to apply it aka not instantaneous) I'm pretty sure you need the AVERAGE velocity, (vo +vf)/t, for the distance that the bullet travelled. So if you try to use this equation, you can't because you don't know the initial velocity (definitely not zero because the gun gives it an initial velocity). So then you could try deltaKE/t

Using average velocity will work but keep in mind that the average velocity is (vo+vf)/t only for linearly changing velocity (constant acceleration).

 

[qw098]

Was the answer 12.5J/s?

 

[Ineedhopenow]

Was the answer 12.5J/s?

Yes