## Work & Energy review video

931287m
Posts: 93
Joined: Mon Mar 11, 2019 1:43 pm

### Work & Energy review video

Hello,

A question in this video asked to solve for the power for a crane lifting 7000 kg at 5 m/s

Using the equation P = (f)(v), the answer is said to be 40,000 W

What force was used for this calculation? I just want to make sure I have a good grasp on this derivative of power since it's new to me

Thanks!
NS_Tutor_Will
Posts: 766
Joined: Fri May 25, 2018 9:15 am

### Re: Work & Energy review video

The best way to think about it, for me at least, is with units.

Power: W = J/s
Work: J = N*m
Force: N = kg*m/s^2
Velocity: m/s

So, to do some analysis...

One way of thinking about power:

Power is work/time, or Joules per second.

The other way, though, is via force and velocity

Power is force * velocity, or newtons times meters per second.

J/s = (N*m)/s = N*(m/s) = force * velocity

I hope this helps!
931287m
Posts: 93
Joined: Mon Mar 11, 2019 1:43 pm

### Re: Work & Energy review video

Hi,

Thank you for the response. I should have been more clear with my question, but I understand how power can equal F*v based on the units, I just don't understand what force value they used in this specific question to get the correct answer.

The information I provided (m=7000 kg, v=5m/s) was the only information given in the original question. How do we use P=F*v to solve for power here?

Thank you
NS_Tutor_Mathias
Posts: 230
Joined: Sat Mar 30, 2019 8:39 pm

### Re: Work & Energy review video

This recent thread is highly similar: viewtopic.php?f=5&t=2293&sid=3e229ef6fa ... a009d5d2bb

Let me know if that answers your question - it seems to be exactly what you're asking about, dimensional analysis of the F*v=P identity.
931287m
Posts: 93
Joined: Mon Mar 11, 2019 1:43 pm

### Re: Work & Energy review video

Hello,

I am fine with the dimensional analysis while obtaining P=F*v

My question is how they got a power of 40,000 W using only m=7000kg & v=5m/s. How do you solve for power when only given a mass & velocity?

P=F*v

P=F*(5m/s)

What value was used for F to get an answer of 40,000 W?

Thank you
NS_Tutor_Mathias
Posts: 230
Joined: Sat Mar 30, 2019 8:39 pm

### Re: Work & Energy review video

Oh wow, I see what you're talking about. I'll make sure this gets looked at. The correct power output for the crane is 350kW, or 3.5*10^6 W.

Code: Select all

``````F = mg = 7000 kg * 10 m/s = 70 000 N
v = 5 m/s
so P = 5 * 70 000 = 350 000 W or 350 kW.
``````
This is a pretty important oversight in the problem, thank you for bringing it up!

As a fun comparison, a riding lawnmower provides about 23 kW (so half of our erroneous crane). A 1970 Volkswagen Beetle in all its air-cooled glory puts out about 45 kW. Somehow I don't see a Beetle pulling 7 metric tons, or about two 2018 Chevy Tahoes, off the ground, do you?
931287m
Posts: 93
Joined: Mon Mar 11, 2019 1:43 pm

### Re: Work & Energy review video

Ahh thank you! I assumed there must have been an error with the question. It had me puzzled for awhile haha

Thanks so much!