# Power\mass ratio

1. Jul 30, 2004

### almohandes

how do i calculate power\mass ratio of something.

2. Jul 30, 2004

### HallsofIvy

Divide power by mass?

Other than that, your question is not very clear. Are you talking about a particular problem in which power is some function of mass?

3. Jul 30, 2004

### almohandes

ok,

the problemo:
The hind legs of a locust are extremely powerful!. it takes off with a speed of 3.0ms-1, the acceleration takes place in 25 milliseconds (25ms). The locusts mass is 2.5g.

the Question states:
What is the power/mass ratio of the locust (in J kg-1)?

4. Jul 30, 2004

to solve this one you need to do a simple calculation.
Power = Force x Velocity
acceleration = v2 - v1 / t
= (0.30m/s - 0m/s) / 0.0025s
= 120m/s^2
F = ma
= (0.0025kg)(120m/s^2)
= 0.3N
P = FV
= (0.3N)(0.30m/s)
= 0.09J/s or W
now the power is 0.09W and mass is 0.0025kg, the ratio is 36:1. Thats my calculation, if anybody finds anything wrong, please correct me, I hope Ive been a help.

5. Jul 31, 2004

### almohandes

yes, thank you.

6. Aug 1, 2004

### Staff: Mentor

The basic idea is correct. But the speed is 3 m/s, not 0.3; and the time is 0.025s, not 0.0025.
Right.
Two problems: (1) as stated above, the speed is 3 m/s, (2) P = FV, where V = average speed, which is 3/2 m/s, not 3. So power = 0.45 J/s.

7. Aug 4, 2004