# Find initial velocity using energy

1. Nov 23, 2003

### physics333

A 1.4 kg box slides 15m across the floor. The coefficient of kinetic friction between the box and the floor is 0.3. I found that friction force in slowing the box was -61.74J. Is this right?

What was the initial velocity of the box?
I worked this problem using Kf = Ki + Wnc and found that Vo =9.4m/s. Is this right? Also I would like to know how to work this problem another way with Energy. Any help with what formula to use or any help at all will be great.

2. Nov 23, 2003

### Ambitwistor

You did it right. I don't know what you mean by working it another way with energy. The way you worked it is the way that involves energy: you used the work-energy theorem.

3. Nov 23, 2003

### physics333

Would anyone else like to take a guess?

4. Nov 24, 2003

### NateTG

Well, this isn't an energy method but
&mu;k=0.3 means that a=0.3g or about 2.94m/s2 so you could use that to get an answer. (9.39 m/s)

5. Nov 24, 2003

### ShawnD

force of friction = (0.3)(1.4)(9.8)
force of friction = 4.116N

energy lost to friction = Fd
energy loss to friction = (4.116)(15)
energy loss to friction = 61.74J

E = (1/2)mv^2
v^2 = 2E/m
v = sqrt(2E/m)
v = sqrt(2(61.74)/(1.4))
v = 9.39m/s

all work shown and it agrees with the previous guy's answer

6. Nov 24, 2003

### Integral

Staff Emeritus
You lose points for and incorrect number of significant digits.

Likewise other responders there is at most 2 good digits in the given information, you cannot give more then that in the answer. A nit picky detail but one you need to be aware of. The common use of calcultors for the most trivial of computations makes this sort of error very common.

7. Nov 24, 2003

### NateTG

Re: Re: Find initial velocity using energy

If you're going to nitpick, do so correctly. The coefficient of kinetic friction has only one significant digit.

8. Nov 24, 2003

### ShawnD

If you are an engineer, you either have the number of digits the question gave or use 3; whichever is greater. If the number starts with a 1, you use at least 4. Angles are to be expressed to at least 2 decimal places.

Last edited: Nov 24, 2003
9. Nov 24, 2003

### Integral

Staff Emeritus
Obviously I am not an engineer.

You know I looked right past that single digit in &mu;

10. Nov 24, 2003

### chroot

Staff Emeritus
Re: Re: Re: Find initial velocity using energy

Ouch!

- Warren