Find initial velocity using energy

[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.

[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.

[physics333]

Would anyone else like to take a guess?

[NateTG]

Well, this isn't an energy method but
μk=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)

[ShawnD]

Originally posted by 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?

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

[Integral]

Originally posted by 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?


You lose points for and incorrect number of significant digits.

Correct answer 62J

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.

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.

[NateTG]

Originally posted by Integral
You lose points for and incorrect number of significant digits.

Correct answer 62J


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

[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.

[Integral]

Obviously I am not an engineer.

You know I looked right past that single digit in μ

[chroot]

Originally posted by NateTG
If you're going to nitpick, do so correctly. The coefficient of kinetic friction has only one significant digit.
Ouch!

- Warren