# Work-energy theorem problem

1. Oct 25, 2008

### kmikias

I am just confuse in this question may be i forgot about work and energy .I just need little help not the answer ,here is the question

1.A block of mass 2.2 kg, which has an initial
velocity of 3.9 m/s at time t = 0, slides on a
horizontal surface.
Calculate the work that must be done on
the block to bring it to rest. Answer in units j.

solution .

Is it work = KE WHICH IS 1/2MV^2

I try to use W = F.d
but i don.t know the acceleration

2. Oct 25, 2008

### Staff: Mentor

Re: work

Good. Use the work-energy theorem: W = ΔKE.

(What's the change in KE?)

3. Oct 25, 2008

### kmikias

Re: work

oh thank you , now i got it, i didn't put negative sign

4. Oct 25, 2008

### kmikias

Re: work

how about If a constant friction force of 11 Newtons is
exerted on the block by the surface, what is
the acceleration? Answer in units of m/s.

solution
F - friction force = mass * acceleration

is that right

5. Oct 26, 2008

### Staff: Mentor

Re: work

Is friction the only horizontal force acting on the block? If so, then all you need is F = m*a, where F is the friction force.

Also: acceleration has units of m/s^2, not m/s (m/s is the units for speed)

6. Oct 26, 2008

### kmikias

Re: work

thank you doc i just forgot to write the square