# How long does it take it increase speed?

1. Feb 5, 2008

### RanHam335

1. The problem statement, all variables and given/known data
A freight train has a mass of 1.2e+7 kg. If the locomotive can exert a constant pull of 8.4e+5 N, how long does it take to increase the speed of the train from rest to 82 km/h?

2. Relevant equations
(mass*velocity)/time=force

3. The attempt at a solution
(1.2e+7 * 82)/h = 8.4e+5

i solved for h
h = (8.4e+5)/(9.84e+8)
h = 8.54e-4

then i had to convert from hours to min.
8.54e-4*60

so i got
5.1e-2 min

this is my first time posting on this forum so i hope i followed the right format.

2. Feb 5, 2008

### rock.freak667

82km/h = $\frac{82*1000m}{3600s}$

3. Feb 5, 2008

### RanHam335

can you explain the above?

4. Feb 5, 2008

### rock.freak667

I just converted the speed from km/h to m/s which is usually what you should do.

82km/h

in 1h train travels 82km
=> in 1h train travels 82*1000m
in 3600s train travels 82000m
in 1s train travels 82000/3600 m

therefore the velocity in m/s is 820/36

5. Feb 5, 2008

### RanHam335

that doesnt answer my question, i think im messing up on the formula. I have one submission left so i dont want to risk it

6. Feb 5, 2008

### rock.freak667

$$F=\frac{mv}{t} \Rightarrow t=\frac{mv}{F}$$

$$t=\frac{(1.2*10^7 kg)*((\frac{82000}{3600} ms^-1)}{8.4*10^5 N}$$

Can you calculate now?

But your answer was $5.1*10^{-2}$minutes. Doesn't that seem like a small amount of time to go from 0 to 82km/h ?

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