Work, Energy and power questions

[pantiome]

A 500w motor is used to pull a 12kg mass along a level surface from rest. If the motor is on for 10 seconds, what is the maximum speed that the mass can reach?
p=fs which i rearranged to s=p/f so to work out the force i did 12*9.81=117.72, so I put into the equation s=500/117.72=4.25
Have i done this question right?

And also another question on my homework which I'm not too sure about is:
A pendulum bob of mass 200g is raised 10cm above its resting position and allowed to swing freely. Calculate (a) its loss of potential energy in returning to its resting position (b) its maximum speed as it returns to its resting position (c) its speed when it is 5cm above resting position.
(a)
deltaEp=mg*h or W=fs
0.2x9.81=1.962
deltaEp=1.962*0.1=0.1962j

(b)
Ek=1/2mass*speed^2
speed^2=Ek / 1/2mass
speed^2=0.1962/0.1
speed^2=1.962
speed=1.4ms^-1

(c)
deltaEp=1.962*0.05
deltaEp=0.0981J
speed^2=0.0981/0.025
speed=1.98ms^-1

thanks in advance :)

 

[Delphi51]

A 500w motor is used to pull a 12kg mass along a level surface from rest. If the motor is on for 10 seconds, what is the maximum speed that the mass can reach?
p=fs which i rearranged to s=p/f so to work out the force i did 12*9.81=117.72, so I put into the equation s=500/117.72=4.25
Have i done this question right?

No. The F in P = F*v is the force pushing the object, not the force of gravity. That formula isn't all that useful in this question. Maybe begin with Power = work/time. No friction?

 

[kerimek]

Your calculations for both questions seem to be correct. However, it is always important to double check your units to make sure they are consistent. In the first question, the units for force should be Newtons (N), not kg. So the correct calculation for force would be 12*9.81 = 117.72 N. And in the second question, the units for potential energy should be Joules (J), not Newtons (N). So the correct calculation for potential energy would be 0.2*9.81*0.1 = 0.1962 J. Other than that, your approach and calculations are correct. Keep up the good work!