determining speed of a motor (1 Viewer)

[Itike]

I have to determine the speed of a motor using PE=mgh and KE= ½mv^2. The motor is being used to lift a 20g mass 25cm into the air. Im not really sure how to do this, but this is how I did it:

PE = 0.02 x 9.8 x 0.25
= 0.049 J

KE = PE
Therefore KE = 0.049 J

0.049 = ½ x 0.02 x v^2
v^2 = 0.049/(0.02 x ½ )
v^2 = 4.9

Therefore v = 2.2 ms-1

I don’t think this right, please help!

 

[zingerburger]

Looks pretty right to me.

 

[Forbidden.]

I thought E_K + E_P = 0 meaning resultant sum of the energies were equal to zero.

(E_K is Kinetic Energy and E_P is Potential Energy)

 

[Kirjava]

I can't see any problems in your working Itike, though I havn't bothered to prove it.

F3nris; potential and kinetic energy are often related, but by no means must their sum total at zero. I'm not entirely sure if you meant that that statement should apply generally, or just as it pertains to Itike's problem- so i'll say no more .

 

[Itike]

Thanks, i guess it must be right then.

 

[alcalder]

I have to determine the speed of a motor using PE=mgh and KE= ½mv^2. The motor is being used to lift a 20g mass 25cm into the air. Im not really sure how to do this, but this is how I did it:

This is a strange question. What is the actual question?

 

[xiao1985]

it is strange... from your working out, it seemed that the motor doesn't have to be there....

 

[Kirjava]

Mm, at second glance I see a problem that I overlooked originally: There is no minimum speed required for the motor to lift the weight! When I looked over it the first time it crossed me as something similar to a projectile motion question, and for that reason his working seemed altogether reasonable.

Something appears to be missing in the question.

 

[thehootman]

Lol thats the speed of the weight ascending. To calculate the RPM you need the diameter of the pulley or whatever.