- #1
PhizKid
- 477
- 1
Homework Statement
A string pulls a 50kg block 10m up and the acceleration of the block is g/10. What is the kinetic energy of the block?
Homework Equations
W = Fd
W = 1/2*mv^2
The Attempt at a Solution
1st attempt (each force work separately)
The work done on the block from the string:
W_s = F_s * 10m
F_s = ma
F_s = 50kg * g/10 m/s^2
W_s = (50kg * g/10 m/s^2) * 10 m = 490 J
The work done on the block by gravity:
W_g = F_g * -10m
F_g = mg
F_g = 50kg * 9.8 m/s^2
W_s = (50kg * 9.8 m/s^2) * -10m = -4,900 J
If I add the net Work done up (-4410 J), the answer is incorrect.
2nd attempt (net forces first)
W_s = F_s * 10m
F_s - mg = ma
F_s = mg + ma
F_s = (10*9.8) + (10*[g/10]) = 107.8 N
W_s = 107.8 * 10m = 1078 J
-mg + F_s = ma
mg = F_s - ma
mg = 107.8 N - (10 * [g/10])
mg = 98
g = 98/10 = 9.8
W_s = 9.8 * 10 = 98 J
1078 + 98 = 1176 J
Both of the answers should be the same but they're not, and both answers are incorrect.
The first method was not using the net force and using each force separately to find the work done then add them together, and the second was finding the net force in the up direction and then the down direction and then adding them together/