I am studying for a test on monday and i am doing alot of the questions that i didnt do. But some of these im not sure if i did right so can you please help me.

1. A clerk pushes a filing cabinet of mass 22 kg across the floor by exerting a horizontal force of magnitude 98 N. The magnitude of the force of kinectic friction is acting on the cabinet is 87 N. The cabinet starts from rest. Use the law of conservation of energy to determine the speed of the cabinet after it moves 1.2m.

Heres what i did but i don't i used the law of conservation of energy.

W= Ek2 - Ek1
Fd=1/2mv^2
Fapplied - Ffk=1/2mv^2
I am not going to show all the steps but i after manipulating that formula i got 1.1 m/s. The answer is right, but i dont know if that's how your supposed to do it.

2. A 0.20 kg mass is hung from a vertical spring of force constant 55 N/m. When the spring is released from its unstretched equilibrium position, the mass is allowed to fall. Use the law of conservation of energy to determine
a)The speed of the mass after it falls 1.5 cm
b)The distance the mass will fall before reversing direction.

I did (a) easily and got the right answer which is 0.48 m/s. But i cant seem to get the right answer to be which is 0.071 m
Here's what i did.

Fe=Fg
kx=mg
x=mg/k
x=0.0356 m

Did i do something wrong. Can you please help me anyone. I think i will have another question when i get to it.

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Just another quick questions. There is probably going to be a question like this on my test, can anyone help me with the answer.

Which has more kinetic energy a slow moving car or a fast moving truck.

I think its a truck because the formula is Ek=1/2mv^2 but there has to be a catch to it.

daniel_i_l
Gold Member
b) when the mass is just about to reverse it's direction it has acceleration even though it has no speed (like a ball thrown in the air just before it starts to fall down) so the sum of forces on it aren't equal. Instead you should use energy with this problem: the total mechanical energy on the bottom (KE,PSE,PGE) is equal to total mechanical energy everywere else.