Physics energy

[chiddler]

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Gravity is the only force acting on a 2 kg masss. The potential energy of the mass increases by 144 J. What is the change in velocity of the mass?
A. Increase in 12 m/s
B. Decrease in 12 m/s.
C. Change in velocity depends upon the initial amount of potential energy.
D. The change in velocity depends upon the initial velocity.

Answer is D.

Why can't it be B? If gravity is the only force, and it is increased, then the energy must come from velocity. The answer says that you must know initial velocity to find out how much speed changes.

Why must I know initial speed?

 

[Pisiform]

I agree, magnitude will be 12 m/s increasing, it doesn't male sense. May be they are emphasizing direction (vector) which we don't know

 

[milski]

144 J increase in PE means that the KE energy decreased with 144. KE is proportional to the square of the velocity but you cannot tell by how much it has changed if its square has changed by 144.

Consider these two cases:

a) vi = 12 m/s, vf = 0 m/s, KEi = 144 J, KEf = 0J
b) vi = 100 m/s, KEi= 10000 J, KEf = 9856 J, vf = sqrt(9856) = 99.28 m/s, deltaV = .72 m/s

In general, you need to solve (vi)^2 - 144 = (vi+change)^2 and you cannot do that if you don't know vi.

 

[Pisiform]

144 J increase in PE means that the KE energy decreased with 144. KE is proportional to the square of the velocity but you cannot tell by how much it has changed if its square has changed by 144.

Consider these two cases:

a) vi = 12 m/s, vf = 0 m/s, KEi = 144 J, KEf = 0J
b) vi = 100 m/s, KEi= 10000 J, KEf = 9856 J, vf = sqrt(9856) = 99.28 m/s, deltaV = .72 m/s

In general, you need to solve (vi)^2 - 144 = (vi+change)^2 and you cannot do that if you don't know vi.

true ,,,, good question though

 

[chiddler]

thanks very much.

 

[orangeblossom]

sorry to bump an old thread, but i dont understand this question either. and the explanation doesnt quite make sense to me

since mass (of 2 kg) and the "1/2" in KE=1/2mv^2 stays the same, shouldnt the (change in velocity)^2 be the same regardless of initial velocity, since the change in KE must be 144 J?

 

[Testosterone]

sorry to bump an old thread, but i dont understand this question either. and the explanation doesnt quite make sense to me

since mass (of 2 kg) and the "1/2" in KE=1/2mv^2 stays the same, shouldnt the (change in velocity)^2 be the same regardless of initial velocity, since the change in KE must be 144 J?

Think about it mathematically. We want to find Vf-Vi

KE=1/2mv^2

Assuming mass stays the same
ΔKE=1/2mΔ(v^2)

Plugging in values, we get
144= (1/2)(2)(Vf^2-Vi^2)

144= (Vf^2)-(Vi^2)

Breaking this down, you can see that Vf-Vi cannot be solved without knowing the value of Vi.