# Problem with acceleration

#### Ugnius

Moved from technical forums, so no template
Hi , im trying to practice working with new exercises and i've met some problems.
Lifting object with mass of 2kg to height of 10m we do 240J mechanical work.
What is acceleration of lifting?

My calculations:
A=Fs=Fh
F=A/h = 240/10 = 24N

a= F/m = 24/2 = 12 m/s^2 , somehow it's not the right answer

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#### garylau

There is a downward mg which compensates the mechanical work done?

your answer is correct if there is no mg force

in fact,part of the mechanical work done is to compensate the work done by the mg force

#### Ugnius

No , that's all what's written in question

#### Dale

Mentor
The word "lifting" implies gravity.

#### garylau

Lifting object with mass of 2kg to height of 10m we do 240J mechanical work.
What is acceleration of lifting?

the final answer is 2 ?

2 m/s^2 yes

#### garylau

in fact

your answer is correct if there is no gravity.

#### garylau

if there is gravity
then you will feel a "resistance" from the gravity

part of your 240J will try to compensate this "resistance" so that it is impossible that the force that you provide will be the same of the case of no gravity force

#### Ugnius

Lifting object with mass of 2kg to height of 10m we do 240J mechanical work.
What is acceleration of lifting?

the final answer is 2 ?
How did you get 2? can you write the equation?
Was it a=F/m - g?

#### haruspex

Science Advisor
Homework Helper
Gold Member
2018 Award
I'm unsure how to interpret the question.

If this is in a terrestrial setting, we can calculate that part of the work that went into overcoming gravity, i.e. into achieving the PE gain. That is less than 240J. Where did the rest go? Since it asks about acceleration, maybe we are supposed to assume there is residual KE, i.e. it is still moving upwards. If so, we can compute the gained velocity, but without knowing how long this took there is no way to find the acceleration.

Alternatively, this is extraterrestrial, and all they want you to find is the local gravitational acceleration, but then that would be 12m/s2, not 2.

#### NascentOxygen

Mentor
If so, we can compute the gained velocity, but without knowing how long this took there is no way to find the acceleration.
The object was lifted 10m.

#### TomHart

Since it asks about acceleration, maybe we are supposed to assume there is residual KE, i.e. it is still moving upwards.
The object was lifted 10m.
Yes, although it isn't stated, I think maybe the solution should assume constant acceleration.

#### haruspex

Science Advisor
Homework Helper
Gold Member
2018 Award
Yes, although it isn't stated, I think maybe the solution should assume constant acceleration.
That does seem to be the most likely error/omission in the question.

#### Dale

Mentor
How did you get 2? can you write the equation?
Was it a=F/m - g?
Yes. That is it.

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