A block coming down the incline

[rudransh verma]

Homework Statement

A block of mass 2kg is pushed down an inclined plane of inclination 37 degree with a force of 20N acting parallel to the incline. It is found that the block moves on the incline with a acceleration of 10 m/s2. If the block started from rest, find the work by the applied force in first sec.

Relevant Equations

$v=u+at$
$v^2=u^2+2as$
$W_{ext}=\Delta E$

$W_{ext}=mgh+KE_f+0=-20(5\sin 37)+(1/2)2(10^2)$
$W_{ext}=-60.18+100=39.81J$

But it’s not consistent with $W=F.d=20*5=100J$
I can’t figure it out.

 

[Doc Al]

But it’s not consistent with $W=F.d=20*5=100J$
I can’t figure it out.

You are asked to find the work done by the applied force. (There are other forces that contribute to the net work done, but who cares?)

 

[rudransh verma]

You are asked to find the work done by the applied force.

That will be the total energy gained or lost by the block. During going down if it has gained some energy then it has also lost.

 

[Doc Al]

That will be the total energy gained or lost by the block.

Nope.

 

[rudransh verma]

Nope.

But I have used the eqn. $W_{ext}=\Delta E$. It is asking for external work.

 

[Doc Al]

But I have used the eqn. $W_{ext}=\Delta E$. It is asking for external work.

From your problem statement:

find the work by the applied force in first sec.

 

[rudransh verma]

From your problem statement:

Is applied force not an external force?
Both ways should produce the same result W=100J

 

[Doc Al]

Is applied force not an external force?

But is it the only external force? If it was, what would the acceleration be?

 

[rudransh verma]

But is it the only external force

Yes, there is no non conservative force(friction) but there is a conservative force(gravity). I simply used the eqn $W_{nc}+W_c+W_{ext}=\Delta KE$

 

[Doc Al]

Yes, there is no non conservative force(friction) but there is a conservative force(gravity). I simply used the eqn $W_{nc}+W_c+W_{ext}=\Delta KE$

I know what you did. You are assuming things without justification.

Now do what I ask: Calculate the acceleration of the block assuming that the only forces are the applied force and gravity.

 

[rudransh verma]

Calculate the acceleration of the block assuming that the only forces are the applied force and gravity.

a=16.01 m/s2 but it’s given 10. Is there friction?

 

[Doc Al]

a=16.01 m/s2 but it’s given 10. Is there friction?

Perhaps, but who cares? The important thing is to not make unjustified assumptions.

You need to learn to solve these sorts of problems in the simplest and most direct manner possible.

 

[rudransh verma]

Perhaps, but who cares? The important thing is to not make unjustified assumptions.

You need to learn to solve these sorts of problems in the simplest and most direct manner possible.

I thought friction was in the third part of the question(there are three parts). Maybe the author just thought that why not pour some friction in it at the end
So we can’t use that eqn. We need to apply the definition of work. And then use this formula to calculate $W_{nc}$.

 

[Doc Al]

We need to apply the definition of work.

That's all you need. You are given the force. You are given the acceleration.

Don't overcomplicate such an easy problem.

 

[rudransh verma]

Don't overcomplicate such an easy problem.

I try to.
So what’s going on is that the gravity is trying to increase the KE of the body while the friction is taking out. Net work is done by the applied force of 100J

 

[Doc Al]

So what’s going on is that the gravity is trying to increase the KE of the body while the friction is taking out.

Most likely. But you don't need to know what other forces act to answer the question.

Net work is done by the applied force of 100J

Don't say "net work". Just say: The work done by the given applied force is 100J.

 

[rudransh verma]

Don't say "net work". Just say: The work done by the given applied force is 100J.