# A block coming down the incline

• rudransh verma
In summary, the conversation discusses finding the work done by an applied force on a block. The equation for external work is used, but it is not consistent with the equation for work done by a force. The acceleration of the block is calculated assuming only the applied force and gravity are present, and the result differs from the given value. The concept of friction is brought up, but the important thing is to not make unjustified assumptions. The simplest and most direct method should be used to solve the problem. Overall, the work done by the given applied force is 100J.
rudransh verma
Gold Member
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.

rudransh verma said:
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?)

Steve4Physics
Doc Al said:
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.

rudransh verma said:
That will be the total energy gained or lost by the block.
Nope.

Doc Al said:
Nope.
But I have used the eqn. ##W_{ext}=\Delta E##. It is asking for external work.

rudransh verma said:
But I have used the eqn. ##W_{ext}=\Delta E##. It is asking for external work.
rudransh verma said:
find the work by the applied force in first sec.

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

rudransh verma said:
Is applied force not an external force?
But is it the only external force? If it was, what would the acceleration be?

Doc Al said:
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##

rudransh verma said:
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.

Doc Al said:
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?

rudransh verma said:
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.

Doc Al said:
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}##.

rudransh verma said:
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.

Doc Al said:
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

rudransh verma said:
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.
rudransh verma said:
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.

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

## What is the definition of "A block coming down the incline"?

A block coming down the incline refers to the motion of a solid object, such as a block or box, as it moves down a sloped surface or ramp due to the force of gravity.

## What factors affect the speed of a block coming down the incline?

The speed of a block coming down the incline is affected by the angle of the incline, the mass of the block, and the force of gravity.

## How does the angle of the incline affect the motion of a block coming down?

The steeper the incline, the faster the block will accelerate and the quicker it will reach the bottom. This is because the component of gravity acting along the incline is greater.

## What is the formula for calculating the acceleration of a block coming down the incline?

The formula for calculating the acceleration of a block coming down the incline is a = g*sin(θ), where a is the acceleration, g is the acceleration due to gravity (9.8 m/s^2), and θ is the angle of the incline.

## How does friction affect the motion of a block coming down the incline?

Friction acts in the opposite direction of motion and can slow down the block as it moves down the incline. The amount of friction depends on the surface of the incline and the material of the block.

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