Homework Help: Pulling a carpet

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1. Feb 28, 2016

Titan97

1. The problem statement, all variables and given/known data
A long pliable carpet is laid on the floor. One end of the carpet is bent back and then pulled backwards with constant velocity $v$. Find the velocity of centre of mass of the moving part and the minimum force required to pull the moving part.

2. Relevant equations
$P=mv$

3. The attempt at a solution
If the end of the carpet that is pulled backwards, moves a distance $2x$, then the bent part will move a distance $x$ and after some calculations, I found that the midpoint shifted by a distance $\frac{3x}{2}$.

• If each part of the carpet is moving with different velocities, then won't the carpet stretch?
• Also, for finding force, $$F=\frac{dM}{dt}v$$ Which $v$ should I use?
• Finally, some points in the carpet will experience a jerk and suddenly aquire a velocity. So I can't use conservation of energy right?

2. Feb 28, 2016

Simon Bridge

The different parts of the carpet are not moving with different velocities - the centre of mass is a geometric location, not physically part of the carpet. The part of the carpet that corresponds to "the middle" keeps changing.

Newton's Law is for the force acting through the centre of mass.

You are adding energy to the carpet via whatever pulls on the carpet.

3. Feb 28, 2016

drvrm

to calculate the force required you better use change in momentum

4. Feb 29, 2016

Titan97

@Simon Bridge it's given in the solution to "observe that it's not P=mv_cm"

5. Feb 29, 2016

Simon Bridge

6. Feb 29, 2016

Titan97

That's is what I did. What is v?

7. Feb 29, 2016

drvrm

the required force must be equal to rate of change of momentum -
write down the the equation and see whether you can calculate the force and velocity-
take a portion of the carpet moved by the action of force.....
how you can calculate v - by energy considerations or impulse.....