OCR G484 Jan 2011?

For 1bii of the Spring 2011 Newtonian World paper we are asked to calculate the momentum of descending air, force provided by the blades, and the mass of a helicopter.

In the previous part I calculated the mass of air to be 6126kg, which was correct. Given that the velocity is 12ms-1, I made the momentum to be 7.4x10^4, again correct.

But on the next part when we're asked to calculated the force provided by the helicopter blades, the mark scheme has done: mv/r.

Surely it should be mv^2/r?

That's the equation provided by the data booklet, and it makes sense, so why are they only using mv not mv^2? Or is it an error?

Thanks again!

Posted from TSR Mobile

Reply 1

joostan

13

Original post by Scitty

For 1bii of the Spring 2011 Newtonian World paper we are asked to calculate the momentum of descending air, force provided by the blades, and the mass of a helicopter.

In the previous part I calculated the mass of air to be 6126kg, which was correct. Given that the velocity is 12ms-1, I made the momentum to be 7.4x10^4, again correct.

But on the next part when we're asked to calculated the force provided by the helicopter blades, the mark scheme has done: mv/r.

Surely it should be mv^2/r?

That's the equation provided by the data booklet, and it makes sense, so why are they only using mv not mv^2? Or is it an error?

Thanks again!

Posted from TSR Mobile

They've actually used

F=\dfrac{mv}{t}

.

F=\dfrac{mv^2}{r}

is the formula for centripetal force - the force towards the centre of a circle required to maintain the circular motion.

The fact that the helicopter blades are rotating is irrelevant to the question.