If you wrap string around a reel and pull gently from the underside of the reel
ID: 1655539 • Letter: I
Question
If you wrap string around a reel and pull gently from the underside of the reel in a horizontal direction, the reel will start to roll. Varying the angle to the horizontal you will find that at some angles the reel will roll towards you and at other angles it will roll away from Investigate some of the parameters that might affect the reels behaviour, such as the two diameters of the reel, the angle of the string to the horizontal, the effect of friction, the mass of the reel rotational motion and the forces that cause it (torques) and attempt the following questions: a) What are the forces that cause the reel to rotate at all? b) What factors might affect the size of these forces? c) Can you predict how these factors might affect the motion of the reel? d) What simple questions come to mind when you see this phenomena e.g. "Does the phenomena occur with all reels or just this special one that we have started off with?"- it is as important to be able to come up with, investigate and answer such qualitative questions as well as any quantitative questions.Explanation / Answer
a)There will be two forces acting, The gravity and the Tension in the string. The gravity pulls it downwards and the tension in the string pulls it upwards causing it to rotate.
b)F(gravity) = mg
F(Tension) = mg cos(theta)
So the angle, the mass will affect the size of forces on the reel.
c)Torque = Force x perpendicular distance from the zxis of rotation
Also, Torque = I alpha
alpha = F x r/I
If the force reduces, the torques acting will also reduce, which will cause angular acceleration to reduce and hence the angular velocity will also reduce.
d)Why did not it fell down? Its just supported with a very light string.
This came to mind.
Yes, force balances and torque due to the forces cause spool to rotate.