Posted by Lily on Sunday, September 30, 2007 at 5:34pm.
I am not going to do this for you. I will be happy to critique your thinking.
This is what i reasoned:
a) if block is moving up the ramp, it means that tension is greater than the sin of mg. if T was half of mg, that means the speed decreases up the ramp because the T force is less.
b) it would increase. i don't understand what the 30 deg has to do with anything, i suppose if i had actual values it would make a difference in the force pulling the mass down... but T is still half of the force pulling the weight down.
c) i suppose it stays the same, because the normal force is the same as sin of theta, which does not effect the horizontal values to the ramp.
im sorry that's a lot to read, does it sound right?
a) Look at the component of weight down the plane: mg*sinTheta= mg/2
if tension is that, the block is at constant velocity , as net force up plane is T-mg/2.
b) if net force is T-mgsinTheta, and theta is greater than 30, the net force is negative, or down the ramp. So it speeds up down the ramp.
c)The question makes no sense. If it is going up the ramp, T is > mgsinTheta, or T is greater than the down the ramp component of weight. Normal force is not in the picture, as it is frictionless. At 45 deg, Normal= down plane force. At greater than 45, down the slope is > normal force, and below 45, normal is greater than down the slope. The question does not make sense.
Your instructor could have done a better job of measuring reasoning on this.
thank you SO much, i got them all right in the first try! the answer to question c is "need more information"