Homework Help: Newton's laws conceptual questions

1. Sep 28, 2006

vu10758

1) An elevator with a person in it is being pulled upwards by a cable. Identify all interaction forces in the system. Consider your system to consist othe cable, elevator, the Earth and the person. The Earth pulls down on the elevator with force Mg and the elevator pulls up the Earth with force Mg. Find the other four pairs of forces.

I got the person pushing down on the elevator and the elevator pushing up with a Normal force.

The cable pulls the elevator up with a tension force and the elevator pulls the cable down with the same force.

The earth pulls the person down with mg and the person pulls the earth up.

Are the 3 I have correct? What is the 4th one?

2) A block is initially held stationary on a ramp by friction. Suddenly a force F is applied and is gradually increased from 0 up until the block is accelerating up the incline. During the entire time period, describe the changes in the magnitude and direction of the frictional force extered by the incline on the block.

I know that if the block's mass stays the same, and it acceleration increase going up, F will increase in magnitude. On the contrary, this acceleration will be against gravity, reducing the downward net force. Since friction |f_k| = (mu_k) *|n|, friction will decrease since n decrease with gravity. Is my reasoning correct? How do I know the magnitude and direction changes though?

2. Sep 29, 2006

andrevdh

1) The weight of the cable

2) Let's call the component of the weight of the block along the ramp Wr. This is the force that is pulling the block down the ramp. Initially the friction balances this force. That is it is the same size as this force and the friction is acting upwards on the block. Your reasoning that the friction is getting smaller as the additional force increases is correct. So eventually this force will be sufficient to support the block without any friction at all. That is eventually the friction will be zero and the additional force will balance the component of the weight of the block on its own. What will happen if the additional force increases beyond this point (remember that the component of the weight stays the same)?

3. Sep 29, 2006

vu10758

One the force increases beyond that point, the block will be pushed up the incline. When the acceleration exceeds the magnitude of gravity, we have acceleration up. Since |f_k| = mu_k * N, the f_k will change direction and actually assists F in that direction. My reasoning seems a little bit strange here. Is it correct?

4. Sep 29, 2006

andrevdh

When the additional force increases beyond the magnitude of the downwards force the box wants to start sliding upwards, but ...

5. Sep 29, 2006

andrevdh

What would happen if the ramp was flat (or at a very slight incline) in such a case? Can the friction act in any direction no matter in which direction the block is pulled?

Last edited: Sep 30, 2006
6. Sep 30, 2006

andrevdh

Change made to previous post.

7. Sep 30, 2006

vu10758

Friction wouldn't be able to act in such situation.

8. Oct 1, 2006

andrevdh

Do you mean to say that if the ramp is flat and the pulling force is increased gradually from zero friction would not be able to oppose the pulling force?