Mechanics M1

Albert and Bella are both standing in a lift. The mass of the lift is 250kg. As the lift moves upward with constant acceleration, the floor of the lift exerts forces of magnitude 678N and 452N respectively on Albert and Bella. The tension in the cable which is pulling the lift upwards is 3955N.
a. Find the acceleration of the lift
b. Find the mass of Albert.

I don’t get how to solve part ‘a’. Am i supposed to assume that their weights are 678N and 452N to find the acceleration??

Reply 1

old_engineer

11

Original post by Mme_Bonii

I don’t get how to solve part ‘a’. Am i supposed to assume that their weights are 678N and 452N to find the acceleration??

No. You are supposed to observe that the upwards force (tension) is accelerating Albert and Bella and the lift. You are told how much of this force is being applied to Albert and Bella. The remainder must be accelerating the lift itself. You know the mass of the lift, so you have all the information you need to find the net upwards force acting on the lift itself, and hence its acceleration.

Reply 2

Mme_Bonii

OP

11

Original post by old_engineer

No. You are supposed to observe that the upwards force (tension) is accelerating Albert and Bella and the lift. You are told how much of this force is being applied to Albert and Bella. The remainder must be accelerating the lift itself. You know the mass of the lift, so you have all the information you need to find the net upwards force acting on the lift itself, and hence its acceleration.

Thank you so much! I get it now.

Would you mind helping me in part ‘a’ of this question as well? We don’t have the upward force, how am i supposed to solve it if that was the case?

A small stone of mass 400g is projected vertically upwards from the bottom of a pond full of water with speed 10m/s. As the stone moves through the water it experiences a constant resistance of magnitude 3N. Assuming that the stone does not reach the surface of the pond, find:
a) the greatest height above the bottom of the pond that the stone reaches.
b) the speed of the stone as it hits the bottom of the pond in its return.
c) the total time taken for the stone to return to its original position on the bottom of the pond.

Reply 3

old_engineer

11

Original post by Mme_Bonii

Thank you so much! I get it now.

Would you mind helping me in part ‘a’ of this question as well? We don’t have the upward force, how am i supposed to solve it if that was the case?

A small stone of mass 400g is projected vertically upwards from the bottom of a pond full of water with speed 10m/s. As the stone moves through the water it experiences a constant resistance of magnitude 3N. Assuming that the stone does not reach the surface of the pond, find:
a) the greatest height above the bottom of the pond that the stone reaches.
b) the speed of the stone as it hits the bottom of the pond in its return.
c) the total time taken for the stone to return to its original position on the bottom of the pond.

There is no upward force. The net downward force is mg + 3N. You know the mass of the stone so you can find its (negative acceleration). A couple of tips:

* Use SUVAT but be careful with direction (e.g. settle on upwards as positive);

* Note that the 3N resistance will change direction and act upwards when the stone starts to travel downwards.

Reply 4

Mme_Bonii

OP

11

Original post by old_engineer

There is no upward force. The net downward force is mg 3N. You know the mass of the stone so you can find its (negative acceleration). A couple of tips:

* Use SUVAT but be careful with direction (e.g. settle on upwards as positive);

* Note that the 3N resistance will change direction and act upwards when the stone starts to travel downwards.