further mechanics PPQS

Question 1

A teacher is demonstrating the principle of conservation of momentum using a flat glass surface and air pucks. Lightweight tubing supplies compressed air to the pucks which is forced out from the bottom of the pucks. This means that the pucks move with very little friction across the glass surface.
Explain , using ideas about molecular movement, how the puck is able to hover a small distance above the glass surface [4]

Answer 1

• air molecules make collisions with the puck and transfer momentum to the puck.
• according to N2L, the change of momentum creates a force on the puck.
• the rate of change of momentum by air molecules colliding with the bottom of the puck is greater than that due to the collisions on the top of the puck.
• the net (upward) force balances the weight of the puck OR the greater air pressure below the puck allows the puck to be supported.

Question 2

A teacher is demonstrating the principle of conservation of momentum using a flat glass surface and air pucks. Lightweight tubing supplies compressed air to the pucks which is forced out from the bottom of the pucks. This means that the pucks move with very little friction across the glass surface.
Applying N2L and 3L of motion to the collision between to pucks leads to the conclusion that momentum is conserved. Justify this statement. [6]

Answer 2

Applying N3L to one puck. A exerts a force on the other puck and vice versa.
- forces equal in magnitude and opposite in direction.
- forces act for the same time
- F △t(A) = - F △t(B)
- N2L: F △t = △p. Since F is a resultant force on each puck.
- total change in momentum= 0 so momentum is conserved.

Question 3

A device called a clutch can be used to connect a motor to a load.
Several magnets are embedded in the plastic disc and it is rotated by the motor.
The motor rotates at 500 revolutions per minute.
Calculate the angular speed (w) of the motor. [2]

Answer 3

w = (500 x 2pi) / 60

52 rad per second

Question 4

Motors usually have a rotating component which can do work W. A motor lifts a load in a time t. The axle of the motor has a radius r and exerts a force F.
The power produced by a motor can be calculated by using the following word equation.
Power - moment of the force exerted by the rotating axle x angular velocity
Derive this equation starting with
P = W/t
[4]

Answer 4

P = force x distance /time
P = force x velocity
P = force x radius x angular speed
Fxr is the moment of F

Question 5

Astronauts in the ISS are often described as being ‘weightless’. Discuss whether the astronauts are ‘weightless’ when they are orbiting the Earth in the ISS.
[4]

Answer 5

Isnt weightless
Because gravitational force keeps them in circular motion OR
Weight provides a centripetal force.
Notion of ‘weight’ is a reaction force from ground
No reaction forces so it ‘feels’ weightless