Find the energy dissipated by frictional forces when the car moves down from X to Y.

Answer: Option B

Using conservation of energy (COE), the total energy of the car at X = total energy at Y.

At X, the car only has gravitational potential energy. If there is no friction, the magnitude of the kinetic energy must be equal to the GPE at X.

But since there is frictional force, there will be some energy wasted as work done against friction, usually in the form of internal (thermal) energy and sound energy.

Hence, GPE at X = KE at Y + WD against friction

Refer to the video explanation

Liquid Pressure - Determine the pressure of the enclosed air in the manometer (pure physics)

Answer: Option A

Refer to the video explanation

Moment double pivot - What are the normal contact forces at P and Q?

Answer: Option A

Firstly, you need to know that at both pivot P and Q, there are normal contact forces acting upwards on the plank.

2 conditions for the plank to be in equilibrium:

1) Sum of clockwise moment = Sum of anticlockwise moment (POM)

2) Net force = 0N (i.e. total upward forces = total downward forces)

To find the value of the normal force, you can choose to take pivot about either P or Q. If you take moment about P, the normal force at P will not be in the Principle of Moments equation Ma = Mc as the line of action of the normal contact forces at P passes through the pivot P, there is no perperdicular distance, hence no moment created. We can determine the value of the normal contact forces at Q.

Once normal force at Q is found, you can use the concept net force = 0N to find the normal force at P straight away, which is faster.