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Questions on Pressure and Moments

Q1. Two syringes are connected together as shown in the diagram below.

A force of 20 N is applied to the piston in syringe A.

(a) Calculate the pressure that the piston in syringe A exerts on the oil.

P = F/A

P = (20 N)/ (0.5 cm2)

a mark for recalling the equation

P = 40 N cm-2

(3 marks)

(b) Calculate the force needed to just prevent the piston in syringe B from moving out.

P = F/A

So F = PA

a mark for rearranging the equation

F = 40 x 5 = 200 N

(3 marks)

(b) The diagram below shows the brake pedal used to operate the brakes in a car.

The foot applies a force of 50 N.

(i) Calculate the force applied to the piston P.

Moment supplied by the foot = 50 N x 20 cm

Moment = 1000 N cm

1000 N cm = Force applied to piston x 5 cm

Force applied to piston = 1000/5

Force applied to piston = 200 N

(4 marks)

(ii) The brake fluid pushes another piston, Q, which is attached to the car's brakes. Piston Q has an area which is eight times larger than piston P. Calculate the force on the car's brakes.

The pressure from piston P would be 200/A

This pressure would be applied to piston Q but the area is 8 times larger:

200/A = F/8A

F = 200 x 8 = 1600 N

 

(2 marks)

Maximum 12 marks