2013SPp2q9 – Pressure and Moment

Solutions:

(a) (i) A bigger force than F can be obtained due to the lever system and hydraulic system.

Lever system: Applying principle of moments, the anticlockwise moment by the F is equal to the clockwise moment by the force on piston A (note that the handle is pushing the piston down, but the piston A is pushing on the handle upwards – action = reaction). As the perpendicular distance from F to the pivot is greater than the perpendicular distance of the force by piston to the pivot, the force on the piston A is greater than F at handle.

Hydraulic system: As the pressure transmitted in the liquid is the same, pressure at piston A = pressure at piston B. As P = F/A and area of piston A is smaller than area of piston B, a larger force is obtained in piston B. Hence these two systems allow the force on piston B to be greater than F at the handle.

(a) (ii) Both liquid and gas molecules are in a continuously random motion. But in liquid, the molecules are closely packed together and able to slide around one another. There is very little empty space between the molecules hence liquid is not compressible. Gas molecules are far apart from one another, hence gas can be easily compressed.

(b) P = F/A = 12000 / 0.060 = 200 000 Pa

(c) Velocity is the vector quantity while speed is a scalar. As the car goes round the bend, the direction of the car changes. Hence velocity is changing even though speed is constant.

Likewise, as the velocity is changing, the car is considered to have an acceleration )and the acceleration is towards the centre of the circle of the bend)

Common Electrical Symbols

Which is better to cool the food? Copper or ice at 0oC - 2008SPP2Q6(b)

Other examples in our daily lives:

In some supermarket, the seafood are placed outside of air-conditioned place. The seafood is kept cold by putting crushed ice covering the seafood to keep the them cold and fresh.

Refer to this Sci Physics question N2008P2Q6(b)

Solutions: 

For the solid that does not melt, when thermal energy is absorbed from the surrounding  food, its temperature starts to rise. So it is not so effective at keeping the food cool.

For ice-pack, when thermal energy is absorbed from the surrounding food, it starts to melt. During melting process, a much larger quantity of thermal energy is absorbed from the food to melt per unit mass of ice, the temperature remains constant at 1oC, and the melting process is long. Hence ice-pack is more effective at keeping the food cool.