# Tag Archives: Topic Energy Work Done Power

## Energy Question with Work Done against Friction

**Solutions: Option A**

## N2007P1Q9 Pure Physics – Energy needed to pump water up a building

An apartment block receives water from a nearby reservoir. A pump is necessary to lift the water into a storage tank at the top of the building.

The gravitational field strength is 10 N / kg. How much energy does the pump supply to lift each kilogram of water into the tank?

**Solutions: 100J**

Water will always find its own level. Hence with or without the pump, the water level in the pipe in the building will be of the same level as the reservoir. Hence, the pump is only needed to pump water up a height of 10m only (instead of the 50m).

Energy, E = mgh = 1 kg x 10 N/kg x 10m =** 100 J**

## My daughter is learning Physics =)

A good example of Conservation of Energy =)

Evan: Chemical Energy to Electrical energy to Gravitational Potential Energy to Kinetic Energy

Levonne: *yawn* ….. I’m so sleepy…… zzzzzzzzzzzzzzzzzzzzz

## Bullet embedded in wooden block

At a training exercise, a policeman fires a pistol. A bullet of mass 10.0 g leaves the pistol horizontally at a speed of 500 ms^{-1}. The bullet strikes and gets embedded in a stationary piece of wood of mass 1.0 kg which was suspended. Immediately after impact, the wood with the embedded bullet moves at a horizontal speed of 5.0 ms^{-1} and swings upwards to a vertical height of h from its initial position before stopping momentarily.

a) What is the initial kinetic energy of the bullet when it leaves the pistol?

b)What is the final kinetic energy of the wood and embedded bullet immediately after impact?

c) By comparing (a) and (b), what is the loss in kinetic energy? Account for this loss.

d) Calculate the vertical height, h, stating any assumption made. Assume the gravitational field strength is 10 N kg^{-1}.

**Solutions:**

a) 1250 J

b) 12.6 J

c) 1240 J. The loss in kinetic energy is converted to heat and sound energy when the bullet strikes the wood and is embedded.

d) 1.25 m

## Work Done is the same for all 3 paths

Work done in bringing an object up to a height, h, is independent on the path. Hence path A, B and C all have the same work done as the vertical height is the same.

Work Done = Gain in GPE

= mgh

= weight x vertical height

## Work Done = Force x Distance moved in the direction of the force

In Physics point of view, WORK DONE = FORCE x DISTANCE moved in the direction of the force.

Hence, there is only work done when a force acting on a object produces a distance moved in the direction of the force.