Work, Energy and Power

• Brainiac• Solar Energy

Work

Work – Sample Qs 1

Work - Sample Qs 2

• A man carries a 50kg bag of cement up a stairs at a steady speed. The vertical height through which he rises is 20m. He exerts an upward force on the bag equal to the weight of the bag. Find the work done by the man on the bag of cement.

• Ans: 9800J

Work – Sample Qs 3

• Find the work done in bringing a 100kg car from rest to 30m/s in 10s.

• Ans: 450,000J

Energy

• Energy (E) is the ability to do work• the amount of energy something has is the

amount of work it can do.• Any time work is done energy is transferred.• Units: Joules (J)• Different Types: chemical, heat, electrical,

sound, nuclear, potential, kinetic

Kinetic Energy (K.E.)

K.E. – Sample Qs 1 & 2• What is the kinetic energy of a race car of mass

1585 kg travelling at 252 km/h• A particle of mass 20kg moving at a speed of

3m/s accelerates to a speed of 30m/s under the action of a force of magnitude 12 N in the direction of the motion. Calculate:– the initial K.E. of the particle– the final K.E. of the particle– the change in kinetic energy of the particle– the distance travelled– the work done by the force

When Work is done, Energy is transferred.

Imagine pushing a car of mass 800kg a distance of 10m.• Work is done and chemical energy in your muscles is

used up.• The car gains kinetic energy as you move it

• Mathematically:–If an object comes to rest, the kinetic energy lost will be equal to the work done in bringing the object to rest.

Sample Qs 3

• A bullet of mass 2g travelling with a speed of 150m/s penetrates 0.6m into a block of wood before coming to rest. Find the force that the wood exerts on the bullet.

Potential Energy (P.E.)

P.E. Sample Qs 1 & 2

• A high jumper of mass 65kg raises her centre of gravity from 1.3m to 2.1m. What is her gain in potential energy?

• A particle of mass 35kg is projected upwards through a vertical height of 1200m. Find the increase in its potential energy.

Principle of Conservation of Energy

• Energy can neither be created nor destroyed, only converted from one form to another.

Relationship between K.E. and P.E. for falling objects

• A stone is held at a height and has a P.E. = 5000J and K.E. = 0.

• When released the stone begins to fall and its P.E. is converted to K.E.

• Just before it hits the ground, P.E. = 0 and K.E. = 5000J.

• All the potential energy has been converted to K.E.

P.E./K.E. Sample Qs 1 & 2

• A stone of mass 4kg is dropped from a height of 200m. Find its kinetic energy and speed just before it hits the ground

• A stone is projected vertically upwards with an initial speed of 40m/s. Find the greatest height reached

Consider an oscillating pendulum

Consider an oscillating pendulum

PE = mgh

KE = ½ mv2

At the top of the oscillation the pendulum bob stops. All it’s energy is PE

At the bottom the bob

has no PE only KE

PE at top=KE at bottom

h

P.E./K.E. Sample Qs 3

A pendulum bob is released from a height. Find the speed of the bob as it passes through a vertical position 40cm below its starting position.

Collisions

• When two bodies collide, kinetic energy is lost in the collision.

• However momentum is always conserved:• Momentum Before = Momentum After

• The loss of kinetic energy is usually converted into sound energy or heat energy.

Newton’s Cradle

• Potential Energy is converted to Kinetic Energy, which is transferred through the spheres until at the end it is converted back to potential energy.

• Why does it stop?• Energy is also converted into sound and heat

Energy Conversions

• Think of examples of different energy conversions.

• Clearly state the conversion taking place.Example:In a filament bulb, current is passed through a

thin tungsten wire, which becomes white hot, which in turn gives out light

Electric Energy to Heat and Light Energy

Efficient Energy Use in the Home

List ideas to reduce energy in the home.www.powerofone.ie

Power

Power

Power Sample Qs 1-3

• A man raises an object of mass 40kg through a vertical height of 20m in 4s. Find his average power.

• A weight lifter raises a mass of 30kg through a height of 0.6m. He does this 50 time in half a minute. Find the average power he develops.

• Calculate the average power developed by a 70kg woman running up a set of stairs, given that each of the 15 steps is 20cm high and it takes her 10s to reach the top.

Efficiency

Efficiency Sample Qs 1 & 2

• The power rating on a food blender is 300W. However it is only 75% efficient. The loss in useful energy is due to heat. Calculate the rate at which heat is generated by the blender.

• A 60kW motor in a crane lifts a mass of 4000kg through a height of 10m in 20s. Calculate the % efficiency of the motor if it is working at full power.