conservation and dissipation of energy

When the pod is released, the pod accelerates upwards.

Before the pod is released the extension of each of the two bungee cords is 8.0m

The spring constant of each bungee cord is 735N/m

The mass of the pod id 240kg

Gravitational field strength = 9.8N/kg

Calculate the maximum height reached by the pod

EQUATIONS:

$$Ee=\frac12ke^2$$

$$Ep=mgh$$

1. Calculate the total elastic potential energy

   for one bungee cord:

   $$Ee=\frac12\times735Nm\times\left(8.0\right)^2$$

   = 23520J

2. For two identical bungee cords

   2 × 23520J = 47040J

3. Calculate maximum height

   $$Ep=Ee,total$$

   $mgh=47040J$ $240\operatorname{kg}\times9.8Nkg\times h=47040J$

   $2352\times h=47040$ $h=\frac{47040}{2352}=20m$

The actual maximum height reached by the pod will be lower than the correct answer to part (b)

Explain why.

air resistance (opposes the motion of the pod upwards)

• (so) not all of the elastic potential energy will be transferred to gravitational potential energy

• allow the energy transfer is not 100% efficient

• allow some energy is transferred to the surroundings

• allow some energy is dissipated

The figure below shows how the power output of the generator varied during one year

A solder power system is installed in the remote village in addition to the hydroelectric generator.

Explain why this improves the reliability of the electricity supply to the village.

Use information from the figure above.

In the summer the power output from the hydroelectric generator is lower but the solar power output would be greater

→ allow power output of hydroelectric generator depends on rainfall and power output of solar power system depends on light intensity

So less variation in total power output (which improves the reliability of the supply)

→ allow electricity supply for total power output

→ allow reference to specific months eg April to September

A student investigated the insulating properties of different materials.

This is the method used:

1. Wrap insulating material around the can

2. Put a fixed volume of boiling water in the can

3. Place the lid on the top of the can

4. Measure time taken for the temperature of the water to decrease by a fixed amount

5. Repeat steps 1-4 using the same thickness of different insulating materials

Identify the independent and dependent variable in this investigation.

Independent variable: type of insulation/material

Dependent variable: time

What is the resolution of a thermometer

The smallest change in temperature that the instrument can display.

Why is a lined thermometer more likely to be misread than a digital thermometer?

Viewing angle affects measurement

→ allow judgement needed in reading the position

→ allow level of liquid may be between lines

→ allow number of lines may be miscounted

The figure below shows a girl doing an experiment to determine her power output by running to the top of some stairs.

The mass of the girl was 60.0kg

The height of the stairs was 175cm

The girl ran to the top of the stairs in 1.40s

gravitational field strength = 9.8N/kg

Calculate the power output of the girl.

h = 1.75m

Ep = 60 × 9.8 × 1.75

Ep = 1029 (J)

P = 1029/1.40

P = 735 (W)

The total power output of the girl was greater than the answer to part (a).

Suggest two reasons why.

Girl increases her kinetic energy

Some energy is wasted in her muscles

or

Some energy is transferred as thermal energy (to surroundings)