A machine lifts 30 tonnes of weed to a height of 15 m in 5 minutes. The input power of the machine is 20 kW. a) Determine the efficiency of the machine. b) Comment on the efficiency and suggest improv

Mass of weed, $m = 30 \text{ tonnes} = 30 \times 1000 \text{ kg} = 30000 \text{ kg}$
Height, $h = 15 \text{ m}$
Time, $t = 5 \text{ minutes} = 5 \times 60 \text{ s} = 300 \text{ s}$
Input power of machine, $P_{input} = 20 \text{ kW} = 20 \times 1000 \text{ W} = 20000 \text{ W}$
Acceleration due to gravity, $g = 9.8 \text{ m/s}^2$

Mass of child 1, $m_1 = 45 \text{ kg}$
Distance of child 1 from pivot, $d_1 = 30 \text{ cm} = 0.30 \text{ m}$
Mass of child 2, $m_2 = 50 \text{ kg}$
Distance of child 2 from pivot, $d_2 = ?$

For the see-saw to be in equilibrium, the clockwise moment must equal the anticlockwise moment.
Moment = Force $\times$ distance from pivot.
Force = mass $\times$ acceleration due to gravity ($F = mg$).
So, $m_1 g d_1 = m_2 g d_2$.
Since $g$ (acceleration due to gravity) is common on both sides, it cancels out:
$m_1 d_1 = m_2 d_2$

The other child has to sit at a distance of $\boxed{0.27 \text{ m}}$ or $\boxed{27 \text{ cm}}$ from the pivot.

ITEM 4
Report on Energy

Energy is simply the ability to do work or cause change. Everything that happens in the world, from a car moving to a plant growing, requires energy. Energy cannot be created or destroyed, but it can change from one form to another.

We get energy from various sources. Some sources are renewable, meaning they can be replenished naturally, like solar energy from the sun, wind energy, and hydroelectric energy from moving water. Other sources are non-renewable, meaning they are limited and take a very long time to form, such as fossil fuels like coal, oil, and natural gas, and nuclear energy from uranium.

Energy exists in many types. For example, there is kinetic energy, which is the energy of movement, like a running person. There is also potential energy, which is stored energy, such as a ball held high above the ground. Other types include thermal energy (heat), chemical energy (in food or fuel), electrical energy (used in homes), light energy (from the sun), and sound energy. Understanding these different forms helps us use energy wisely.