Physics: conservation of energy (grade 11) · lesson

Kinetic and potential energy

💡 Kinetic energy is energy of motion (½mv²), and gravitational potential energy is energy of height (mgh).

KE = ½mv². Speed is squared, so doubling speed makes four times the kinetic energy. A 2.0 kg ball at 3.0 m/s has ½ × 2.0 × 9 = 9 J.

PE = mgh. Lifting a 5.0 kg box 4.0 m with g = 10 m/s² stores 5.0 × 10 × 4.0 = 200 J.

A book on a high shelf has potential energy but no kinetic energy, because it is not moving.

🧠 Let's work it out

What do you think comes next?

💡 Without friction, potential energy lost equals kinetic energy gained, so total energy stays the same.

A roller coaster with 50,000 J of potential energy at the top has 50,000 J of kinetic energy at the bottom, if there is no friction.

Potential energy is proportional to height. Halfway down a hill, half the energy has become kinetic, so kinetic and potential are equal.

🧠 Let's work it out

What do you think comes next?

💡 Devices work by turning one form of energy into another, step by step.

At a dam, high water has potential energy. Falling water gains kinetic energy. The water spins a turbine and generator, which make electrical energy.

In a flashlight, chemical energy in the battery drives an electric current. The bulb turns that into light and some heat.

🧠 How to think about it

Start with the stored energy, then follow it through each device until it reaches its final form. Each step needs the one before it.

💡 Energy that seems lost becomes thermal energy and sound, and efficiency tells how much stays useful.

A bouncing ball goes lower each time because some energy becomes heat and sound. The energy is not destroyed.

Efficiency = useful energy out ÷ energy in. A motor that uses 500 J to give a load 400 J of potential energy is 400 ÷ 500 = 80% efficient.

🧠 Let's work it out

What do you think comes next?

💡 Power is how fast energy is used or work is done.

Power = work ÷ time. Its unit is the watt (W), one joule per second.

Two motors can do the same work, but the faster one has more power.

🧠 How to think about it

600 J of work in 3.0 s: 600 ÷ 3.0 = 200 W. Check units: joules per second are watts.