Forces and energy (grade 7) · lesson

Kinetic energy

💡 Kinetic energy grows with mass and grows even faster with speed, since speed is squared.

Kinetic energy is the energy of motion. Anything moving has it. Anything at rest has none.

More mass means more kinetic energy. A fast truck has more than a fast bike.

Speed matters even more. The rule is KE = ½ × mass × speed². If you double the speed, the energy is 2 × 2 = 4 times as big.

🧠 Let's work it out

What do you think comes next?

💡 Potential energy is stored energy that depends on position, like height above the ground or how close two magnets are.

Gravitational potential energy is stored by height. The higher an object is, the more it stores. A heavier object at the same height stores more.

Other forces store energy too. Push two magnets that repel close together, and energy is stored between them. Squeezing a spring does the same.

When stored energy is let go, it usually becomes motion. Let go of the magnets, and they fly apart.

🧠 Let's work it out

Ask what changed: mass or position?
A book moved to a higher shelf has the same mass but more height, so it has more potential energy.
How fast it was lifted does not matter once it rests there.

What do you think comes next?

💡 As objects rise and fall, energy changes back and forth between potential and kinetic.

A falling ball loses height and gains speed. Its potential energy turns into kinetic energy.

A ball thrown up does the opposite. It slows as it rises, so kinetic energy turns into potential energy. At the top, it stops for an instant.

A ball rolling down a higher ramp starts with more potential energy. So it is moving faster at the bottom.

🧠 Let's work it out

What do you think comes next?

💡 When objects push on each other, energy can move from one to the other, but the total is never destroyed.

When a rolling cart hits a box, the cart slows down and the box starts moving. Energy moved from the cart to the box.

Some energy also becomes sound and heat. None of it is destroyed.

🧠 Let's work it out

What do you think comes next?