Forces and energy (grade 8) · lesson

Forces come in pairs

💡 When one object pushes on another, the second pushes back just as hard in the opposite direction.

This is Newton's third law. Push on a wall, and the wall pushes back on you. That push can roll you backward on skates.

A rocket pushes hot gas down. The gas pushes the rocket up. It works even in space, where there is nothing else to push on.

In a crash, the car and the truck push on each other with equal force. But the car has less mass, so its motion changes more.

🧠 Let's work it out

Name both objects in the pair.
'The rocket pushes the gas' and 'the gas pushes the rocket' are equal and opposite.
Then use mass to see which one's motion changes more.

What do you think comes next?

Newton's third law: every push or pull comes with an equal and opposite push or pull

🔗 Khan Academy: Motion and forces · Khan Academy ↗

Force, mass, and motion

💡 An unbalanced force changes motion, and the same force speeds up a smaller mass more.

When forces are balanced, the net force is zero. A book on a table does not move. Gravity pulls down and the table pushes up just as hard.

When forces are unbalanced, motion changes. The object speeds up, slows down, or turns.

Newton's second law says acceleration = force ÷ mass. The same push speeds up an empty cart more than a full one.

🧠 Let's work it out

Plug in the numbers.
A net force of 10 N on a 2 kg cart: a = 10 ÷ 2 = 5 m/s².
Double the mass with the same force, and the acceleration is cut in half.

What do you think comes next?

net force: the total of all forces on an object
acceleration: how fast an object's speed or direction changes
newton (N): the unit for measuring force

🔗 PhET simulation: Forces and Motion: Basics · PhET Interactive Simulations ↗

Gravity

💡 Gravity pulls all objects together, and it is stronger with more mass and less distance.

Every object pulls on every other object. This pull is gravity.

Bigger masses pull harder. Objects closer together pull harder.

Your mass is the same everywhere, but your weight is not. Weight is the pull of gravity on you. The moon has much less mass than Earth, so it pulls less. A 1 kg mass weighs about 9.8 N on Earth but only about 1.6 N on the moon.

🧠 Let's work it out

Compare pairs one factor at a time.
Same distance, more mass: stronger pull.
Same mass, less distance: stronger pull.

What do you think comes next?

gravity: the pull that all objects have on each other
weight: the force of gravity on an object

Magnets and electromagnets

💡 Like magnetic poles repel and opposite poles attract, and an electromagnet gets stronger with more loops and more current.

Every magnet has a north pole and a south pole. Two north poles push apart. A north and a south pull together.

Electric current in a wire makes a magnetic field. Wrap wire around an iron nail, and you have an electromagnet.

More loops of wire make it stronger. More current, such as from a second battery, also makes it stronger. An iron core helps. Plastic does not.

🧠 Let's work it out

To make an electromagnet stronger, ask what adds more magnetism.
Each loop adds to the field.
More current makes each loop stronger.
Removing loops does the opposite.

What do you think comes next?

repel: push away
attract: pull together
electromagnet: a magnet made by electric current in a coil of wire

🔗 PhET simulation: Magnets and Electromagnets · PhET Interactive Simulations ↗

Waves and light

💡 A taller wave carries more energy, and light that hits a surface can be reflected or absorbed.

A wave's height is its amplitude. A wave with a larger amplitude carries more energy. That is why a big ocean wave can knock you over.

When light hits a surface, it can bounce off or be absorbed. Absorbed light turns into thermal energy.

Dark colors absorb more light. Light colors reflect more. So a black shirt in the sun gets warmer than a white one.

🧠 Let's work it out

Keep everything else the same and look at one change.
Two waves have the same wavelength, but one is twice as tall.
The only difference is amplitude.

What do you think comes next?

amplitude: how tall a wave is from its middle to its top
wavelength: the distance from one wave top to the next
absorb: take in
reflect: bounce off

🔗 PhET simulation: Wave on a String · PhET Interactive Simulations ↗