Physics: motion and Newton's second law (grade 11) · lesson

Speed and acceleration

💡 Speed is distance per time, and acceleration is change in velocity per time.

Speed = distance ÷ time. A car that goes 150 m in 6.0 s moves at 25 m/s. Velocity is speed in a given direction.

Acceleration = change in velocity ÷ time. Speeding up, slowing down, and turning are all accelerations.

🧠 Let's work it out

A cart goes from 2.0 m/s to 10 m/s in 4.0 s.

Change in velocity = 10 − 2.0 = 8.0 m/s.
Acceleration = 8.0 ÷ 4.0 = 2 m/s².

What do you think comes next?

velocity: speed in a given direction
acceleration: how fast velocity changes

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Net force

💡 The net force is all the forces combined, with direction taken into account.

Forces in the same direction add. Forces in opposite directions subtract, and the net force points toward the larger one.

A 30 N push to the right and 12 N of friction to the left give 18 N to the right.

🧠 Let's work it out

If you know the acceleration, you can find a missing force.
A 4.0 kg box speeds up at 5.0 m/s², so the net force is 20 N.
If the pull is 30 N, friction must be 30 − 20 = 10 N.

What do you think comes next?

net force: the total of all forces on an object

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Newton's second law

💡 Net force equals mass times acceleration (F = ma).

A bigger net force gives a bigger acceleration. A bigger mass gives a smaller acceleration for the same force.

Rearrange as needed: a = F ÷ m, or m = F ÷ a. Use kilograms, m/s², and newtons.

🧠 Let's work it out

Look for a pattern in data.
With the same force, 1 kg → 6 m/s², 2 kg → 3 m/s², 3 kg → 2 m/s².
Mass × acceleration is 6 every time, so doubling the mass halves the acceleration.

What do you think comes next?

newton (N): the unit of force; 1 N speeds up 1 kg by 1 m/s²

Newton's first law

💡 With zero net force, an object keeps the same velocity: at rest, or moving straight at a steady speed.

Moving objects don't need a force to keep moving. A puck on smooth ice slides on because almost no net force acts to stop it.

Zero net force does not mean zero motion. A car at a steady 25 m/s in a straight line has zero net force: the engine's push balances friction and drag.

Turning at a steady speed is still acceleration, because the direction changes. So it needs a net force.

🧠 Let's work it out

Ask: are both speed and direction staying the same?
If yes, the net force is zero.
If either is changing, a net force is acting.

What do you think comes next?

inertia: an object's tendency to keep doing what it is doing

Mass and weight

💡 Mass is the amount of matter and stays the same; weight is the pull of gravity and changes from place to place.

Mass is measured in kilograms and does not depend on location. Weight is a force, measured in newtons.

The Moon's gravity is about one-sixth of Earth's. A 70 kg astronaut still has 70 kg of mass there, but she weighs about one-sixth as much.

🧠 Let's work it out

Ask whether the amount of matter changed.
Going to the Moon adds or removes no matter, so mass stays the same.
Only gravity's pull changes.

What do you think comes next?