Relative Motion – KS3 Tutorial

What is Relative Motion?

Relative motion is how one object appears to move when you compare it to another object.

🔍 Imagine you’re sitting in a car next to another car. If both cars move at the same speed, they look still — but they’re actually moving!

It’s not just about how fast you’re moving — it’s how fast you’re moving compared to another moving thing.

🧍‍♂️ Imagine This:

You’re walking at 2 m/s. Another student walks past you at 4 m/s in the same direction.
To you, it feels like they’re only moving at 2 m/s (not 4).
That’s relative motion in action.

📗 Why Do We Use Relative Motion?

To work out how fast things are getting closer or further apart
To compare moving objects (cars, people, trains, etc.)
Especially useful in transport, racing, or passing situations

🔄 Two Situations in Relative Motion

🔹 1. Objects moving in opposite directions (e.g., two trains facing each other):

👉 Rule: Add their speeds
📌 Example:

Train A = 40 km/h
Train B = 60 km/h (coming toward A)
Relative speed = 40 + 60 = 100 km/h
➡ They come together faster.

You are on the red train and see the blue train coming toward you.
It looks like the blue train is moving at 100 km/h,
because you are moving at 40 km/h, and the blue train is moving at 60 km/h in the opposite direction.
This is why the relative speed is 40 + 60 = 100 km/h — they come together faster.

🔹 2. Objects moving in the same direction (e.g., car overtaking a tractor):

👉 Rule: Subtract their speeds
📌 Example:

Car = 30 mph
Tractor = 20 mph (both moving forward)
Relative speed = 30 – 20 = 10 mph
➡ The car pulls away slowly.

📊 Summary Chart

Direction	Rule to Use	Example	Result
Opposite directions	Add the speeds	30 + 25	55 km/h
Same direction	Subtract the speeds	30 – 20	10 mph

🧪 Try It Yourself

Two cyclists ride towards each other: One at 12 km/h, the other at 8 km/h.
➤ Relative speed = ________
✅ Answer: km/h
A bus (50 km/h) overtakes a scooter (30 km/h).
➤ Relative speed = ________
✅ Answer: km/h