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Describing motion for Class 9: which fibub activity to use, and how

For students · Updated · For Class 9

A Class 9 guide to the fibub activities for the chapter on describing motion around us: distance, velocity, acceleration, motion graphs and the equations of motion, with habits and questions to check yourself.

Motion is everywhere in your day: the bus to school, a ball you kick, a cyclist on the road. In Class 9 you start to describe it with numbers, and that is where many students get stuck. Which is distance and which is displacement? Why is a slope the velocity? This guide shows which free activity to use for each idea. Every tool runs in your browser and works offline once the page has loaded. You will not need an account, and nothing you type is sent anywhere.

What you will learn

  • The difference between distance (the whole path you travel) and displacement (how far you end up from the start, with a direction).
  • Speed and velocity, and why velocity needs a direction.
  • Acceleration, the rate at which velocity changes.
  • How to read distance-time and velocity-time graphs, using the slope and the area.
  • The three equations of motion for steady acceleration, and how to use them to solve problems.

Which activity to use when

  • Distance and displacement: use the distance and displacement simulator. Walk a path on the grid and compare the length of your path with the straight line back to where you began. Choose the Class 9 level.
  • Graphs: use motion graphs live. Drive a car with the slider and watch the distance-time and velocity-time graphs draw themselves. Try to match a target graph.
  • Equations of motion: use equations of motion. Drag a velocity-time graph and see the rectangle and triangle under it add up to the displacement. Then use Solve a problem to find the missing values.
  • Your own motion: use the video motion tracker. Film a ball rolling with a ruler in shot, open the video, type its frames per second, click both ends of the ruler to set the scale, then click the ball frame by frame. The tool gives its speed and acceleration from your own video.

How to use them properly

  • Predict first. Before you press Start (or Play), write down what you think the graph will look like. Then check. Being wrong and seeing why is how the idea sticks.
  • Always write units. Metres, seconds and metres per second are different from each other. A number without a unit is not an answer.
  • Check your answers. Use a second method. If the equation gives 20 m/s, check that the slope or area on the graph gives the same.
  • Watch the sign. Decide which direction is positive, and keep to it. A negative acceleration points the negative way. If the object is moving the positive way, it slows down; if it is moving the negative way, it speeds up.
  • Use the right idea for the question. Distance questions need the whole path. Displacement questions need the straight line from start to finish.

A worked example

A car starts from rest and reaches 20 m/s in 5 s. Its acceleration is a = (20 − 0) ÷ 5 = 4 m/s². Its displacement is s = ut + ½at² = 0 + ½ × 4 × 5² = 50 m. Enter u = 0, v = 20 m/s and t = 5 s in Solve a problem and the page finds the same acceleration and distance.

A walk of 600 m east and then 800 m north, taking 10 minutes, has distance 1,400 m. The straight line back to the start is √(600² + 800²) = 1,000 m, so the displacement is 1,000 m. The average speed is 1,400 ÷ 600 = 2.33 m/s and the average velocity is 1,000 ÷ 600 = 1.67 m/s.

Try this at home (safe)

  • Walk a square. Pace out a 10-step square in a clear corridor or a field, away from roads and stairs. Note your distance and your displacement at the end. What is the displacement when you return to the start?
  • Roll a ball down a ramp. Use a book as the ramp and a ruler as the track. Film it with your phone from the side, with the ruler in the shot, and track it with the video tracker. Ask an adult to help, keep the ramp low, and do it on the floor so nothing can fall far.
  • Time a bus. With an adult, and using a watch, note the time for a short bus ride between two stops whose distance you know. Work out the average speed. Stay seated and never go near the road to measure.

Check yourself

  1. A bus travels 3 km east, then 4 km north, in 15 minutes. What is its distance and its displacement? Answer: distance 7 km; displacement 5 km (the straight line, by Pythagoras).
  2. A cyclist moves at a steady 5 m/s for 10 s on a straight road. How far does she go? Answer: 5 × 10 = 50 m. On a velocity-time graph this is the area of a rectangle, 5 by 10.
  3. A car moves round a circular track at a steady 10 m/s. Is it accelerating? Answer: yes. Its speed is steady, but its direction keeps changing, and a change in velocity is an acceleration.

If an answer surprised you, try a different number in the simulator for that idea.

Tools in this article

Motion graphs live #L8 Drive a car along a road with a slider or pick a ready-made motion, and watch its x-t, v-t and a-t graphs draw themselves; read the slope (velocity, acceleration) and the shaded area (displacement), then try to match a target graph. Equations of motion #L9 Drag a velocity-time graph and watch the rectangle and triangle under it add up to the displacement; see v = u + at, s = ut + ½at² and v² = u² + 2as with your numbers; solve any problem from three known values; and find a car's stopping distance. Video motion tracker #T641 Open a phone video, set the scale with a ruler in the shot, click the object in each frame and get x(t), y(t), velocity and acceleration, with straight-line and parabola fits (g from a thrown ball). The video is played in your browser and never uploaded. Distance and displacement #L143 Tap a walk on a map and see the distance you walked next to the straight-line displacement, with average speed and average velocity. Then run round a circular track at a steady speed and watch the velocity arrow turn: uniform circular motion.

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