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      Changing speed

      I can describe how the speed of an object changes throughout the time that a resultant force is acting on it.

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      Lesson 6 of 10
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      • Year 8

      Changing speed

      I can describe how the speed of an object changes throughout the time that a resultant force is acting on it.

      Lesson details

      Key learning points

      1. A resultant force can change the speed of an object.
      2. The speed of an object changes throughout the time a resultant force is acting on it.
      3. The greater the gradient (steeper the line) on a distance-time graph, the faster the movement.

      Keywords

      • Resultant force - The overall effect of a set of forces acting on an object.

      • Speed - A measurement of how fast something is moving, found from the distance travelled divided by the time taken.

      • Dynamics trolley - A small wheeled cart used in motion experiments.

      • Acceleration - A change in speed or direction of movement.

      Common misconception

      Pupils can think that change in speed is instantaneous due to forces, not over a period of time.

      Demonstrate that objects continue to accelerate while the force is acting. Make the force easy to understand (e.g. a pulling string).

      Pupils can be allowed to observe acceleration due to a constant force using the set up shown in the lesson (falling masses dragging a trolley). This will let them see the increase in speed clearly while the force is acting.
      Teacher tip

      Equipment

      Content guidance

      • Risk assessment required - equipment

      Supervision

      Adult supervision required

      Licence

      This content is © Oak National Academy Limited (2026), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

      Lesson video

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      Prior knowledge starter quiz

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      6 Questions

      Q1.
      Which of these is a force measurement?

      30 kg
      30 m
      30 m/s
      Correct answer: 30 N
      30 J

      Q2.
      What feature of a distance-time graphs shows the speed?

      An image in a quiz
      The horizontal axis.
      The verical axis.
      The gridlines.
      Correct answer: The gradient of the line.
      The number of divisions on each axis.

      Q3.
      Which of these show how forces should be represented on diagram?

      Correct answer: Is drawn as an arrow.
      Has a special colour for each type of force.
      Correct answer: Starts at the point where the force acts.
      Correct answer: Points in the direction the force acts.
      Must always have a pair drawn with it.

      Q4.
      Ben pushes a shopping trolley so it is moving quickly but then falls and lets go. Which of these forces still act on the trolley after he has let go?

      An image in a quiz
      Ben's push.
      A driving force keeping the trolley moving.
      Correct answer: A friction force due to the ground.
      Correct answer: A normal contact force due to the ground.
      Correct answer: A gravitational force due to Earth.

      Q5.
      A student is measuring the average speed of a trolley between two points marked on a ramp. Which measurements do they need to make to find the speed?

      An image in a quiz
      The mass of the trolley.
      The angle of the ramp.
      Correct answer: The time for the trolley to pass between the markers.
      The time the trolley took to reach the markers.
      Correct answer: The distance between the two markers.

      Q6.
      What is the average speed of this object?

      An image in a quiz
      0 m/s
      Correct answer: 0.15 m/s
      6.6 m/s
      51 m/s
      69 m/s

      Assessment exit quiz

      Download quiz pdf

      6 Questions

      Q1.
      Match the key word with the description.

      An image in a quiz
      Correct Answer:resultant force,The sum of forces on an object, accounting for direction.
      Correct Answer:gradient,The steepness of a line on a graph.
      Correct Answer:speed,Distance divided by time.
      Correct Answer:dynamics trolley,A wheeled device used in motion experiments.
      Correct Answer:acceleration,A change in speed.

      Q2.
      Which of the following are acceleration?

      A change in the mass of an obejct.
      Correct answer: An increase in speed.
      Correct answer: A decrease in speed.
      Moving at constant speed in one direction.
      A change in distance.

      Q3.
      A person drags a box along the ground using a force of 25 N towards the right. A frictional force of 20 N acts on the box as it is dragged. What is the resultant force on the box?

      An image in a quiz
      45 N to the right
      45 N to the left
      Correct answer: 5 N to the right
      5 N to the left
      0 N

      Q4.
      A sprinter takes part in a 100 m race. Which statement describes how the forces change during the race?

      An image in a quiz
      The sprinter is at the same speed throughout the race as the force is constant.
      The sprinter's speed will only increase at the start of the race.
      Correct answer: The sprinter's speed will change as long as there is a resultant force acting.
      The sprinter produces a forwards force only at the start of the race.
      The forces on the sprinter are always balanced so the speed stays constant.

      Q5.
      The figure shows a distance-time graph for a car. What does the graph show?

      An image in a quiz
      The speed of the car is increasing.
      Correct answer: The speed of the car is decreasing.
      The forces on the car are balanced.
      There is a resultant force in the same direction as the car's movement.
      Correct answer: There is a resultant force in the opposite direction to the car's movement.

      Q6.
      A firework rocket is launched vertically and it produces a large thrust for 5 s then the fuel runs out. Which answers correctly describe the rocket's motion?

      Correct answer: The rocket will accelerate upwards for the first 5 s.
      The rocket will stop after 5 s.
      Correct answer: After 5 s the speed of the rocket will start to decrease.
      After 5 s the rocket will start to move downwards.
      The rocket's speed will still be increasing after 5 s.

      Lesson appears in

      UnitScience / Moving by force

      Science

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