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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.

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New
New
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.

Download all resources
Share activities with pupils

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.

Common misconception

Students 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).

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 of speed.

Students 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

Content guidance

  • Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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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
Q2 UYSEG Q4 UYSEG Q5 UYSEG Q6 UYSEG

Exit quiz

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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.

The sum of forces on an object, accounting for direction.

Correct Answer:gradient,The steepness of a line on a graph.

The steepness of a line on a graph.

Correct Answer:speed,Distance divided by time.

Distance divided by time.

Correct Answer:dynamics trolley,A wheeled device used in motion experiments.

A wheeled device used in motion experiments.

Correct Answer:acceleration,A change in speed.

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.
Q1 UYSEG Q3 UYSEG Q5 UYSEG