Calculating from displacement-time graphs (v = s ÷ t)
I can take readings from displacement-time graphs to find distance, displacement, speed and velocity.
Calculating from displacement-time graphs (v = s ÷ t)
I can take readings from displacement-time graphs to find distance, displacement, speed and velocity.
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Lesson details
Key learning points
- The velocity of an object is equal to the gradient of a displacement-time graph.
- A direction in which velocity and displacement are measured needs to be chosen.
- A negative gradient on a displacement-time graph (sloping downwards) represents velocity in the opposite direction.
- A negative value for displacement represents a position ‘behind’ the starting position.
Keywords
Displacement-time graph - A displacement-time graph shows the change in displacement over a time period. The displacement along a straight line can be positive or negative.
Gradient - The gradient on a displacement-time graph gives the instantaneous velocity.
Instantaneous velocity - The instantaneous velocity is the velocity of an object at a particular moment in time.
Common misconception
A pair of coordinates on a s-t graph can be used to calculate the instantaneous speed or velocity at that point, but that only works for a straight line graph through the origin.
Ask pupils to calculate the velocity of a graph that represents a steady velocity on a line that does not go through the origin, and to check to see if dividing the values at any point on the line gives the right value for velocity.
Equipment
Licence
Lesson video
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Prior knowledge starter quiz
6 Questions
Q1.Which of the following are features of a typical displacement–time graph?
Q2.Match the quantity to the correct description.
a vector measured in m
a vector measured in m/s
a scalar measured in m
a scalar measured in m/s
Q3.Which of the following are examples of velocity?
Q4.Which of the following features of a displacement–time graph indicates an object travelling at the greatest speed?
Q5.Calculate the speed of a car that travels 500 m in 25 s.
Include the unit in your answer.
Q6.How long will a fish travelling at 2.5 m/s take to travel 1.5 km?
Assessment exit quiz
6 Questions
Q1.Match the key words and phrases to the correct descriptions.
a graph showing distance in a given direction over a period of time
the steepness of a line or slope
how fast an object moves in a given direction
how fast an object moves in a given direction over a period of time
Q2.A pupil follows these movement instructions: move 5 m left, then 4 m left, then 6 m right and finally 3 m left.
Which of the following statements are correct?
Q3.The figure shows a displacement-time graph.
What is the total distance travelled during the 50 s?
Q4.The figure shows a displacement-time graph.
Calculate the average velocity for the first 20 s.
Q5.The figure shows a displacement-time graph.
Calculate the average velocity between 30 s and 50 s.
Q6.The figure shows a displacement time graph.
At which of these times is the speed greatest?
