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Forces and motion

Motion and speed

Motion and speed

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Explainer Video

Tutor: Ayesha

Summary

Motion and speed

In a nutshell

The speed of an object tells you how fast it is travelling. By looking at the relationship between distance and time, you can find out the speed of an object and sometimes even the way in which the speed of the object is changing. 

​

Equations

WORD EQUATION

SYMBOL EQUATION

​speed=distancetimespeed = \frac{distance}{time}speed=timedistance​​​
​s=dts = \frac{d}{t}s=td​​​


Variable definitions 

QUANTITY NAME

SYMBOL

UNIT NAME

UNIT

​speedspeedspeed​​
​sss​​
​metre per secondmetre \space per \space secondmetre per second​​
​m/sm/sm/s​​
​distancedistancedistance​​
​ddd​​
​metremetremetre​​
​mmm​​
​timetimetime​​
​ttt​​
​secondsecondsecond​​
​sss​​



Speed calculations

You can find the average speed of the object if you know the distance it has travelled and the time it took for the object to travel that far.


​speed=distancetimespeed = \frac{distance}{time}speed=timedistance​


​s=dts = \frac{d}{t}s=td​​​


If you need to calculate the distance travelled instead, you can rearrange this equation to make distance the subject. Or if you need to find the time taken to travel a certain distance, you can rearrange for time. 


Example

A car travels at a speed of 3 m/s3 \space m/s3 m/s for a total of 10 seconds10 \space seconds10 seconds​. How far has the car travelled in metres? 

 

Begin by writing out the quantities you been given:

​speed=3 m/sspeed = 3 \space m/sspeed=3 m/s​​

​time=10 stime = 10 \space stime=10 s​​


Rearrange the equation to find distance travelled: 

distance=speed×timedistance = speed \times timedistance=speed×time​​


Substitute the values into the equation:

​distance=3×10distance = 3 \times 10distance=3×10


The distance travelled by the car is 30 m‾\underline{30 \space m}30 m​​. 



Distance-time graphs

A distance-time graph shows you how the speed of an object changes over time. Distance is plotted on the xxx-axis and time is plotted on the yyy-axis. The slope, also called the gradient, tells you the speed of the object. The shape of the distance-time graph can also tell you about how the motion of the object is changing.


Science; Forces and motion; KS3 Year 7; Motion and speed

A
Distance

B
Time

​

1.
If a distance-time graph has a straight line section, it means that the object is moving with a constant speed. If it is uphill, then the object is moving away from the starting point. If it is downhill, then the object is moving back towards the starting point. The steeper the straight line, the faster the object is moving.
2.
If a distance-time graph has a flat section, it means that the object has stopped moving.
3.
If a distance-time graph has a curve in it, then this means that the object is changing speed. If it is a steeping curve, then the object is getting faster or, in other words, accelerating. If it is a curve that is levelling off, then the object is slowing down or decelerating.


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Learn with Basics

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Speed: Formula and units

Unit 1

Speed: Formula and units

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Motion and speed

Unit 2

Motion and speed

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FAQs - Frequently Asked Questions

What does a curved distance-time graph mean?

If a distance-time graph has a curve in it, then this means that the object is changing speed.

How can I find the average speed?

You can find the average speed of the object if you know the distance it has travelled and the time it took for the object to travel that far.

What is a distance-time graph?

A distance-time graph shows you how the speed of an object changes over time, with distance plotted on the y-axis and time plotted on the x-axis.

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