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Motion
• How do you know when something is moving?
• Objects must be compared to a “reference point”, something that is not moving.
• When an object changes position over time relative to a reference point, it is said to be “moving”.
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What is speed?
Speed is a measure of how quickly an object gets from one place to another.
This jet is traveling at 350 m/s. This means the jet travels 350 meters every second.
This truck is traveling at 60 mph. This means the truck travels 60 miles every hour.
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The speed of an object does not depend on the direction in which it is traveling. The velocity of an object is the speed and direction in which it is moving.
The car is traveling north with a velocity of 10 m/s.
How is velocity different to speed?
As the car goes round the corner, the speed of the car remains constant but the velocity changes.
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How is speed calculated?
The speed of an object can be calculated using this equation:
distance traveled
time takenspeed =
Speed is measured in meters per second (m/s).
Distance traveled is measured in meters (m).
Time taken is measured in seconds (s).
The standard unit for speed in physics is m/s, but other units, such as kilometers per hour (km/h), are more convenient when measuring the speed of vehicles. Why is this?
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Calculating speed question
A train takes 100 seconds to travel 1,500 m. What is the speed of the train?
= 15 m/s
speed =distance
time
= 1,500
100
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Using a formula triangle
A formula triangle helps you to rearrange a formula. The formula triangle for speed (s), distance (d) and time (t) is shown below.
x
Cover the quantity that you are trying to work out, which gives the rearranged formula needed for the calculation.
So to find speed (s), cover up s…
…which gives the formula…
s =d
t
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Calculating speed question
A car travels at 25 m/s for 3 minutes. How far does it travel?
= 4,500 m = 4.5 km
speed =distance
time
= 25 x 180
distance = speed x time
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Speed, distance, time calculations
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The acceleration of an object is a measure of how quickly its velocity changes.
A train accelerates in a straight line from rest. As it does, its velocity increases.
The brakes on this motorcycle are causing it to slow down. This is negative acceleration or deceleration.
What is acceleration?
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The acceleration of an object can be calculated using this equation:
change in speed
time takenacceleration =
Acceleration is measured in meters per second per second (m/s2).
Change in speed is measured in meters per second (m/s).
Time taken is measured in seconds (s).
How is acceleration calculated?
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A race car accelerates from rest to a speed of 60 m/s in a time of 4 seconds. What is the acceleration of the car?
= 15 m/s2
change in speed
time takenacceleration =
= 60
4
Acceleration problem
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Using a formula triangle
A formula triangle helps you to rearrange a formula. The formula triangle for acceleration (a), speed (s) and time (t) is shown below.
x
Cover the quantity that you are trying to work out, which gives the rearranged formula needed for the calculation.
So to find acceleration (a), cover up a…
…which gives the formula…
a =s
t
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A hungry cheetah spots a gazelle and decides to chase it. The cheetah accelerates at 10 m/s2 from rest until it reaches 20 m/s. How long did this take?
acceleration = change in speedtime taken
= 2010
= 2 s
Acceleration problem
time taken = change in speedacceleration
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Acceleration problems calculations
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Practice Problems
1. A car moves 240 meters west in 120 seconds. What is the velocity?
2. A car travels 1000 m in 5 s, what is the speed?
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Practice Problems
3. A boy walks 500 m in 10 seconds, what is the speed?
4. An object travels at a velocity of 18.99 m/s for 15.63 seconds, what is the displacement?
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Practice Problems
5. A cat walks 10 m east and then 5 m west in 12 seconds. Find the velocity.
