Post on 23-Dec-2015
Motion and speed• How can we calculate how fast an object
is moving?
• How long will a journey take at a particular speed?
• Speed is a measure of how fast something is moving.
• The speed of an object tells us the distance it travels each second.
SPEED• An Olympic sprinter
will run about 10 meters every second. We say they are running at a speed of 10 meters per second. This can be written as 10 m/s.
Calculating speed
• To calculate speed:
• the distance travelled is divided by the time taken to travel that distance.
• During the journey the speed may change, meaning that the speed calculated is the average speed.
Calculating speed: Example
• average speed = distance travelled ÷ time taken
• A horse, takes 12 s to trot a distance of 96 m. Calculate the average speed of the horse during this part of its journey. information:
• distance travelled = 96 m
• time taken = 12 s.
Calculating speed: Example
• information:
• distance travelled = 96 m
• time taken = 12 s.
• calculation:
• average speed = distance travelled ÷ time taken =96m÷12s= 8m/s
• Answer:8m/s
Calculating Distance Travelled
• To calculate the distance travelled you need to know the time taken for the journey
• and the average speed during the journey.
• You then calculate the distance travelled using the following equation: distance travelled = average speed x time taken
Calculating Distance Travelled
• Example
• A girl walks at an average speed of 2 m/s for 80 s. Calculate how far she walked in that time.
• information:
• average speed = 2 rn/s
• time taken = 80 s.
Calculating Distance Travelled
• calculation:
• distance travelled = average speed x time taken
• = 2 m/s x 80 s = 160 m.
Time Taken
• To calculate the time taken to travel a given distance when the average speed is known, use the following equation
• time taken = distance travelled ÷ average speed
Example Time Travelled
• A ferry transports cars across a river at an average speed of 4 m/s
• The river is 640 m wide
• Calculate the time that it takes for the ferry to cross the river.
• information:
• distance travelled = 640 m
• average speed = 4 m/s.
• information:
• distance travelled = 640 m
• average speed = 4 m/s.
• calculation:
• time taken = distance travelled÷ average speed=
• 640 m ÷ 4 s = 160 s.
Other units of speed
• It is not always a good idea to measure speeds in metres per second.
• A snail making its way along a garden path will be moving very slowly and it would be better to measure its speed in millimetres per second.
• For a car travelling along a motorway at high speed it would be better to measure the speed in miles per hour or kilometres per hour
• The same equation is used to calculate the speed, but the appropritate units for distance and time must be used.
• To find the speed of the snail (mm/s) the distance should be measured in millimetres time in seconds.