Forces and motion

How and How Fast

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

A sneeze travels at 160 km/hr

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.