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What is speed?. Speed is a measure of how far an object moves in a given time. This car is travelling at 60 mph. This means the car travels 60 miles every hour. This jet is travelling at 350 m/s. This means the jet travels 350 metres every second. How is velocity different to speed?.
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What is speed? Speed is a measure of how far an object moves in a given time. This car is travelling at 60mph. This means the car travels 60 miles every hour. This jet is travelling at350m/s. This means the jet travels 350 metres every second.
How is velocity different to speed? The speed of an object does not depend on the direction in which it is travelling. The velocity of an object is the speed and direction in which it is moving. The car is travelling north with a velocity of 10m/s. As the car goes round the corner, the speed of the car remains constant but the velocity changes.
distance travelled time taken speed = How is speed calculated? The speed of an object can be calculated using this equation: • Distance travelled is measured in metres (m). • Time taken is measured in seconds (s). • Speed is measured in metres per second (m/s). The standard unit for speed in physics is m/s, but other units such as kilometres per hour (km/h) are more convenient when measuring the speed of vehicles. Why is this?
distance time speed = = 1,500 100 Calculating speed question A train takes 100 seconds to travel 1,500 m. What is the speed of the train? = 15m/s
d s = t x 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. 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…
distance time speed = Calculating speed question A car travels at 25m/s for 3 minutes. How far does it travel? distance = speed x time = 25 x 180 = 4,500m = 4.5km
The gradient of the line in adistance–time graph equals the speed. distance distance time time Calculating speed from the gradient The slope of a graph is called the gradient. It is difficult to calculate the gradient of ‘realistic’ graphs because the line is curved. Simple graphs use straight lines only, making it easy to calculate the gradient.
B 70 60 50 40 distance (m) 30 20 A 10 0 0 1 2 3 4 5 6 7 8 9 time (s) What’s the speed? What is the speed of the object between points A and B? • the object has moved 60m (70- 10) • it took 3s to move this distance (6 - 3) • speed = distance/time = 60/3 = 20m/s
What is acceleration? 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.
change in speed time taken acceleration = How is acceleration calculated? The acceleration of an object can be calculated using this equation: • Change in speed is measured in metres per second (m/s). • Time taken is measured in seconds (s). • Acceleration is measured in metres per second per second (m/s2).
change in speed time taken acceleration = 60 4 = Acceleration problem A racing car accelerates from rest to a speed of 60m/s in a time of 4 seconds. What is the acceleration of the car? = 15m/s2
s a = t x 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. 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…
change in speed time taken acceleration = 20 10 = change in speed acceleration time taken = Acceleration problem A hungry cheetah spots a gazelle and decides to chase it. The cheetah accelerates at 10m/s2 from rest until it reaches 20m/s. How long did this take? = 2s
This gradient of the line in a speed–time graph represents acceleration speed speed time time Calculating acceleration from the gradient How can the acceleration of an object be calculated from a speed–time graph? • If the gradient goes up, the object has a positive acceleration. • If the gradient goes down, the object has a negative acceleration, or deceleration.
35 30 B 25 20 speed (m/s) 15 10 A 5 0 0 1 2 3 4 5 6 7 8 time (s) What’s the acceleration? What is the acceleration of the object between points A and B? • the object’s speed has increased by 20m/s (25 - 5) • it took 4s to change speed (6 - 2) • acceleration = speed/time = 20/4 = 5m/s2
Speed and safety Why are speed limits important?
stopping distance = thinking distance + braking distance Why have speed limits? Speed limits are an important part of road safety. They aim to prevent drivers from driving at speeds that are unsuitable and unsafe. The speed limit of a particularroad depends on a range offactors, such as how straight or curved it is, and its location. The faster a vehicle is driving, the longer it will take to stop –the overall distance this takes is the stopping distance.
other drugs and some medicines alcohol distractions, such asmobile phones tiredness speed What affects thinking distance? The thinking distance is the distance a vehicle travels in the time it takes for a driver to react to a situation and apply the brakes. What factors will affect thinking distance?
weather condition of tyres/brakes condition of road speed What affects braking distance? The braking distance is the distance a vehicle takes to stop once the driver has applied the brakes. What factors will affect braking distance?
How do speed cameras work? There are several types of speed camera., They use different methods to calculate the speed of a vehicle. Gatso speed cameras use radar to detect the speed of a vehicle, then take two photos (half a second apart) to provide visual evidence. Lines marked on the road indicate how far the vehicle has travelled in that time. Truvelo speed cameras are activated by pressure detector cables in the road. The cables are 10cm apart and a computer calculates how long it takes the vehicle to pass from one to another, and therefore the speed of the vehicle.
Glossary (1/2) • acceleration – A measure of how an object’s velocity changes over time. It usually refers to an object that is speeding up. • braking distance – The distance it takes for a vehicle to stop once its brakes have been applied. • deceleration – Negative acceleration, i.e. slowing down. • non-uniform – Speed or acceleration that is constantly changing. • speed – A measure of how far an object moves in a given time.
Glossary (2/2) • stopping distance – The total distance it takes for a vehicle to stop, i.e. thinking distance plus braking distance. • thinking distance – The distance it takes for a driver to react to a situation and apply the brakes. • uniform – Speed or acceleration that is constant and unchanging. • velocity – A measure of the speed and direction of a moving object.
