Speed, Distance, Time.

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Speed, distance and time The faster something moves, the greater the distance it covers in a given time, or the less time it takes to cover a given distance. For example, a car that is driven 100km in 2 hours is going faster than one covering 60km in 2 hours, and the fastest 100m sprinter finishes in the least time. Calculating speed To work out speed the following two things must be known: * The distance travelled * The time taken to travel that distance Equation This equation shows the relationship between speed, distance and time. speed (metres per second, m/s) = distance (metre, m) / time (seconds, s) Example: A person runs 200m in 20s speed = distance/time speed = 200m/20s speed = 10m/s distance = speed x time time = distance / speed This graph shows acceleration: This graph shows deceleration: Use this triangle to help you rearrange the equation to: change in speed = acceleration x time taken time taken = change in speed / acceleration Speed-time graphs A speed-time graph tells us how the speed of an object changes over time. Acceleration is caused by a force, eg gravity or friction, acting on a moving object. The stopping distance of a car depends on two factors - thinking distance and braking distance. Under normal driving conditions, the thinking and braking distance depend on the speed of the car. Forces, mass and acceleration When an object accelerates it does so because an extra force is acting on it. For example: * Gravity causes a falling apple to speed up as it falls. * Air resistance causes a parachutist to slow down just after opening the parachute. * The brake pads on a bike cause extra friction, which slows the bike down. The relationship between force, mass and acceleration For the same mass: * more force causes more acceleration * less force causes less acceleration For the

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