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YEAR 8 SCIENCE

CHAPTER 13: SPEEDCHAPTER 13: SPEEDSpeed is a measure of the distance covered by an object in a certain time. Speed is given by the formula: Distance travelled Time taken The units for speed are m/s and km/h. Speed can be measured using: (i) Speedometer (ii) Speed trap gun (iii) Stopwatch (iv) Light gates

CHAPTER 13: SPEEDThe speedometerThe speedometer in a vehicle is connected by a cable to a wheel shaft.When the wheel shaft turns, the wire in the cable also turns. At the other end of the wire is a magnet. It spins around when the wire turns. The magnet is surrounded by a circular metal cup which is affected by the spinning magnet. CHAPTER 13: SPEEDAs the speed of the spinning magnet increases, the cup also turns faster. The hairspring holds the cup to prevent it from turning too much. The pointer turns with the cup and moves across the scale of the speedometer dial.CHAPTER 13: SPEED

Speedometer CHAPTER 13: SPEED

Speedometer dial CHAPTER 13: SPEEDThe speed trap gunThe speed trap gun is a radar gun.When the gun is fired at an approaching vehicle, a beam of radio waves travels to it through air. The radio waves are reflected off the front of the vehicle and returns to a receiver on the gun. A computer in the gun compares the time difference between sending and receiving the beam. The computer then calculates the vehicles speed.

CHAPTER 13: SPEED

Speed trap gunCHAPTER 13: SPEEDThe stopwatch The stopwatch is started as the object passed the start line and stopped when the object passed the finish line. The distance travelled from the start line to the finish line is measured and divided by the time measured by the stopwatch. This gives the speed of the object. CHAPTER 13: SPEED

Stopwatch CHAPTER 13: SPEEDLight gates In a light gate, a beam of light shines onto a light-sensitive switch.When the beam of the light gate used at the start of a speed test is broken by an object passing through it, the switch starts an electronic stopwatch.When the beam of the light gate used at the finish of the speed test is broken by the object passing through it, it causes the stopwatch to be stopped. CHAPTER 13: SPEEDThe speed of the object is then calculated by dividing the distance between the light gates by the time as measured by the electronic stopwatch.

Light gatesCHAPTER 13: SPEEDA distance/time graph shows the distance travelled by an object over a period of time. The distance travelled by the object is recorded on the vertical axis and the time taken for the object to travel the distance is recorded on the horizontal axis. The gradient of the graph gives the speed of the object. The greater the gradient, the greater the speed of the object.CHAPTER 13: SPEED

Distance/time graphStationary Constant speedConstant speed(object moving to back to its starting point)Graph AGraph BCHAPTER 13: SPEEDCalculations involving distance/time graph.1. Calculate the speed for Graph A. Speed = Distance travelled Time taken = 10 2 = 5 m/s

CHAPTER 13: SPEED2. Calculate the speed in the first 4 seconds for Graph B. Speed = Distance travelled Time taken = 8 4 = 2 m/sCHAPTER 13: SPEED3. Calculate the speed from 4 seconds to 7 seconds for Graph B. Speed = Distance travelled Time taken = 0 3 = 017CHAPTER 13: SPEED4. Calculate the speed for the last 3 seconds for Graph B. Speed = Distance travelled Time taken = 8 3 = 2 2/3 m/s

CHAPTER 13: SPEEDWhen something moves it goes in a particular direction.When the speed and the direction of movement are given together this is called velocity.