Why does a cars speedometer show instantaneous speed?

Complete step by step answer:
The basic design of the speedometer is an analog instrument where the pointer of the speedometer may take up any position on the scale and sweeps through every value from zero to full scale as the car accelerates from rest to full speed. However, now-a-days digital speedometers are extensively used in the market.An analog representation is the measured quantity (in this case, speed) which is converted into another physical quantity (pointer
position) in a continuous way that means the value of the measured quantity is continuously represented by the value of the analogue, and no minimum change or step is required in the measured quantity to cause a change in its analogue. At any moment, the amount by which the pointer has moved from zero is said to be an `analog’ of the speed of the car. This measurement is at a particular moment, or you can say at a particular instant; hence the speedometer measures the instantaneous speed of the car or other vehicles.
Hence, option (C) is correct.

Note: A speedometer is a gauge that measures and displays the instantaneous speed of a vehicle. The speed is defined as the total distance travelled by a body divided by the total time.

When German Formula One racing driver Michael Schumacher broke the Australian Grand Prix lap record in 2004, he completed a 5.303 km lap in 84.125 seconds.

His average speed was:

However, he was able to speed down the straight at speeds of up to 320 km/h.

Clearly, the average speed does not provide much information about the speed at any particular instant during the race.

Keeping track of the speed

The full story of each lap of Michael Schumacher's race could be more accurately told if his average speed was measured over many short intervals throughout the event. For example, if stopwatches were placed at every 100-metre point along the track, his average speed for each 100-metre section of the circuit could then be calculated. On the other hand, if stopwatches were placed every metre along the track, his average speed for each 1-metre section could be calculated. By using more stopwatches and placing them closer together, a more accurate estimate of his instantaneous speed can be obtained. The instantaneous speed is the speed at any particular instant of time.

WHEN TIME TICKS AWAY

A ticker timer provides a simple way of recording motion in a laboratory. When the ticker timer is connected to an AC power supply, its vibrating arm strikes its base 50 times every second. Paper ticker tape attached to the moving object is pulled through the timer. A disc of carbon paper between the paper tape and the vibrating arm ensures that a black dot is left on the paper 50 times every second; that is, a black dot is made every fiftieth of a second.

The average speed between each pair of dots can be determined by dividing the distance between the dots by the time interval. To make calculating the speed easier, every fifth dot can be marked, as shown in the diagram below. Each of the marked intervals on the tape represents five-fiftieths of a second — that is, 0.1 seconds. The average speed during the first interval on the tape shown in the figure below is:

Motion can be recorded with a ticker timer.

Each marked interval represents a time of 0.1 s.

Motion detectors

Sonic motion detectors are used on the bumpers of cars to help the driver detect the distance between the car and another object.

In many classrooms, ticker timers have been replaced with sonic motion detectors. These devices send out pulses of ultrasound at a frequency of about 40 kHz and then detect the reflected pulses from the moving object. The time taken for the pulses to return allows the device to calculate the distance between itself and the object. A small computer in the motion detector allows it to calculate the speed of the object.

THERE MUST BE BETTER WAYS!

Speedometers

The speedometer inside a vehicle has a pointer that rotates further to the right as the wheels of the vehicle turn faster. It provides a measure of the instantaneous speed.

Car speedometers provide a measure of instantaneous speed.

Older speedometers use a rotating magnet that rotates at the same rate as the car's wheels. The rotating magnet creates an electric current to flow in a device connected to the base of the pointer. As the car's speed increases, the magnet rotates faster, the electric current increases and the pointer rotates further to the right.

Newer electronic speedometers use a rotating toothed wheel that interrupts a stationary magnetic field. An electronic sensor detects the interruptions and sends a series of pulses to a computer, which calculates the speed using the frequency of the pulses.

Car speedometers are not 100 per cent accurate. In Australia, an error of up to 10 per cent is common. Speedometers are manufactured according to the diameter of the tyres on the vehicle. Any change in that diameter will make the reading on the speedometer inaccurate.

Speed and road safety

One of the major causes of road accidents and subsequent fatalities and injuries is excessive speed or driving at speeds that are unsafe for the road or weather conditions. Speed limits and speed advisories are set in an effort to minimise such accidents. The police use three different methods to monitor driving speeds as accurately as possible to ensure that speeding drivers are penalised.

• Radar guns and mobile radar units in police cars send out radio waves. The radio waves are reflected from the moving vehicle. However, the frequency of the waves (see OVERARCHING IDEAS: Stability and change: The changing universe) is changed owing to the movement of the vehicle. The change in the frequency, called the Doppler effect, depends on the speed of the moving vehicle. The altered waves are detected by the radar gun or mobile unit. Radar provides a measure of the instantaneous speed. One type of radar unit is linked to speed cameras that automatically photograph any vehicle that the radar reveals is travelling above the speed limit.

• Laser guns send out pulses of light that are reflected by the target moving vehicle. The time taken for each pulse to return is recorded and compared with that of previous pulses. This allows the average speed over a very small time interval to be calculated. Laser guns are useful when traffic is heavy because they can target single vehicles with the narrow light beams. Radio waves spread out, and in heavy traffic it is difficult to tell which car reflected the waves.

• Digitectors consist of two cables laid across the road at a measured distance from each other. Each cable contains a small microphone that detects the sound of a moving vehicle as it crosses the cable. The measured time interval between the sounds is used to calculate the average speed of the vehicle between the cables. Although digitectors were phased out after the 1980s, they are regaining popularity as an alternative to radar and laser guns.

AFL coaches and sports scientists use GPS locators to track the movement of players around the field. The locators are strapped to the upper back of players. A computer is used to analyse the data to provide information about distance covered, speed, time spent moving at different speeds, maximum speed and acceleration. A built-in sensor also monitors heart rate.

The global positioning system

The global positioning system (GPS) uses radio signals from at least four of up to thirty-two satellites orbiting the Earth to accurately map your position, whether you are in a vehicle or on foot. Like radar guns, GPS navigation devices use the Doppler effect to calculate instantaneous speed, usually about once every second.

Ticker timer tapes

KEY INQUIRY SKILLS:

• planning and conducting

• processing and analysing data and information

Equipment:

ticker timer

power supply

scissors

G-clamp

ticker tape (in 60 cm lengths)

• Clamp the ticker timer firmly to the edge of a table or bench so that you will be able to pull 60 cm of ticker tape through it. Connect the ticker timer to the AC terminals of the power supply and set the voltage as instructed by your teacher.

• Thread one end of the ticker tape through the ticker timer so that it goes under the carbon paper disc.

• Turn on the power supply and check that the ticker timer leaves a black mark on the ticker tape.

• Hold the end of the ticker tape and walk away from the ticker timer so that the ticker tape moves through at a steady speed.

• Remove the ticker tape and mark off the first clear dot made and every fifth dot after the first. (There should be four dots between each of the marked-off dots on the ticker tape.) Measure the distance travelled during each 0.1 s interval and write it on your tape. Label the intervals as interval 1, interval 2, interval 3 etc.

• Cut your ticker tape into 0.1 s intervals and glue the strips in order onto a sheet of paper. Each strip shows the distance travelled during a 0.1 s time interval. The graph therefore shows how the speed changes with time.

DISCUSS AND EXPLAIN

1. How much time elapsed between the printing of the first clear dot and the last dot marked off?
2. Calculate the average speed for the motion that took place between the printing of the first clear dot and the last marked dot.
3. Calculate the average speed during each 0.1 s interval.
4. Did you succeed in keeping your speed steady?

   Using ticker tape to plot a graph

REMEMBER

1. Explain the difference between instantaneous speed and average speed. Use an example to support your explanation.

2. Which methods are used by the police to measure:

   1. average speed

   2. instantaneous speed?

3. Make a list of reasons why a speedometer reading might not be accurate. Include in your list anything that could change the diameter of the vehicle's tyres.

4. After being phased out, digitectors are making a comeback in police detection of speeding drivers. Suggest advantages they might have over radar and laser guns.

5. Calculate the average speed during the second and third 0.1 s intervals of the ticker tape shown in INQUIRY: INVESTIGATION 8.1.

6. Use data-logging equipment with a motion detector or light gates to record the motion of a toy car or cart down a slope. Use the software to produce a graph of distance versus time and a graph of speed versus time. Comment on the shape of your graphs.

   Why does speedometer measure instantaneous speed?

   As the interval over which an average speed is measured becomes shorter and shorter, so this speed becomes closer and closer to the instantaneous speed. What you see on the speedometer of a car is the speed at that instant or moment – the instantaneous speed.

   Does the car speedometer measure instantaneous speed?

   The speedometer of a car reveals information about the instantaneous speed of your car. It shows your speed at a particular instant in time.

   What is the instantaneous speed reading in the speedometer?

   At a given instant time what we read from the speedometer is instantaneous speed.

   Why do speedometers Cannot accurately measure the instantaneous velocity of a car?

   A car speedometer measures only speed. It does not give any information about the direction, and so does not measure velocity. 2. By definition, if an object has a constant velocity, then both the object's speed and its direction of motion are constant.