MeasurementPhysics is based on measurementWhat is the time interval between two clicks of a counter?

What is the temperature of the air?

What is the speed of typhoon?

Measurement

How fast?

How long?

How high?

We start learning physics by learning how to measure……..

Time

TimeTo study motion, we need to find out how

quickly an object changes its position.

Besides watches, can you think up other devices which can help us to measure and record the time and motion?

Can they measure very short time intervals?

Any other method / device?

For example

i. by photos

Any other method / device?

For example

ii. by video

we use a ticker-tape timer to record the motion

In the laboratory,

A ticker-tape timer -- is basically a vibrator,-- puts black dots on a tape

Experiment

The ticker-tape timer

1. Set up the ticker-tape timer.

2. Put a length of paper tape through it under the carbon paper disc.

3. Slowly pull the tape for about 5 s (record with stop watch)

4. Count the number of ticks on the tape

5. Calculate the time interval for 1 tick,

There are 246 dots marked on the tape for 5 s. How many ticks do the timer make in 1 second?

For example

In 5 s, 245 ticks are marked.

Number of ticks per second = 245 /5

= 49

A ticker-tape timer -- is basically a vibrator,-- puts black dots on a tape at the rate of 50 ticks per second, 50Hz

A ticker-tape timer -- is basically a vibrator,-- puts black dots on a tape at the rate of 50 ticks per second, 50Hz

The error is due to

1. A miscount of the dots which are too close together2. The reaction time in using the stop-watch.

A ticker-tape timer -- is basically a vibrator,-- puts black dots on a tape at the rate of 50 ticks per second, 50Hz

What is the time between these two adjacent dots?

Time interval between two dots= 1/ 50 s

= 0.02 s

The time interval between 2 adjacent dots on the tape is called a ‘tick’

Time interval between two dots= 1/ 50 s

= 0.02 s

1 tick = 0.02s

By counting the dots marked on the tape, we can work out the time taken.

4 ticks!

Time interval = 0.02s x 4 = 0.08s

Besides, a ticker-tape time can use to record the motion (speed).

Studying motion

Analyze a ticker-tape

According to the distribution of the dots, describe the motions recorded on the following

tapes.

START END

START END

START END

START END

Constant motion

Constant motion

Speed increasing

Speed decreasing

1 .

A t a p e i s p r o d u c e d b y a t i c k e r - t a p e t i m e r w h i c h t i c k s a t a f r e q u e n c y o f 1 0 0 H z .( a ) W h a t i s t h e t o t a l t i m e o f t h e t a p e ?( b ) W h a t i s t h e a v e r a g e s p e e d o f t h e t a p e ?

1.(a)total time = 8 1

100 = 0.08 s

(b)average speed = distance

time = 76

008

.

. = 95 cm s1 = 0.95 m s

1

2. The speed of a car is 50 km h 1.

(a) What is the speed of the car in m s

1 ?(b) Calculate the distance travelled of the car in 15 s.

2. (a) speed = 50 km h1 = 501000m

6060s

= 13.9 m s

1

(b) distance travelled = speed time = 13.9 15 = 208.5 m