1 4.1 Time The SI unit of time: second (s) 1Units of time No. of... Definition: 1 second is a certain number of vibrations of ‘light wave’ emitted by a cesium atom. (This precise definition is not required at this stage) Symbol of time: t

2 4.1 Time 2Measuring time intervals Stop-watch is a common instrument for measuring time intervals. Some stop-watches are accurate to 0.02 s. aStop-watches and reaction time

3 4.1 Time average ~ 0.2 s short eventlarge error This time is called reaction time. Seeing an event Starting/stopping stop- watch by hand time 2Measuring time intervals aStop-watches and reaction time

4 4.1 Time Example (On the back of p.1, notes) Suppose that using a stop-watch, Mr. Chow measured the time of fall of a duster from a height. The time recorded: 1.1s Mr. Chow’s reaction time: 0.2s What is the possible percentage error?

5 4.1 Time Solution Possible error in starting the stop-watch= Possible error in stopping the Stop-watch= Total possible error = 0.2s 0.2s+0.2s=0.4s

6 4.1 Time Solution (continue) So that the possible percentage error = (0.4s/1.1s) × 100% = 36% Large % error! reading unacceptable

7 4.1 Time Is a stop-watch suitable to measure the time interval of tossing a coin (~ 1 s)? NO 2Measuring time intervals aStop-watches and reaction time

8 4.1 Time Percentage error in timing Question ? Example 1 e.g. 1

9 4.1 Time To avoid reaction time error... blocking light beam 2Measuring time intervals bElectronic timing (book 1B, p.4) use devices that automatically start & stop timing

Time 2Measuring time intervals Timer scaler accurate to the nearest 10 –3 s devices that start & stop timing automatically bElectronic timing

Time 1A ____ is the average time for…A ____ is the average time for… 2Which of the following is/are…Which of the following is/are… Check-point 1

Time A __________ is the average time for the Moon to complete 1 revolution round the Earth. A __________ is the average time for the Earth to complete 1 revolution round the Sun. A __________ is the average time for the Earth to complete 1 rotation about its own axis. day month year Q1A _____ is the average...

Time Q2Which of the following... (T/F) The timer-scaler can measure time intervals to the nearest millisecond. The timer-scaler can avoid reaction time error. Are there advantages for using timer- scaler (controlled by light gates) over stop-watch (controlled by hands)?

Time The End

Time Example 2 (Book 1B. P.4, Example 1) Percentage error in timing A stop-watch is used for measuring a pendulum （單擺） to swing 1 cycle. The reading = 1 s Reaction time = 0.2 s

Time (a)If you times 1 cycle, what is the possible percentage error? Possible error (starting) = 0.2 s Example 2 Possible error (stopping)= 0.2 s Percentage error in timing Total possible error = = 0.4 s = (0.4 / 1) × 100% = 40% Large % error! reading unacceptable Possible percentage error (1 cycle)

Time (b)If you times 10 cycles and takes the average, what is the possible percentage error? Example 1 Time for 10 cycles  10 s Percentage error in timing = (0.4 / 10) × 100% = 4% Possible percentage error (10 cycles)

Time Return