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Wonders of Physics

Ticker-tape Timer – Finding acceleration

27 Comments


The diagram shows a strip of paper tape that has been pulled under a vibrating arm by a car moving to the left. The arm is vibrating regularly, making 50 dots per second.  What was the acceleration of the car?

ticker tape

Solutions: View the video tutorial to understand where to take time interval.

The arm vibrating 50 dots per second = frequency of 50 Hz   (50 holes are produced in 1 second)

Hence, the period, T = 1/f = 1 / 50 = 0.02s     (every 0.02s, a hole is created on the tape)

To find acceleration, a, we need to find initial velocity, u,and the final velocity, v.

u = dist / time = 0.02m/0.02s = 1 m/s

v = dist / time = 0.04m/0.02s = 2 m/s

To find acceleration, we can use a = (v-u)/t , but the time, t, taken for the increase in velocity is where most students will make a mistake.

Many will take 7 intervals to calculate the t, which is wrong.

But the intervals should be 6, starting from the middle of first interval and middle of last interval.
This will give a more accurate acceleration.

Hence, t = 6 x 0.02s = 0.12s a = (v-u)/t = (2-1)/0.12 = 8.33 m/s2

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27 thoughts on “Ticker-tape Timer – Finding acceleration

  1. Why 6 intervals, I dont get it. Help me!

    • 6 intervals for the time of acceleration as you have to consider the middle of initial velocity section and the middle of the final velocity section. This will be more accurate. Hope it helps.

    • well the velocity of an accelerating object is always changing at every point, so we take the middle place where it is the average velocity between the two points to make it more accurate.

  2. starting from the middle of the first interval?
    would that be counted as the first official interval? if so, if counted, there are 7 intervals/vertical slashes made.

    • For finding acceleration it is different. To make it more accurate, the time intervals start and end at the middle of the initial motion and final motion sections.

  3. This is correct. You can also use final time minus initial time.
    This will give you 7 X 0.02 = 0.14s minus 0.02s = 0.12s.
    My question – do you use this “change in time” to calculate accelleration even if the dots were evenly spaced – constant velocity?

  4. I counted seven intervals…

  5. how do you find the average speed?

  6. thanks .it had become difficult for me to to calculate acceleration.

  7. Sorry,i would like to know why taking middle of first interval and middle of last interval will be more accurate? If we take the middle interval,does it mean that the velocity calculated is only true on the middle point ? If it is,could you guys please let me know why ?

  8. Sorry,i am also confused that why we dont need to calculate the time before hitting the first dot.I mean that isnt that the ticker timer machine will also need 0.02 second of time interval to hit the first dot on the tape,so before the when the first dot is formed,there is already 0.02s used to produce the first dot? But we are ignoring the 0.02 second when calculating.Does it contain any physics theory?If it does,sincerely asking for the explanation.Thanks a lot ^^

    • When we find the initial speed (velocity) we use the 1st 2 dots. The speed within the 1st 2 dots is increasing. Hence the initial speed is the average in that interval (I.e. like the middle speed). Hence when we consider the initial time for the initial speed before the acceleration, it will be more accurate to consider from the centre of the interval between the 2 dots. Likewise for the final speed and final time. Hope it helps 🙂

  9. Oh,I see,it really helps a lot to me ^^ .But what if the velocity between the 1st 2 dots is constant ? Do we still take the middle point’s speed of the interval ? Or can we take the whole interval instead of half interval during our calculation? So we would not have to minus 1 from the total number of ticker tape when calculating the time.Is it correct? Thank you in advance ^^
    And can you also help me to explain this question:

    Sorry,i am also confused that why we dont need to calculate the time before hitting the first dot.I mean that isnt that the ticker timer machine will also need 0.02 second of time interval to hit the first dot on the tape,so before the when the first dot is formed,there is already 0.02s used to produce the first dot? But we are ignoring the 0.02 second when calculating.Does it contain any physics theory?If it does,sincerely asking for the explanation.Thanks a lot ^^

    THANKSSS

    • Hmm I don’t really understand your question. When finding acceleration the method is also the same. If the velocity is constant there is no acceleration and the intervals are the same.

      Even if the give the distance between 1st n 3rd point to let u find the initial speed, when find the time interval between initial final and final velocity, u will still take the middle ie. The 2nd dot. Cos it’s between 1st and 3rd dots.

      Unless finding the average speed for the car, say they give u the distance from 1st to 10th dot is 20 cm. Then u take distance 10cm divided by time (9 interval x 0.02s)

      U can assume the first dot is made as time =0s

  10. How do you calculate the period of a ticker timer?

  11. Good lesson.A lot has been learnt. Ttanks

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