Time the swing — questions

1. identify

A pendulum bob passes a marker under its lowest point moving to the right. Which of these is one complete oscillation? Then say why the marker is put at the lowest point and not at the end of the swing.

Circle one answer.

• From the marker moving right to the far right end of the swing
• From the marker moving right to the next pass of the marker, moving left
• From the marker moving right until it next passes the marker moving right
• From the right end of the swing to the left end

Why put the marker at the lowest point?

Nudge (only if you are stuck): One oscillation ends where it started, moving the same way.

2. calculate

A pupil times 20 oscillations of a pendulum: the stop-clock reads 28.40 s. Calculate the period of the pendulum to 3 significant figures.

Period of the pendulum: ______________ unit ________

Nudge (only if you are stuck): The period is the time for one oscillation. How many oscillations does 28.40 s cover?

3. identify / explain

A pupil has a hand-held digital stop-clock that shows hundredths of a second. Her reaction time is about 0.2 s. Which of these can she time well by hand? Then explain your choice.

Circle one answer.

• A ball falling 0.5 m (about 0.3 s)
• A camera flash
• Twenty swings of a pendulum (about 25 s)
• One bounce of a ball on a table

Why can she time this one well?

Nudge (only if you are stuck): Compare each time with a reaction time of about 0.2 s. Which one is long enough for 0.2 s not to matter much?

4. state

A pupil’s notebook says: “Pendulum 25 cm: twenty swings 20.1 seconds, so one swing 1.0 seconds.” Write the three column headings for a results table of these readings, each with its quantity and unit (in metres and seconds).

Column 1 heading

Column 2 heading

Column 3 heading

Nudge (only if you are stuck): Each heading is a quantity, a slash, then its unit. Which unit does the length need: cm or m?

5. identify

A pupil wants to measure the period of a pendulum as accurately as possible with a hand-held stop-clock. Choose every method that makes the period more accurate.

Tick every correct statement.

• Time 20 oscillations and divide by 20
• Start and stop the clock as the bob passes a marker at its lowest point
• Say “one” as you start the clock
• Repeat the timing and take the mean
• Start and stop the clock at the end of the swing, where the bob stops

Why does timing 20 oscillations help?

Nudge (only if you are stuck): Which of these share the reaction-time error over more swings, or make the moment easier to judge?

6. calculate

Three timings of 20 oscillations of the same pendulum are 25.32 s, 25.48 s and 25.40 s. (a) Calculate the mean time for 20 oscillations, to 4 significant figures. (b) Calculate the period, to 3 significant figures.

(a) Mean time for 20 oscillations: ______________ unit ________

(b) Period: ______________ unit ________

Nudge (only if you are stuck): Add the three times and divide by 3. Then divide by the number of oscillations.

7. explain

In the experiment, three timings of a single swing of the same pendulum were 1.43 s, 1.10 s and 1.38 s, but the pendulum’s period did not change. Explain why the readings differ, and why timing 20 swings gives a much more precise period.

Nudge (only if you are stuck): Who pressed the stop-clock? How big is a reaction time compared with one swing?

8. plot / estimate

Use the four results in the lesson table (length 0.100, 0.200, 0.400 and 0.500 m; period 0.63, 0.90, 1.27 and 1.42 s). On graph paper, (a) choose scales and plot period against length, (b) draw the best line through the points, and (c) use your graph to estimate the period for a length of 0.300 m.

(a) Axes and scales you chose

(b) The line you drew, and why

(c) Period for 0.300 m / s

Nudge (only if you are stuck): Which quantity did you change? It goes on the horizontal axis. Do the points lie on a straight line?

9. state / explain

Match each timing job to the best timer, using each timer once. Then explain why a hand-held stop-clock is no good for the steel ball.

Write the number of the matching option.

1. A clock or watch with a second hand
2. A hand-held stop-clock
3. Light gates and an electronic timer

• A kettle of water coming to the boil: ______
• A friend running 100 m: ______
• A steel ball falling 0.50 m: ______

Why not a stop-clock for the falling ball?

Nudge (only if you are stuck): Compare each time with a reaction time of about 0.2 s.

10. calculate / state

Water drips from a leaking tap at a steady rate. A pupil starts a stop-clock at one drip, counting it as zero, and stops it at the 50th drip after that: the clock reads 31.0 s. (a) Calculate the time between two drips. (b) She repeats the timing twice more. Write the column headings for a table of her three timings and the time between drips.

(a) Time between two drips, with working

(b) Column headings

Nudge (only if you are stuck): How many gaps between drips did she time? Then: quantity / unit for each heading.