PAG 01.1 - Comparing Methods of Determining g

Question 1

What is meant by free-fall?

Determining g

Determining g

PAG 01.1 - Comparing Methods of Determining g

Answer 1

An object is said to be falling in free-fall if the only force acting on it is gravity. This means that no resistive forces are acting (or in practice they are considered
negligible).

Question 2

What is ‘g’?

Determining g

PAG 01.1 - Comparing Methods of Determining g

Answer 2

Gravitational Field Strength (in our case, on the surface of Earth)

Question 3

Why can the SUVAT equations be used in this experiment?

PAG 01.1 - Comparing Methods of Determining g

Answer 3

The SUVAT equations can be used since the object will fall with uniform acceleration. This is because the force of gravity is constant at the Earth’s surface.

Question 4

When plotting a graph of t² against h, how is ‘g’ determined?

PAG 01.1 - Comparing Methods of Determining g

Answer 4

The gradient of the graph will be t²/h. Consequently, the acceleration (‘g’) will be equal to 2/gradient. This comes from the equation s = ut + ½ at², where s=h, a=g and u=0.

Question 5

When plotting a graph of v² against h, how is ‘g’ determined?

PAG 01.1 - Comparing Methods of Determining g

Answer 5

The gradient of the graph will be v²/h. Consequently, the acceleration (‘g’) will be equal to half the gradient. This comes from the equation v² = u² - 2as, where s=h, a=g and u=0.

Question 6

Describe how an electromagnet system can be used to determine ‘g’.

PAG 01.1 - Comparing Methods of Determining g

Answer 6

A magnetic ball bearing can be released by an electromagnet clamped at a known height. The timing system starts when the electromagnet is switched off, and the timer is stopped when the ball lands on the finish pad.

Question 7

When using a clamp stand in this experiment, what safety precaution should be taken?

PAG 01.1 - Comparing Methods of Determining g

Answer 7

The clamp stand should have a counterweight or G-clamp attached to its base to provide a moment to prevent it toppling over.

Question 8

What safety precaution should be taken when using an electromagnet?

PAG 01.1 - Comparing Methods of Determining g

Answer 8

Electromagnets heat up over time. To reduce this heating effect, you should switch it off when not in use.

Question 9

Suggest how light-gates could be positioned to ensure that the ball or dowel falls directly through them.

PAG 01.1 - Comparing Methods of Determining g

Answer 9

A plumb line could be used to demonstrate the expected path of the object. This allows the light-gates to be positioned in appropriate places so that the ball will fall through them.

Question 10

Why is it advantageous to use a small ball-bearing over a larger ball?

PAG 01.1 - Comparing Methods of Determining g

Answer 10

The smaller the ball, the smaller the effects of air resistance. In the case of a small ball-bearing these effects can be considered negligible.

Question 11

Why should there be a gap between the release position and the first light-gate?

PAG 01.1 - Comparing Methods of Determining g

Answer 11

There should be a gap to ensure that the time over which the ball is passing through the light gate is negligible (the ball is moving sufficiently quick enough).

Question 12

Explain why this experiment would not be valid if the air resistance acting on the ball wasn’t negligible.

PAG 01.1 - Comparing Methods of Determining g

Answer 12

The ball wouldn’t be in free-fall since the acceleration would not be purely due to the force of gravity. The acceleration would also be variable since air resistance increases with speed, and so the uniform acceleration equations couldn’t be used.

Question 13

Suggest why your obtained value of ‘g’ may not be the same as the accepted value.

PAG 01.1 - Comparing Methods of Determining g

Answer 13

• Delays in the timing equipment (if using a stop clock, this will be human reaction time)
• Resistive forces are acting
• Errors in height measurements, such as measuring from different positions on the ball each time

Question 14

What is the advantage of using light-gates over a stop-clock in this experiment?

PAG 01.1 - Comparing Methods of Determining g

Answer 14

Using light-gates should result in a lower uncertainty in your time measurements. A stop-clock will involve human reaction times.

Question 15

How could your results be improved?

PAG 01.1 - Comparing Methods of Determining g

Answer 15

You should take repeat readings at each height and then calculate the mean time taken. You should also ensure that height measurements are taken from the same position each time.

Question 16

How should you calculate the uncertainty in your time readings?

PAG 01.1 - Comparing Methods of Determining g

Answer 16

The uncertainty in time will be equal to half the range of your time readings. This can then be converted into a percentage uncertainty.

Question 17

How do you determine the percentage uncertainty in t²?

PAG 01.1 - Comparing Methods of Determining g

Answer 17

To calculate the percentage uncertainty for a variable that is squared, you double the percentage uncertainty of the variable itself. In this case the percentage uncertainty in t² is double the percentage uncertainty in t.

Question 18

When plotting a graph how should you determine the scales for the axes?

PAG 01.1 - Comparing Methods of Determining g

Answer 18

The scales should be chosen so that the graph fills at least half the available space. Using numbers that split easily into the
squares on page (such as multiples of 5) will also make plotting simpler.

Question 19

What is the minimum number of repeat readings you should take in this experiment?

PAG 01.1 - Comparing Methods of Determining g

Answer 19

You should take at least 3 repeat readings at each height. This allows for anomalous results to be more easily identified.

Question 20

What is the equation used to convert an uncertainty into a percentage uncertainty?

PAG 01.1 - Comparing Methods of Determining g

Answer 20

Percentage Uncertainty = (Uncertainty/Mean Value) x 100%

Question 21

How can the percentage difference between your value of ‘g’ and the accepted value be calculated?

PAG 01.1 - Comparing Methods of Determining g

Answer 21

[(Your Value - 9.81)/9.81] x 100%

Question 22

Would you expect your value of ‘g’ to be greater or lower than the accepted value?

PAG 01.1 - Comparing Methods of Determining g

Answer 22

You will most likely obtain a value that is lower than the accepted value, due to air resistance reducing the downwards resultant force acting on the object.