Experiments

Question 0

Explain what experiment Galileo developed to investigate why a ball rolling down a slope speeds up

Answer 0

• he positioned bells on a slope at lengthening intervals so the bells rang at equal, 0.5s intervals as the ball hit them
• to calculate the average speed of the ball between each bell he divided the bell’s distance from the start by the time it took the ball to get there
• as we are calculating the average speed between each 0.5s interval, to enter the graph onto a velocity-time graph we plot the middle of each interval (0.25s)
• the line is straight showing the ball’s velocity is increasing by equal amounts in equal time periods
• this is known as uniform acceleration

Question 1

Explain a typical lab experiment to investigate the relationship between force, mass and acceleration…

Answer 1

• a nylon line with a mass hanger on one side is attached to a trolley with mass weights on it
• the trolley is positioned on a slope and the nylon line runs over a pulley
• a digital camera taking pictures at equal intervals, or a set of light gates or data logger can be used to find the speed at the top and near the end

Question 2

How can you model terminal velocity in a lab?

Answer 2

Use a tall measuring cylinder filled with water and drop small diameter (1-2 mm) glass beads into it

Or…

Use a thicker liquid such as glycerine or oil and use small diameter ball bearings

Measure the terminal velocity using a light gate and a data logger

Question 3

How would you set up a circuit to investigate Ohm’s Law?

Answer 3

• gather a voltmeter, an ammeter, a battery, a variable resistor and a switch
• set them up in series EXCEPT the voltmeter, which is connected in parallel, using a piece of wire to split the series
• with the switch closed, the readings on the ammeter and voltmeter are noted
• the value of the variable resistors is then altered and new readings are taken
• repeat this process 6 times
• place results in a graph of current (I) against voltage (V)
• the graph should be a straight line, telling us that the current flowing through the piece of wire is directly proportional to the voltage applied across its ends

Question 4

What famous toy can be used to demonstrate the actions of longitudinal waves, and how?

Answer 4

• the slinky
• one end of the slinky is pushed backwards and forwards
• this causes sections of the spring to bunch up (compressions)
• and sections of the spring to stretch out (rarefactions)
• these points of bunching and stretching look as if they are travelling down the length of the spring from the hand of the person doing the moving backwards and forwards

Question 5

Explain a crude way of measuring the speed of sound..

Answer 5

• find a large flat wall that allows you to hear an echo
• clap your hands at regular intervals so that the echo is heard halfway between each clap
• once a good rhythm has been established, time 20 claps
• measure the distance to the wall and calculate how far the sound has travelled by doubling that figure and multiplying by 20
• speed of sound = distance in metres / time in seconds

Question 6

Briefly outline an experiment to show how sounds does not exist in a vacuum, and explain the conclusion of this experiment..

Answer 6

• secure a bell in a jar with a vacuum pump attached to the bottom
• at first you can see and hear the bell ringing, however if you remove the air using the vacuum pump you can still see the bell ring but it cannot be heard
• this shows that light waves can travel through a vacuum but sound waves cannot

Question 7

How can you use resonance to measure the speed of sound?

Answer 7

• gather a resonance tube and some tuning forks
• a resonance tube is a tube with a water reservoir. The height of the water in the tube can be adjusted to change the length of the tube, as the sound waves will reflect at the waters surface
• strike a tuning fork at a known frequency then hold it above open end of the tube
• the water column is adjusted until the loudest sound can be heard
• the first resonance will be heard when the length of the tube is equal to a quarter of the wavelength
• you can check your result by lowering the water to find the next resonance, at 3/4 of the wavelength
• they speed of sound = frequency (f) x wavelength (λ)

Question 8

How do you measure the speed of sound with an oscilloscope?

Answer 8

• gather two microphones, an oscilloscope, a signal generator and a loud speaker
• connect the microphones to the oscilloscope and the signal generator to the speaker
• set the signal generator to give a sound with a frequency of about 1 kHz
• start with the microphones close together and observe how the two traces on the oscilloscope compare
• then, move one microphone further away from the loudspeaker until it is one complete wavelength away from the first (you know you have got to this point when the traces on the oscilloscope screen are exactly above one another)
• measure the distance between the microphones to get the wavelength of the sound and use the oscilloscope screen to find an accurate value for the frequency
• the speed of sound can be calculate with the formula v = f x λ

Question 9

Describe an experiment that could be set up in a school lab to investigate refraction..

Answer 9

The apparatus and materials gathered for each student or group would be:

• semicircular perspex or glass block
• lamp, stand, housing and holder
• single slit
• power supply for lamp
• scissors
• white paper
• card and suitable adhesive

The procedure is as follows:

• stick the card to the flat face of the semicircular block so only a vertical slit is exposed at the middle of the flat face
• place the block onto the sheet of paper and direct a ray streak onto the slit
• observe the ray tracks and measure the angles
• change the angle with which the ray strikes the flat face and record the angles of incidence again
• repeat as required
• values of sin i / sin r can be calculated for each pair of readings, or a graph plotted of sin i against sin r and the gradient calculated can give the refractive index
• students may also direct the ray into the curved face to observe refraction and total internal reflection