Acceleration of Freefall Using Electromagnets & Light Gates
Aim of the Investigations
 The overall aim of these investigations is to calculate the value of the acceleration due to gravity, g
 The first two experiments both use the method of dropping an object and either timing its fall, or finding the final velocity
 Both use the SUVAT equations to produce a straight line graph
 The third experiment, using the ramp and trolley, is based on the inclined ramp experiment done by Galileo when he proved that all objects fall at the same rate, regardless of weight
Electromagnet Method
Variables
 Independent variable = height, h
 Dependent variable = time, t
 Control variables:
 Same object being dropped
 Same electromagnet and trap door switching system
Apparatus
 Metre rule, ball bearing, electromagnet, electronic timer, trapdoor, plumb line
Apparatus used to measure g using the electromagnet method
 Resolution of measuring equipment:
 Metre ruler = 1 mm
 Timer = 0.01 s
Method
 By using the plumb line to find the vertical drop, position the trap door switch directly underneath the electromagnet.
 Check that the ball bearing triggers both the trap door switch and the timer when it is released.
 When the equipment is set up correctly;
 As the current to the magnet switches off, the ball drops and the timer starts
 When the ball hits the trapdoor, the timer stops
 The reading on the timer indicates the time it takes for the ball to fall a distance, h
 Measure the distance from the bottom of the ball bearing to the trap door switch with a metre ruler and record this distance as height, h
 Increase h (eg. by 5 cm) and repeat the experiment. At least 5 – 10 values for h should be used
 Repeat this method at least 3 times for each value of h and calculate an average t for each
Table of Results
Analysis of Results
 The acceleration is found by using one of the SUVAT equations and rearranging it to create a straight line graph (y = mx + c)
 The known quantities are

 Displacement s = h
 Time taken = t
 Initial velocity = u
 Acceleration a = g
 The missing SUVAT value is final velocity, v
 Therefore use
 Replace a with g and s with h and then rearrange to fit the equation of a straight line
(since initial velocity, u = 0)
 The above equation shows that if h is plotted on the yaxis and t^{2} on the xaxis the graph will produce a straight line with gradient = g
Evaluating the experiment
Systematic Errors:
 Residue magnetism after the electromagnet is switched off may cause t to be recorded as longer than it should be
Random Errors:
 Large uncertainty in h from using a metre rule with a precision of 1 mm
 Parallax error from reading h
 The ball may not fall accurately down the centre of the trap door
 Random errors are reduced through repeating the experiment for each value of h at least 35 times and finding an average time, t
Safety Considerations
 The electromagnetic requires current
 Care must be taken to not have any water near it
 To reduce the risk of electrocution, only switch on the current to the electromagnet once everything is set up
 A cushion or a soft surface must be used to catch the ballbearing so it doesn’t roll off / damage the surface
 The tall clamp stand needs to be attached to a surface with a G clamp to keep it stable
Card and Light Gates Method
Variables
 Independent variable = height, h
 Dependent variable = final velocity, v
 Control variables:
 Same card being dropped
 All other equipment is the same
Apparatus
 Metre rule, clear tube with large enough diameter for card to fall cleanly through it, card, blutack, light gate, data logger, plumb line
Method
 Clamp the clear tube vertically using the plumb line as a guide
 Attach the light gate about 20 cm above the bench
 Clamp the metre ruler vertically next to the tube so that the vertical distance from the top of the tube to the light gate can be accurately measured
 Record the distance between the light gate and the top of the tube as height, h
 Cut a piece of card to approximately 10 cm, measure this length precisely and enter it into the data logger as the distance
 Weight the card slightly at one end (a large paperclip or small pieces of blutack can be used)
 Hold the card at the top of the tube and release it so that it falls inside the tube
 The data logger will record velocity
 Repeat this measurement from the same height two more times
 Move the light gate up by 5 cm, record the new height, h, and drop the card three more times, recording the velocity each time
 Repeat for five more values of height
Analysis of Results
 The acceleration is found by using one of the SUVAT equations and rearranging it to create a straight line graph (y = mx + c)
 The known quantities are
 Displacement s = h
 Initial velocity = u
 Final velocity = v
 Acceleration a = g
 The missing SUVAT value is time, t
 Therefore use
 Replace a with g and s with h and then rearrange to fit the equation of a straight line
(since initial velocity, u = 0)
 The above equation shows that if v^{2} is plotted on the yaxis and 2h on the xaxis the graph will produce a straight line with gradient = g
Evaluating the experiment
Systematic Errors:
 The metre ruler needs to be fixed vertically and close to the tube
 All height measurements are taken at eye level to avoid parallax errors
Random Errors:
 Large uncertainty in h from using a metre rule with a precision of 1 mm
 Parallax error from reading h
 The card may fall against the sides of the tube, slowing it down
 Dropping the card from the top of the tube can introduce parallax errors
 Random errors are reduced through repeating the experiment for each value of h at least 35 times and finding an average time, t
Safety Considerations
 The tall clamp stand needs to be attached to a surface with a G clamp to keep it stable
Acceleration of Freefall Using a Ramp & Trolley
 This method of finding acceleration due to freefall uses the SUVAT equations, but applies them to a trolley rolling down an inclined ramp.
Variables
 Independent variable = velocity of the trolley, v
 Dependent variable = time, t
 Control variables:
 Height of ramp must be constant
 Same trolley being used
Apparatus
 Inclined ramp
 Trolley with ≅10 cm card attached
 Light gate and computer or datalogger
 Stopwatch
 Block to prevent slipping
Method
 Carefully cut a piece of card so that it is between 5 – 10 cm in length, and has a height which can break the beam of a light gate as the trolley passes through.
 Measure and record the length, d, of the card
 Record this in the datalogging software
 Attach the card to the trolley and roll the trolley past the light gate checking the beam is broken by the card
 Adjust the height of the light gate as needed.
 Start the timing on the software, making sure it is set to record instantaneous velocity
 Release the trolley and simultaneously start the stopwatch.
 As the card passes the light gate stop the stopwatch
 record the time, t
 Repeat procedure 3 times, discard anomalies and calculate mean t
 This reduces errors
 Repeat the procedure at least 5 times, varying the height the trolley is dropped from for each reading
 This causes a variation in v which is recorded by the lightgate, and t which is recorded using the stopwatch.
Analysis of Results
 The acceleration is found by using one of the SUVAT equations and rearranging it to create a straight line graph (y = mx + c)
 The known quantities are
 Time taken, t = average t
 Initial velocity, u = 0 (the trolley starts from rest)
 Final velocity v = v (recorded by the lightgate)
 Acceleration a = g
 The missing SUVAT value is displacement, s
 Therefore use
 This matches the equation of a straight line
 y = velocity, v
 x = average time, t
 gradient = acceleration, a
 yintercept = initial velocity, u
 Plot a graph of v against average t
 The gradient will be the acceleration
 This acceleration is provided by gravity, and so will give a value for g
Evaluating the experiment
Systematic Errors:
 Make sure for each repeat reading the trolley is released from the same point
 The card should be measured carefully so value d is accurate
Random Errors:
 Large uncertainty in d from using a ruler with a precision of 1 mm
 Reaction time when starting and stopping the stopwatch
 Random errors are reduced through repeating the experiment for each value of v at least 3 times and finding an average time, t
 The card may hit the light gate
 Discard a result where this occurs
 They trolley may not travel straight down the ramp
 Discard a result where this occurs
Safety Considerations
 The trolley may fly off the end of the ramp
 Use a block or tray at the bottom of the ramp to prevent this
Exam Tip
This experiment can be modified by using the light gate to record time through the gate.
You can then use the time from the light gate to calculate the velocity, v of the trolley by calculating with v = d/t where d is the length of the card and the time is the time on the light gate.
However, most lightgate software should allow you to eliminate this step.