Module 3 Practicals

Question 1

Determine g
- Method (5)
- Graphs and Calculations (2)
- Safety (2)
- Validity (6)

Answer 1

• Attach two light gates to a clamp stand
• Measure the distance between the light gates (h) with a metre ruler
• Switch on an electromagnet and attach a ball bearing
• Switch off electromagnet and measure time taken (t) for the ball bearing to pass through the light gates using a data logger connected to the light gates
• Change the distance between the light gates and repeat
• Plot a graph of 2h against t2
• • s=ut+½ at2 → 2h = gt2 (y = mx)
• g = the gradient of the graph
• Place a pad beneath the light gates to avoid the ball bearing bouncing upwards and causing injury
• Place a counterweight on the clamp stand to avoid it tipping over
• Place the light gates as close as possible to the electromagnet
• If the distance of fall is too large or the ball is too small, air resistance may have a noticeable effect
• Keep the distance between the electromagnet and the upper light gate the same each time
• • So the ball bearing reaches the upper light gate with the same speed each time
• Repeat the experiment 3 times for each height and find a mean
• Using light gates and data loggers are more accurate than using a stopwatch due to reaction times
• Adjust the current of the electromagnet so that it only just supports the ball bearing as to high a current would result in a delay in release

Question 2

Determine the Terminal Velocity of a Falling Object
- Method (2)
- Graphs and Calculations (1)

Answer 2

• Use light gates connected to a data logger
• Time when each of the two light beams are broken by the object is recorded
• Velocity = distance between two light gates/time taken

Question 3

Determine the Terminal Velocity of an Object in a Liquid
- Method (3)
- Graphs and Calculations (3)
- Safety (1)
- Validity (3)

Answer 3

• Wrap elastic bands around tube of viscous liquid at set intervals measured by ruler
• Drop ball into tub and record the time it reaches each band using a stopclock
• Use a strong magnet to remove the ball bearing from the bottom of the tube
• Calculate time taken to travel between consecutive bands
• • calculate average time for each experiment
• Calculate mean velocity = distance/mean time of the bearing between each set of bands
• Plot a graph of velocity against cumulative time
• • The velocity which the graph tends to is the terminal velocity
• Use a viscous liquid that doesn’t cause skin irritation
• Repeat 4 times to reduce effect of random errors
• Use a teller tube to allow bearing to travel at terminal velocity for longer
• Using larger intervals between band reduced percentage uncertainty in both distance and time between bands

Question 4

Finding the Stopping Distances for a Range of Starting Velocities
- Method (6)
- Graphs and Calculations (2)
- Validity (2)

Answer 4

• Put an interrupter card on top of block of wood and measure its length using a metre ruler
• Set up a light gate so that it records the average starting velocity of the block moving through it (around 2cm away from start position)
• Record the block starting position
• Push the block of wood through the light gate
• Measure the distance from the light gate to where the wood stopped
• Return block to its starting position and push it again with a different velocity
• Initial velocity = length of interrupter card/time on data logger
• Plot a graph of sopping distance against starting velocity2
• • Should be straight line through the origin as:
• • Ek of block = ½ mv2 = force x stopping distance (as all EK converted to thermal energy by friction)
• Interrupter card allows the distance moved through the light gate to be fixed
• Surface block is pushed on and block material should stay constant to frictional force varies as little as possible

Question 5

Finding the Centre of Mass/Gravity of an Object
- Method (5)

Answer 5

• Make three holes in random places around the edge of the object
• Freely suspend the object from a clamp stand at once of the holes
• • Let it swing and come to rest
• Use a plumb line to draw a vertical line downward on object from point of suspension to bottom of object
• Repeat from different points of the object
• The centre of mass will be the point where all the lines drawn intersect

Question 6

Determine the Young’s Modulus of a Metal
- Method (6)
- Graphs and Calculations (6)
- Safety (3)
- Validity (2)

Answer 6

• Measure diameter of wire (repeat x3) with a micrometer
• Clamp wire horizontally across a table with masses hanging off it via a pulley
• Attach a metre ruler to the table and place a marker on the wire where 0cm is
• Measure the original length of the wire from the clamp stand to the marker
• Attach a mass to the end and record total mass and new position of marker
• Repeat this six times
• Calculate the mean diameter of the wire and so find the average cross sectional area of the wire
• Calculate the force exerted on the wire each time using F = mass x g
• Calculate the extension of the wire each time using extension = new position of marker - original length
• Calculate the stress (F/A) and strain (x/L) for each reading
• Plot a stress-strain graph and draw a line of best fit
• The gradient of the line of bets fit is Young’s modulus
• Wear eye protection as wire could snap and fly off
• Place some cushioning beneath the masses so they will not bounce back up if the wire snaps and releases them
• Place horse shoe shaped protectors over wire to prevent flying off if it snaps
• Make original length of the wire as long as possible to reduce the uncertainty
• Make sure wire is relatively thin as thinner the wire, greater the extension
• • A larger extension will reduce the uncertainty

Question 7

Investigating a Property of Plastic
- Method (7)
- Graphs and Calculations (3)
- Safety (1)
- Validity (1)

Answer 7

• Use a guillotine to slice a plastic bag lengthways and widthways
• Hole punch one end of the plastic strip
• Attach plastic strip to a clamp stand and measure its original length while taut with a ruler
• Attach a 100g mass to the plastic from hole punch and measure new length
• Repeat with more masses
• Apply this method to other strips that are widthways/lengthways
• Repeat the experiment recording the new length after removing masses to get data for unloading as well as loading
• Calculate extension using: extension = new length - original length
• Calculate the force applied using: Force = mass x g
• Plot a graph of force against extension for loading and unloading
• Cushion the floor below so masses do not bounce back up if they fall
• Using a hole punch means the force isn’t evenly distributed through the strip but is concentrated near the hole

Question 8

Investigating Springs in Series and Parallel
- Method (3)
- Graphs and Calculations (4)
- Safety (2)
- Validity (2)

Answer 8

• Attach 2 springs in series/parallel and measure original length using a ruler
• Attach the mass and measure the new length of the springs
• Add more springs and repeat
• Calculate extension for each combination using: new length - original length
• Find the spring constant for each combination using: k = Force applied (mass x g) / extension of combination
• Series:
• • In theory: 1/kcombination = 1/k1 + 1/k2 + …
• • • Since xcombined = x1 + x2
• • • F=kx so F/kcombined = F/k1 + F/k2 and divide by F
• Parallel:
• • In theory: kcombination = k1 + k2 + …
• • • Since Fcombined = F1 + F2
• • • F=kx so xkcombined = xk1 + xk2 and divide by x
• Do not extern too high a force on the springs so the springs don’t break
• Wear eye protection
• Using too high a force can permanently deform the springs
• Always measure from the same point - use a marker