Mod 1 Standard Answers Flashcards
Measuring Cross-Sectional Area
Measure the diameter using a digital calliper at six different positions, and at six different angles. Then calculate a mean average diameter.
Calculate the cross-sectional area using A = πd^2 / 4.
Light Gates (measuring average speed over a fixed distance)
• Connect two light gates to an electronic timer. When the object passes the first light gate, the timer will start. When the object passes through the second light gate, the timer will stop.
• Measure the distance that the object has travelled between the two light gates using a meter ruler.
• Using speed = distance / time, calculate the average speed of the object.
Light Gate (measuring instantaneous speed of an object)
• Connect a light gate to an electronic timer. When the object first intercepts the light gate’s beam, the timer will start. Once the object has passed through the light gate and the light gate’s beam is re-established, the time will then stop.
• Using this time of intercept and the length of the object, calculate the speed of the object by speed = length of the object / time of intercept.
Electromagnet and Trapdoor/Pressure Pad (alternative to two light gates)
• Connect an electromagnet and trapdoor to an electronic timer. When the electromagnet releases the metal ball, the timer will start. When metal ball hits the trapdoor/pressure pad, the timer will then stop.
• Measure the distance the ball has fallen between the electromagnet and the trapdoor, using a meter ruler.
• Using speed = distance / time, calculate the average speed of the object.
• Note: There may be a delay time between the electromagnet releasing the metal ball and the starting of the electronic timer.
Measuring Time Period (SHM)
• Measure the time taken for n oscillations using a stopwatch. Then divide this time by n to calculate the time period of the simple harmonic motion (n 10).
• By increasing the time interval that we are measuring over, the percentage uncertainty of the value of T obtained is reduced.
• Use the equilibrium position as the reference position: Start and stop the stopwatch when the object passes this position.
• Use a fiducial marker to indicate this reference position. Ensure that you keep the marker at eye line to avoid parallax error.
• The object is moving fastest at the equilibrium position, so by starting/stopping the stopwatch at this point, we reduce the uncertainty of this measurement.
• Use a position/motion sensor and datalogger to eliminate the random error (reaction time starting/stopping stopwatch).
Measuring Time Period (SHM) for Small Angles of Swing (ie. not able to time for multiple oscillations)
• Use a protractor to measure the angle of swing (relative to the equilibrium position). For small angles, ensure that this angle does not exceed 10°.
• Use a position sensor and datalogger to measure the time for one full oscillation. The position sensor improves the accuracy of the time measurement.
• Or, use a slow motion camera with an onscreen timer to determine the time taken for one full oscillation (this technique is useful for other experiments where we experience difficulty in determining the time taken for one oscillation by eye).
Experiments Using Visible Light
• The experiment should be carried out in a dark room. This allows the experimenter to determine more accurately:
i) When the light is on/off
ii) The path/position of the light
Eliminating Parallax Errors
• Place a set square against a metre ruler, in line with the edge of the spring and your eye line. This will eliminate the parallax error in your measurements of length/extension.
Increasing / Decreasing Temperature
• Some thermal energy would be lost to the beaker/container and the surrounding air. Insulate the beaker and use a lid to reduce this.
• The liquid will be heated from one position, causing an incorrect temperature measurement. Continuously stirring the liquid ensures that there is a uniform temperature throughout.
• Using a digital thermometer, which has a greater resolution, reduces the percentage uncertainty in the temperature measurement.
• We assume that the liquid is being heated at a constant rate.
• Water baths can be used to investigate the effects of temperature on an electrical component. Add ice and continuously stir the water to decrease the temperature of the liquid uniformly by small intervals at a constant rate.
Data Loggers
• Data loggers automatically take many, continuous readings.
• If a data logger is connected to a computer, it can display readings by plotting a graph on the computer screen.
• If used in an electrical circuit, connect a data logger into the ammeter/voltmeter in the circuit diagram.
• Note: Data loggers do not necessarily have a greater resolution. So, data loggers do not reduce the percentage uncertainty of our measurements.
Graphs with Error Bars
• Error bars indicate the absolute uncertainty of the measured value.
• The worst acceptable line must pass through the vertical part of every error bar.
• Calculating the percentage difference between the gradients of the line of best first and the worst acceptable line will give you the percentage uncertainty in the gradient’s value.
Determining the Position Where an Object Lands
• Place a sand pit where the object is due to land. The object will mark the position where it first makes impact with the ground in the sand.
• Or, paint the object that is being thrown. The paint on the object will mark the position where the object first contacts the ground.
Taking Measurements…
• Measure [quantity] of the [object] with [equipment].
• Use [equation] to calculate the [quantity] of the [object].
• Vary X to change the value of Y, keeping Z constant.
• Record a minimum of 6 pairs of values for X and Y.
• Take multiple repeats of the measurement of X, and then calculate a mean average value for X.
• Take measurements of larger values of X to reduce the percentage uncertainty.
Plotting Graphs…
• Plot a graph of Y against X.
• Draw a line of best fit through the data points.
• The line of best fit is a straight line through the origin (if Y and X are directly proportional).
• Write “y = mx + c” underneath the formula and associate different parts of the formula with “m” and “c”.
• Gradient = “m”, and y-intercept = “c”.
• Use a big triangle to calculate the gradient.
Working Safely…
• Light: Avoid looking directly at the light source. Avoid shining the light source at another person / in own eye. Wear laser / light safety goggles.
• Diode/LED: A fixed resistor is placed in series with the diode/LED. This limits the current through the diode/LED to ensure that it does not overheat and break.
• Circuits: Switch off in between measurements to avoid overheating. Avoid touching hot components/wires.
• Taut Wire/Spring: Safety goggles should be worn in case wire/spring snaps and injures eyes.
