9.2 Space Practicals

# 9.2 Space Practicals

## PROJECTILE MOTION PRACTICAL

### General Note:

This practical is an example of the sort of practical that fulfils Syllabus point 9.2.2 column 3 dot point 2 – “perform a first-hand investigation, gather information and analyse data to calculate initial and final velocity, maximum height reached, range and time of flight of a projectile for a range of situations by using simulations, data loggers and computer analysis”.

This practical can be adapted to include a projectile launcher that can be swivelled to provide various angles of launch other than horizontal.  A simple car launcher from a child’s racing set can be used.

A video camera could be used to assist in the analysis of the motion.  Perhaps there are digital cameras available now that allow multiple exposures to be taken.  In that case, single images could be analysed on a computer, provided there is a clear scale visible in the photos.

There is a very good practical described in “Physics Contexts 2” by D Heffernan et al published by Longman – Investigation 1.2A Projectile Motion.  This is similar to the practical described below but uses a timing pad attached to the data logger to measure time of flight.  This is OK if your timing pad has a response time that is short enough to register the impact of the projectile as it lands and bounces off the pad.

Note also that the Syllabus requires the use of simulations as well as actual hands on practicals when studying projectile motion.  I suggest the use of the following applet.  There are others available.

- Click on this link and then page down until you find the Projectile Motion Applet.

## PRACTICAL

### Aim:

To measure the initial velocity and time of flight of a projectile using a data logger and computer technology.

### Method:

¨      Set up the apparatus as shown in the photograph below.  Note that the 30cm ruler is included only to give an idea of the size of the apparatus.

¨      A steel ball (about 2 cm diameter) is released from various heights along the launching track.  An Allen key pushed through holes drilled in the side of the launching track can be used as a stop for the ball and can be easily withdrawn to set the ball in motion down the track.  The final few centimetres of our track is horizontal.

¨      A light gate is positioned at the bottom end of the launching track.  Another is positioned where the projectile is seen to land.

¨      The light gates are plugged into the digital inputs of the data logger and the data logger attached to a computer.  We use the Data Harvest Data Logger and the Sensing Science Laboratory software (the “Measuring Motion on a Runway” component).  Make sure you set up the software parameters correctly before doing each part of this practical.

¨      Measure the speed of the ball as it leaves the end of the launching track using the light gate at the end of the track.  This can be done for different start positions of the ball.  The data collected can be shown as a table of values and a bar graph and can be saved for later analysis.  By placing a sheet of graph paper over a sheet of carbon paper along the landing strip, the range of the ball can also be measured from the position of the carbon dots made by the ball as it lands.

¨      Measure the time of flight of the ball by measuring the time for the ball to pass between the two light gates.  The positioning of the second light gate is done initially by trial and error.  Even then, every now and then the ball will bounce over the second light gate.  That’s why the use of the timing pad mentioned above would be better if you get that to work properly.

¨      Analyse the data collected in whatever way you see as appropriate.  (a) For example, you might get the students to predict whether the time of flight for the ball launched horizontally at various speeds is the same as the time for the ball to simply fall from rest from the same height as the end of the track?  Calculate the latter and compare the value obtained to the measured value for the time of flight.  Use this to stress the independence of the horizontal and vertical components of the motion.  (b) Have a look at how the horizontal velocity affects the range.  Calculate the theoretical range from some of your initial velocity and time of flight figures and check that it matches the actual range achieved (within experimental limits).  (c) If you adapt the apparatus to allow projectiles to be launched at angles to the horizontal, get the students to predict and experimentally determine whether the time of flight for a ball launched at a given angle is different to that for a ball launched horizontally at the same speed and from the same height.  Discuss this.

¨      Consider also any sources of error that are present in the experiment.  AND please don’t let students get away with the term “human error”.  Most errors are caused by humans.  They need to be specific.  For example, when measuring the speed of the ball leaving the end of the track, is the light gate positioned correctly to view the diameter of the ball or is it too high or low in which case it will give a false speed reading.  It will assume the “length” of the ball passing through the gate is the diameter that you entered into the software, but in fact may be registering just the very top of the ball as it cuts into and out of the beam.

EXAMPLE OF PRAC WORKSHEET FOLLOWS:

RESULTS OF PROJECTILE MOTION PRAC. (Syllabus point 9.2.2 column 3 dot point 2)

Height of horizontal launch position = _______

Part 1 : Measurement of Initial Velocity & Range Using Data Logger Attached To Computer

Table No.1 : Initial Velocity & Range Data – Horizontal Launch

 Trial No. Initial Velocity, ux (m/s) Range (m) 1 2 3 4 5 Average:

Part 2 : Measurement of Time of Flight Using Data Logger Attached To Computer

Table No.2 : Time of Flight Data – Horizontal Launch

 Trial No. Time of Flight (s) 1 2 3 4 5 Average:

Analysis:

·       Calculate the theoretical time of flight for the ball.

·       Compare the theoretical and experimental time of flight data.  Is this what you expected?  Explain.

·       Determine the theoretical range of the ball using the theoretical time of flight and the average measured initial horizontal velocity.

·       Compare the theoretical and measured values of the range.

·       Outline the major sources of error in this experiment.

·       Evaluate the reliability and accuracy of this experiment.

·       If we used a launch track that could be adjusted to launch projectiles at a known angle q above the horizontal, we could use the data logger to measure the initial launch velocity v0 of the projectile.  Describe how you would then calculate (from q & v0) the following quantities: maximum height reached, time of flight, range and final velocity of the projectile as it hits the ground.

·       Outline an experimental procedure that would enable us to measure the maximum height reached by a projectile launched as described in the last dot point.

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