Applicable Levels : GCSE
Applicable Examination Boards : AQA, EDEXCEL, OCR21C , WJEC
AIM
To investigate the effect of force on the acceleration of a trolley.
APPARATUS
A
trolley, a white plastic track, a board, two light gates, a suitable
computer interface, a pulley, thread, slotted mass hanger and seven
additional slotted masses (mass 10g each) and some leads. (A motion
sensor can replace the two light gates)
METHOD
Set up your apparatus as shown in the diagram
and set your interface unit to measure acceleration. Compensate for
friction by tilting the track slightly so that the trolley runs
steadily down with no increase in speed when there is no force pulling
it.
Start with all of your seven masses on the trolley and the mass
hanger on the thread hanging down. The mass to be accelerated (Total
mass in the table below) is the mass of the trolley and seven slotted
masses and the mass hanger while the accelerating force is the weight
of the mass hanger (0.1 N).
Allow the trolley to accelerate down the track and record its acceleration – repeat this reading and take an average.
Repeat the procedure by taking one slotted mass off the trolley and
adding it to the mass hanger – the accelerating force is now 0.2N (same
total mass). Carry on until you have only one mass left on the trolley.
SAMPLE RESULTS
Assume a trolley mass of 0.08 kg and a perfectly friction compensated runway. Sample table of Results
| Accelerating force (N) | Total mass (kg) | Acceleration (m/s/s) |
|---|
| 0.10 | 0.16 | 0.63 |
| 0.20 | 0.16 | 1.25 |
| 0.30 | 0.16 | 1.88 |
| 0.40 | 0.16 | 2.50 |
| 0.50 | 0.16 | 3.20 |
| 0.60 | 0.16 | 3.80 |
| 0.70 | 0.16 | 4.80 |
| 0.80 | 0.16 | 5.00 |
SAMPLE GRAPH
Plot a graph of acceleration (y-axis) against accelerating force (x-axis).
Sample graph
EXPERIMENT NOTES
This ‘interchange of
masses’ is done to ensure that the TOTAL accelerated mass of the system
remains constant since we are studying the variation of acceleration
with the accelerating force. Remember that the trolley and the
suspended masses are accelerating – NOT just the trolley.
If the pupil forgets to friction compensate then the graph will not
pass through the origin. The straight line obtained will be below the
line i.e. negative intercept. If the pupil overcompensates then the
pupil’s intercept will be positive.
Make sure that the pupils catch the trolley before it falls of
the end of the runway onto the floor! If the accelerating force is too
large the acceleration will be too great and the trolley will career
along the runway and be difficult to stop at the far end.
The mask is simply a strip of cardboard of known width (1cm
suggested) stuck to the trolley so that it will cut the light beam when
it passes though each light gate.
The two light gates can be replaced by a motion sensor. This would
simplify the practical but make it unlikely to be a class experiment.
No mask would be needed for that version.
Some teachers may prefer to use the ‘double mask’similar
to that shown here. If this is done only one light gate will be needed.
If using this it is important to refer to your own light gate and
interface instructions.
USEFUL INFORMATION
Stress
that when an unbalanced force (OR resultant force OR accelerating
force) acts on an object various things may happen – stretch, squash,
turn or change its speed. It is the last of these that we will deal
with in this section of work. It is not that an unbalanced force will
keep an object moving it is that it will ALTER its speed giving an
acceleration or retardation.
Talk about the idea of acceleration and that a resultant force is
needed to stop or start something moving or to change its speed once it
is moving.
Talk about examples of accelerating forces.
Refer to road safety and the need for a reduction in the
acceleration in a crash. Small deceleration means a long time and a
small force. Also forces involved in deceleration such as catching a
ball or jumping onto hard and soft ground – a big deceleration on hard
ground but a small one on soft ground.
Forces on astronauts at take off and on pilots in high speed jets
‘pulling’ as much as 5 or 6 g (this means five or six times the
acceleration of gravity (g = about 10 m/s2 and so 5g would be an acceleration of 50 m/s2), dragsters, grasshoppers jumping, forces in muscles in jumping and sprinting (mention importance of the warm up and warm down)
Ask the class about cars they know about – collect data on different cars performance from local garages (time for 0 – 60 mph).
There is a difficulty with the use of the word velocity here –
circular motion at constant speed but a force is still needed since the
velocity is changing. With any luck this will not be a problem at this
stage with directed questions.
If a 10 litre bus engine were fitted in a Mini - the acceleration
would be impressive, but an 80 kg male sprinter would not accelerate
off the starting blocks very quickly if his legs could only develop the
same force as those of a grasshopper. However the grasshopper’s legs
give a great acceleration to a grasshopper!
APPLICATIONS
Air bags will deploy if the deceleration is greater than 60 m/s 2.
The acceleration of a bullet is between 2.5x10 5 and 1.5x10 6 m/s2.
Take off acceleration of a manned rocket is around 30 m/s2.
Some animals can experience large accelerations, a perch may reach 33 ms-2, a bush baby 180 ms-2,
a woodpecker 1000 ms-2 when pecking, a flea jumping may achieve 1400 ms-2!
EXAM BOARD REFERENCES
| AQA | P2 12.2 |
| EDEXCEL | P2-9 |
| OCR21C | P4.2 |
| WJEC | P2 7 a,b |
QUESTIONS
- Write up your experiment with a diagram, a brief account of the experiment, and the results.
- Comment on the results, using the terms accurate, precise, random error, systematic error, and reliability.
- Random errors due to mode of release, systematic errors due to friction and rotation of pulley and slope of runway, results should be reliable (similar to other groups) and close to the true value.
- State the independent variable, the dependent variable and all the control variables
- independent variable = accelerating force,
dependent variable = acceleration,
control variables = total mass of trolley and slotted masses also slope of runway.
- With the help of your graph state the conclusion of your experiment.
- Straight line graph through the origin expected showing acceleration proportional to accelerating force. Line above origin slope over-compensated for friction.
- In another group the experiment is completed with only six slotted masses. How will this affect the acceleration of the trolley?
- Accelerating force same but total mass smaller so acceleration larger.
- Acceleration also depends on mass. How would you alter the experiment to find out how acceleration depends on mass? State how the variables (Question 3.) have changed.
- Keep the accelerating force constant, vary the mass on the trolley. Independent variable-mass, dependent variable-acceleration, control variables-Accelerating force
- Predict, scientifically guess, how acceleration depends on mass.
- Double mass, halve acceleration, Acceleration is inversely proportional to mass.
- A mass of 2kg is accelerated by a resultant or accelerating force of 10 N. Calculate the acceleration of the trolley.
- Acceleration=Force/Mass = 10/2 =5m/s/s
- A car of mass 1000kg accelerates from rest to 30m/s in 5s. Calculate the acceleration.
- Acceleration=change of speed/time =30/5=6m/s/s
the accelerating or resultant force Force = Mass x Acceleration = 1000 x 6 =6000N.
- The same car accelerates from rest to 30m/s over a distance of 90m. Calculate
- the average speed
- av speed = (initial speed + final speed)/2 = 30/2 = 15m/s
- the time taken to accelerate to 30m/s
- time = distance/av speed = 90/15=6s
- the acceleration
- = change of speed /time = 30/6 = 5m/s/s
- the accelerating or resultant force
- = mass x acceleration = 1000 x 5 =5000N