1.1 method of determining g (light gates)
- Set up the apparatus as shown in the
diagram, connecting the light gates to a
data logger and as close to the
electromagnet as possible. - The position of the lower light gate
should be adjusted such that the height
(h) is 0.75 m, measured using the
metre ruler. - Turn on the electromagnet and attach the ball bearing.
- Switch off the electromagnet, and note the time taken for the bearing to fall between
the light gates (t) as recorded by the data logger. - Reduce h by 0.05 m by moving the lower light gate upwards and repeat the above
two steps, reducing h by 0.05 m each time down to 0.25 m. - Repeat the experiment twice more to find mean values of t for each value of h.
1.1 calculations for estimating g
1) Plot a graph of 2h against t^2
and draw a line of best fit.
2) The gradient of the line of best
fit will be g.
3) This is derived using one of the constant acceleration formula below:
s = ut + (0.5)at^2
h = 0.5g(t^2) [as h is distance travelled, g is acceleration, u is 0] 2
2h = g(t^2)
y = mx
1.1 safety for estimating g (using light gates)
● Use a counterweight or clamp the stand to the table to avoid it toppling over and
causing injury.
● The ball bearing is cushioned by a pad at the bottom of the clamp so it does not
bounce upwards and cause injury.
1.1 method of determining g (using stopwatch)
- Using the metre ruler, measure a height (h) of 1.0 m. Place the soft pad at the bottom
of the ruler. - Hold the tennis ball so the its bottom half is at the 1.0 m mark (it may be useful to
work in pairs here, one holding the ruler, one holding the ball). - Release the ball and simultaneously switch on the stopwatch and switch it off as
soon as the ball hits the soft pad. Note the time taken for the ball to hit the ground (t)
as recorded by the stopwatch. - Reduce h by 0.05 m and repeat the above two steps, reducing h by 0.05 m each time
down to 0.50 m. - Repeat the experiment twice more to find mean values of t for each value of h.
1.1 safety for estimating g (stopwatch)
The ball bearing is cushioned by a pad at the bottom of the clamp so it does not
bounce upwards and cause injury.
1.1 notes for estimating g (stopwatch)
● The metre ruler must be kept perpendicular to the ground, you could use a set
square to make sure this is the case.
● The tennis ball will experience a large amount of air resistance which may affect your
calculation of g, therefore the tennis ball can be swapped out for a ball bearing to
improve results.
● Reaction times will hugely affect the recorded times (t), making the results less
accurate
PAG 1.2 Investigating terminal velocity
- Wrap elastic bands around the tube of viscous liquid at set intervals measured by the
ruler. - Drop the ball into the tube and record the time it reaches each band (it will help to
use a lap feature on the stopwatch here). - Repeat 4 times to reduce the effect of random errors and use the strong magnet to
remove the ball bearing from the bottom of the tube
calculations for investigating terminal velocity
● Calculate the time taken to travel between consecutive bands and calculate the
average of this time for each experiment.
● Use the equation speed = distance/time to find the average velocity of the bearing
between each set of bands.
● Plot a graph of velocity against time. The velocity to which the graph tends to is the
terminal velocity.
1.2 investigating terminal velocity safety
Use a viscous liquid that doesn’t cause skin irritation.
1.2 investigating terminal velocity notes
● Using a taller tube allows the bearing to travel at its terminal velocity for longer.
● Using larger intervals for the bands reduces the percentage uncertainty in both the
distance and time between the bands.
● Terminal velocity occurs when the weight of the bearing is equal to the drag force
due to the fluid, as there is no resultant force on the bearing, it travels at a constant
velocity.
1.3 Investigating initial speed and stopping distance method
- A vehicle is modelled by the block of wood which is pushed and decelerates due to friction
with the surface it moves on. - Glue the 10 x 10 cm interruptor card to the side of the block of wood so that the time for the
width of the card to pass through the gate is recorded. The interruptor card allows the
distance moved through the light gate to be fixed, as it registers with the light gate easily
without the light gate interrupting the block’s motion. - Set up the light gate such that it records the average starting velocity of the block moving
through it (speed = 0.1 m/time for card to move through in seconds). - Record the starting position of the block and position the light gate 2 cm after this point.
- Push the block and record the position at which it stops.
- Record the average starting velocity and the corresponding distance between the light gate
and the stopping point (stopping distance), in a table.
1.3 Investigating initial speed and stopping distance calculations
● Find the stopping distance for a range of starting velocities.
● Plot a graph of stopping distance against starting velocity squared, this should be a straight
line through the origin as:
- Ek of block = 0.5m(v^2)
- Work done by friction = force x stopping distance
- As all the kinetic energy is converted to thermal energy by friction
0.5m(v^2) = force x stopping distance
Mass and ½ are both constants, stopping force is assumed to be constant as the block travels
relatively slowly. In reality, the frictional force increases with velocity. However, assuming it is
constant:
v^2 ∝ stopping distance
1.3 Investigating initial speed and stopping distance notes
● The surface the block is pushed on and the block material should stay constant so that the
frictional force varies as little as possible.
