free fall and motion
TRANSCRIPT
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1.3c Free fall and motion graphs
We
Are
Learning
To
•Acceleration due to gravity, g; detailed experimental methods of
measuring g are not required.
•Terminal speed.
•Representation by graphical methods of uniform and non-uniform
acceleration; interpretation of velocity-time and displacement-time
graphs for uniform and nonuniform acceleration; significance of
areas and gradients.
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Starter
Recap gravity – lots of demos!
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On the Earth gravity pulls with a force of 10 Newtons
for every kilogram (10 N/kg).
Weight (N) = mass (kg) x gravitational field strength (N/kg)
A bag of sugar
has a mass of
1kg. The
Earth’s gravitypulls it with a
force of 10N
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Galileo Galilei
1564 - 1642
On Earth a free-falling object has an acceleration
of 9.8 ms-2
a = g
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water
air
Self sealingneedles
Heavytennis ball
Normaltennis ball
Galileo – Tennis Balls
Note: Ask students what will happen when you drop two tennis balls. Both hit thefloor as expected. Then pass the tennisballs to a student. One is heavier than theother. One tennis ball has been filled withwater. This will challenge their thinking
that heavier objects fall faster.
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Note: Stand on a desk and ask students to observe which object hitsthe ground first, (1) two oranges (2) an orange and a grape. The orangeand the grape should hit the ground at the same time.
Misconception: heavier objects fall faster than lighter objects.
Galileo – Orange and Grape
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Note: get two pieces of paper, crumple up the one piece and then drop themboth at the same time. The crumpled up paper hits the floor first eventhough its got the same mass as the other! Why?
Galileo – Paper
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Note: Cut out a circular piece of paper slightly smaller than a real coin. Dropthem both and the coin will hit the floor first (the air resistance on the coin
takes longer to build up, so it reaches a higher terminal velocity). Then drop thecoin and the paper coin together (paper coin above real coin), they both hit theground at the same time. Why?
Galileo – Coin and Paper Trick
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Coin and Paper explanation
• Before they fall:The coin has a larger weight, but they start
accelerating at the same rate.
• Shortly after drop:The paper quickly reaches its terminal velocity
The coin remains accelerating
• Just before the hit the
ground:The air resistance on the coin has increased, but
It is still accelerating!
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Note: Invert the tube containing the coin and feather (with air in first).Then use the vacuum pump to extract the air. Invert again and thefeather falls at the same rate as the coin.
Galileo – ‘Guinea and Feather’
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Note: Put a 1 kg mass onto a analogue weighing scale and then jumpoff a table holding the scales. Choose a weight to give a significantdeflection of the needle. Students observe the dial on the scale as
you fall with it. The dial should move anti-clockwise throughout thefall.
Safety: Take care jumping off the table (do not allow a student to
jump off the table). Make sure that the weight and pan is securedto the balance.
Free Fall – Weighing Scales
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Note: Students are asked to predict what will happen to a swinging
pendulum if it is allowed to freefall.
Hold a pendulum at an angle and then jump off a table. Studentsobserve the pendulum as it falls. The pendulum should be ‘frozen’ asit falls.
Safety: take care jumping off the table (do not allow a student to jump off the table).
Free Fall - Pendulum
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Note: The sound toy has a box inside that makes a noisein the tube as the box falls inside the tube. Stand on atable and jump off carefully as you invert the toy.Observe the noise of the toy as the teacher falls.
Safety: Take care jumping off the table (do not allow astudent to jump off the table).
Free Fall – Noisy Toy
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Note: Fill a bottle (with a small hole in theside) with water. Observe the stream ofwater coming out of the bottle as thebottle is dropped into a dustbin.
Free Fall – Bottle With Hole
Held
Dropped
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weightlessness
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Practical – finding the
acceleration due to gravity
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Student practical- finding the acc
due to gravity g PHOTO
Light gate
Light gate
Vernier
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Terminal Velocity
• Note – this can be skipped until Newton’s
Laws are covered – it may be clearer then!
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Skydiver
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Terminal VelocityConsider a skydiver:
1) At the start of his jump the airresistance is _______ so he
_______ downwards.
2) As his speed increases his airresistance will _______
3) Eventually the air resistance will be
big enough to _______ theskydiver’s weight. At this pointthe forces are balanced so hisspeed becomes ________ - this iscalled TERMINAL VELOCITY
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SUVAT problems
Object fallingObject being thrown
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Ground
Object falling
due to gravity
Object being thrown
upwards (against gravity)
g = +
s = +v = +
g = -
s = -
v = -
A coin as released at rest at the top of a ell It took 1 6 s to it
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A coin was released at rest at the top of a well. It took 1.6 s to it
the bottom of the well. Calculate:
v = u + at
s = (u + v)t2
s = ut + ½ at2
v2 = u2 + 2as
(i) the distance fallen by the coin
(ii) its speed just before impact
v =
u = 0
s = ?
t = 1.6
a = -9.8
v = ?u = 0
s =
t = 1.6
a = -9.8
s = ut + ½ at2
s = -12.5 m(- indicates 12.5 m downwards)
v = u + at
v = -15.7ms-1
(- indicates downward velocity)
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Homework
Questions page 121
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C id b ll th di tl d d
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Consider a ball thrown directly upwards andcaught when it returns. The ball rises to amaximum height of 2 metres
What would the following
graphs look like?
1. Distance-time2. Displacement-time
3. Velocity-time
Think about what the gradient of the
line represents on each graph and
what the area underneath the line
represents on a velocity-time graph)
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Difference between a distance-time graph
and a displacement-time graph
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displacement
time
Maximum height
Displacement-time graph for an object projected upwards
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distance
time
Maximum height
Distance-time graph for an object projected upwards
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Difference between a speed-time graph
and a velocity-time graph
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velocity
time
Velocity-time graph for an object projected upwards