The motion of the objects can be shown with the
help of distance - time and speed – time graphs.
Object at rest (stationary object)
Object moving with constant speed
Object increasing speed non - uniformly (accelerating)
Object decreasing speed constantly
Object decreasing speed non - uniformly (decelerating non uniformly)
Note:
The gradient of the distance-time graph represents the
speed of the object.
Example Question:
The graph shown below represents the distance traveled by
a car plotted against time.
(a) Describe the motion of the car between point A and E.
(b) How far has the car travelled at the end of 30seconds?
(c) Calculate the speed of the car between point B and C?
(e) What is the distance travelled between A and D?
(f)What is the average speed between A and D?
Object at rest (stationary object)
Object moving with constant speed
Object moving with uniform acceleration
Object moving with non - uniformly acceleration
Object decelerating uniformly
Object decelerating non - uniformly
Note:
• the gradient of the speed-time graph is equal to
the acceleration of the object.
• the area under the speed-time graph represents
the distance travelled by the object.
Example question:
The speed – time graph below shows the motion of a
lorry.
(a) Describe the motion of the lorry.
First the lorry accelerates non – uniformly, then it moves
with uniform acceleration. After some time speed of lorry
decreases and moves with constant speed, finally the lorry
decelerates uniformly to the rest.
(b) Calculate the acceleration of the lorry between 2s and 4s.
a=
𝑣 −𝑢
𝑡
=
30 −10
2
= 10 m/s²
(c) Calculate distance travelled by the lorry between 5s and
7s.
A = l x b = 40 x 2 = 80m
Free fall
When an object is released near to the surface of the Earth,
it falls under the influence of gravity. This is known as free
fall.
 The acceleration of free fall or acceleration due to gravity
is denoted by g.
 The acceleration due to gravity is 10 m/s2.
 This is true in the absence of air resistance.
The experiment below shows, all the air has been removed
from the tube. Both feather and lead ball dropped from same
height and reaches bottom of the tube at the same time. This
is because all the objects in the absence of air resistance,
accelerates towards the earth at same rate that is10m/s2.
TERMINAL VELOCITY
When any object falls through air, due to the gravitational
force or weight the object will exerts a resistive force. This
resistive force increases as the object moves faster.
Resistive force acts in the opposite direction to which the
object is moving. And after some time the resistive force
and weight of the object becomes equals, so the object
moves with constant speed, this is known as terminal
velocity.
Consider a parachutist jumping from aircraft.
Consider a parachutist jumping from
aircraft.
When the parachutist jumps from the
aircraft, his speed increase downward due
to weight of his body or gravitational force
As his speed increases his air resistance
will also increases in the opposite
direction of fall.
After sometime air resistance will be big
enough to balance the parachutist’s
weight. At this point the forces are
balanced so his speed becomes uniform this is called terminal velocity.
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When he opens his parachute the air
resistance suddenly increases and the
speed decreases.
After some time again air resistance
decreases until it balances his weight.
The parachutist has now reached a
new, lower terminal velocity.
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Speed-time graph of a parachutist is shown below…
speed
Speed
decreasing
Parachute opens – diver
slows down
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal
velocity reached
Diver hits the ground
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