Transcript File - singhscience

Smart Teach
P2 Foundation Topic 3
In this session, we will cover:
Energy
Momentum
Stopping Distance
Work Done
‘Work done’ is when a force is used to move an
object.
work done (J) = force x distance.
Work done is always measured in Joules (J). Also,
work done (J)= energy transferred (J)
You will not be asked to rearrange the equation.
Exam examples
The weight of the egg is 0.6 N.
Calculate the work done on the egg to lift it up by 20
m. State the unit. (3)
work done on egg = . . . . . . . . . . . . . . . . . unit . . . . . . .
.........
Exam examples-Look how similar they are!
Work Done (J)= Force (N) x Distance (m)
Q1. The brakes exert a force of 2000 N.
A car takes 9 m to stop after the driver applies the brakes.
Calculate how much work is done by this force. (2)
Q2. The braking force on another car is 600 N. The force acts
for a distance of 15 m. Calculate the work done by the
braking force. (2)
Q3. A car engine produces an average driving force of 1200
N. The car travels 8.0 m.
Calculate the work done by the force over this distance. (2)
Q1
Q2
Q3
Types
of
energy:
• Kinetic-When an object is moving
• Potential-When an object is moved against
gravity,
i.e. being held above the ground
How to work out the
Kinetic Energy of an
object
Kinetic energy = 1/2 x mass x
2
speed
Exam example:
The car has a mass of 1400 kg and travels at a
velocity of 25 m/s.
Calculate the kinetic energy of the car.
Exam example:
The car has a mass of 1400 kg and travels at a
velocity of 25 m/s.
Calculate the kinetic energy of the car.
Exam example:
A student stands on the ground with an egg in his
hand. He throws the egg vertically upwards.
The egg rises to a height of 10 m.
Then the egg falls and lands on the ground.
Describe the energy changes of the egg during this
sequence of events. (6)
Throwing egg At the top the egg
As it falls, the
up Kinetic
has Potential
potential energy
energy changes energy (stopped
changes into
into Potential
moving)
kinetic energy
energy
Exam model answer=6 marks
Mark scheme ‘a detailed description of a sequence of
relevant energy changes or transfers’
Kinetic energy is transferred into potential energy as
the egg rises. This then changes back into kinetic
energy as it falls back down.
Momentum
A moving object has momentum. This is the tendency
of the object to keep moving in the same direction.
Momentum is a vector quantity as it has both a force
and a direction.
momentum (kg m/s) = mass (kg) × velocity (m/s)
A heavy object going very fast will have a large
momentum
Q1.
The photograph shows a man dropping an egg inside a padded box
from a height. He is investigating to see if the padding stops the egg
from breaking.
The velocity of the container was 18 m/s as it hit the floor.
The mass of the container was 0.5 kg.
Calculate the momentum of the container. (1)
momentum (kg m/s) = mass (kg) × velocity (m/s)
units = kg m/s
Your go...
Q1. The velocity of the aircraft when it lands is 75
m/s. The mass of the aircraft is 130 000 kg.
Calculate the momentum of the aircraft.(2)
momentum = . . . . . . . . . . . . . . . . . . . . . . kg m/s
Q2. A car has a mass of 800 kg. It has a velocity of
3.0 m/s. Calculate the momentum of the car. (2)
momentum of car = . . . . . . . . . . . . . . . . . . . . . . kg
m/s
Q1
Q2
What about when momentum changes?
If momentum changes quickly, there will
be a large force on the object. This
happens in car crashes.
The longer it takes for momentum to
change, the less force there is on the
object.
How is this knowledge tested in the
exam?
Q. A pilot begins to land an aircraft.
When the aircraft lands, the momentum of each
passenger also changes.
Explain why it is more comfortable for a passenger if
the aircraft takes a longer time to slow down.
Model answer: There will be less force on the
passenger because the change in momentum is
slower
Stopping Distance
Thinking distance + Braking distance = Stopping Distanc
You may be tested on:
-Factors that affect thinking distance or braking
distance
-Ability to read stopping distance graphs
Thinking distance is affected by reaction time.
REALLY IMPORTANT!!!!
Increasing the reaction time means the time to
react is longer, so the reactions are slower!
Do not get this confused
Which of these situations can increase the reaction time of a driver?
They are asking, which ones make your reactions slower
A
B
C
D
an icy road
worn tyres on his car
stopping for a cup of coffee
driving for a long time without taking a break
How is your knowledge assessed?
As speed increases, stopping
distance increases.
The chart shows the thinking, braking and stopping distances for an
average car and driver stopping from 50 miles per hour as shown in the
Highway Code.
It also shows the thinking, braking and stopping distances for drivers of
cars A and B, both stopping from 50 miles per hour.
A and B are different cars on different roads.
Use the factors that can affect thinking and braking distances to explain
the differences in stopping distances for cars A and B. (6)
Use the factors that can affect thinking and braking distances to explain
the differences in stopping distances for cars A and B. (6)