Transcript P2_Additional_Physics Revision

P2 Additional Physics
P2 1.1 Distance-time graphs
• How can we tell from a distance-time graph if an object is
stationary or moving at constant speed?
• How do we calculate speed of a body?
• The slope on a distance-time graph represents speed.
• Speed (metre/second, m/s) = distance travelled (m)
time taken (s)
Question: A car travels 1800m in 60s.
a) What is its speed?
b) how far would it travel at this speed in 300 s?
P2 1.2 Velocity and acceleration
• What is the difference between speed and velocity?
• What is acceleration and what are its units?
• What is deceleration? = negative acceleration
• Velocity is speed in a given direction (units – m/s).
• Two objects may travel at the same speed but may have
different velocities.
• Acceleration of an object is the change in its velocity per
second (units – m/s2)
• Acceleration = change in velocity (m/s)
Time taken for the change (seconds)
• Question:
• The velocity of a car increased from 8m/s to 28m/s in 8s
without change of direction. Calculate a) the change in velocity
and b) its acceleration.
P2 1.3 Velocity time graphs
• How can we tell from a velocity-time graph if an object is
accelerating or decelerating?
• What does the area under a velocity-time graph represent?
The slope of the line on a velocity-time graph represents
acceleration. (NOTE: the line on a distance-time graph
represents speed).
The area under the line on a velocity-time graph represents
distance travelled.
5 minute task:
Answer Q1 and 2 (if time) on page 189.
P2 1.3 Velocity time graphs
• How can we tell from a velocity-time graph if an object is
accelerating or decelerating?
• What does the area under a velocity-time graph represent?
The slope of the line on a velocity-time graph represents
acceleration. (NOTE: the line on a distance-time graph
represents speed).
The area under the line on a velocity-time graph represents
distance travelled.
5 minute task:
Answer Q1 and 2 (if time) on page 189.
P2 1.4 Using graphs
• How can we calculate speed from a distance-time graph?
• How can we calculate distance from a velocity-time graph?
• How can we calculate acceleration from a velocity-time graph?
Distance-time graph: The slope of the line = speed
Slope of line = the height of the triangle
the base of the triangle
Velocity-time graph: The slope of the line = acceleration
Slope of line = height of the triangle
base of the triangle
Remember, the area under a velocity-time graph represents
the distance travelled. (Questions on page 191)
P2 2.1 Forces between objects
What is the unit of force?
What can we say about the forces acting on two interacting
objects?
When two objects interact, they always exert equal and
opposite forces on each other.
The unit of force is Newtons (N)
Question:
Name the 4 main forces acting on an object.
What is the resultant force?
P2 2.2 Resultant force
What is a resultant force?
What happens if the resultant force on an object is zero?
What happens if the resultant force on an object is not zero?
We can work out the effect of the forces on an object by
replacing them with a single force called the resultant force.
When the resultant force is zero, the object:
-remains stationary OR
- Moves at constant speed in the same direction
3 minute task:
Copy Key points table on page 199.
P2 2.2 Resultant force
What is a resultant force?
What happens if the resultant force on an object is zero?
What happens if the resultant force on an object is not zero?
We can work out the effect of the forces on an object by
replacing them with a single force called the resultant force.
When the resultant force is zero, the object:
-remains stationary OR
- Moves at constant speed in the same direction
3 minute task:
Copy Key points table on page 199.
P2 2.3 Force and acceleration
How is resultant force, acceleration and mass related to each
other?
Resultant force (N) = mass (kg) x acceleration (m/s2)
F = ma
Question:
Calculate the resultant force on an object with a mass 6.0kg when
it has an acceleration of 3.0m/s2.
P2 2.4 On the road
What is the resultant force on a vehicle travelling at constant
velocity?
What does the stopping distance of a vehicle depend on?
What factors can increase the stopping distance of a vehicle?
For any car travelling at constant velocity, the resultant force on
it is zero.
The braking force needed to stop a vehicle depends on a) the
velocity of the vehicle and b) the mass of the vehicle.
Stopping distance = thinking distance + the braking distance
Factors affecting stopping distances:
Tiredness, alcohol, drugs, how fast the vehicle is travelling,
adverse road conditions and poorly maintained vehicles.
(See diagram on page 202 and read Key points on page 203)
P2 2.5 Falling objects
What is the difference between weight and mass?
What is terminal velocity?
The weight of an object is the force of gravity on it (Unit –
Newtons, N)
The mass of an object is the quantity of matter in it (Unit –
kilograms, kg)
Gravitational field strength on Earth = The force of gravity on a
1kg object on Earth.
Weight (N) = mass (kg) x gravitational field strength (N/kg)
As an object falls, its acceleration decreases as the drag force
starts to increase. The object starts to travel at constant
velocity – this is called terminal velocity.
P2 3.1 Energy and Work
What do we mean by the word ‘work’ in science?
What is the relationship between work and energy?
What happens to work done against frictional forces?
‘Work’ is done on an object if it moved by a force.
Work done = energy transferred
Work done (Joules, J) = force (N) X distance moved (m)
Work done to overcome friction is mainly transformed into heat
energy (p. 211)
Question:
A student (weighing 450N) steps on a box of height 0.2m.
Calculate the gain of gravitational potential energy of the
student.
P2 3.2 Kinetic energy
What are kinetic energy and elastic potential energy?
How does the kinetic energy of an object depend on its speed?
How can we calculate kinetic energy?
Kinetic energy (J) = ½ x mass (kg) x speed2 (m/s)2
Elastic potential energy is the energy stored in an elastic
object when work is done on it to change its shape.
Question:
A car moving at constant speed has 360000J of kinetic
energy. When the driver applies the brakes, the car stops
in a distance of 95m.
a) Calculate the force that stops the vehicle.
b) The speed of the car was 30m/s when its kinetic energy
was 360000J. Calculate its mass.
P2 3.3 Momentum
How can we calculate momentum?
What is its unit?
What happens to the total momentum of two objects when they
collide?
Momentum of a moving object = its mass x velocity
Unit is kilogram metre/second (kg m/s)
Questions:
Answer Q2 on page 215.
More on Momentum: Read pages 216 – 219.
C2 3.2 Masses of atoms and moles
How can we compare the mass of atoms?
How can we calculate the mass of compounds from the
elements they are made from?
The mass of a single atom is immeasurable so we focus on the
relative masses of atoms of different elements i.e. relative
atomic masses.
We use the mass of C-12 as a standard atom and calculate masses
of other elements relative to this.
We can use this information to work out the number of moles of a
substance.
You need to work out the number of moles to work out how much is
really there. Remember R.A.M and R.F.Ms are relative values.