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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?

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Presentation on theme: "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?"— Presentation transcript:

1 P2 Additional Physics

2 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?

3 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/s 2 ) 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.

4 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.

5 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.

6 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)

7 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?

8 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.

9 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.

10 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/s 2 ) F = ma Question: Calculate the resultant force on an object with a mass 6.0kg when it has an acceleration of 3.0m/s 2.

11 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)

12 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.

13 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.

14 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 speed 2 (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 J 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 J. Calculate its mass.

15 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.

16 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.

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