Physics 2.2

• explain why an object moving in a circle with a constant speed has a changing velocity (qualitative only)

  an object moving in a circle at a constant speed is still accelerating even though its speed does not change. it is constantly changing direction, so its velocity is constantly changing. to do this a force directed towards the centre of the circle acts on the object
• recall and apply newton's third law

  pairs in forces arise when objects interact. in an interaction pair each force acts on a different object, the forces are the same size and type (eg. contant/non-contact) and the forces act in opposite directions. this is newtons third law. sometimes this law is written as "for every action there is an equal and opposite reaction." it is more useful to think of the law as: "forces always come in pairs"
• explain, with reference to examples, the definition of power as the rate at which energy is transferred


  power tells you the rate at which energy is transferred. a powerful person can run upstairs in a short time. if both people are the same weight then they do the same amount of work. each person transfers the same amount of energy to a gravity sotre. the more powerful tranfers the energy in a shorter time.equation: power (w) = work done (j) / time (s)
• calculate relevant values of stored energy and energy transfers; convert between newton-metres and joules


  newton is a derived unit and 1 n = 1 kgm/s^2. the joule is also a derived unit. if you multiply force and distance the unit of the quantity you can calculate is the newton-metre. this is the same as the unit of kinetic energy. one newton-metre is the same as one joule.
• use the relationship between work done, force, and distance moved along the line of action of the force and describe the energy transfer involved

  doing work in science is about using forces to transfer energy between stores. yoou use aa force to lift your suitcase into a car, and you do work against gravity. you have shifted energy to a gravitational store. when you pull your suitcase along you are doing work against friction. energy is shifted to a thermal store. you must measure the distance along the line of the force. equation: work done (j) = force (n) * distance (m)
• define momentum and describe examples of momentum in collisions

  in science, momentum is a quantity that depends on mass and velocity. momentum is a vector so when you do calculattions involving momentum you need to take account of direction.in an elastic collions no energy is tranferred to other stores. the energy in the kinetic store stays the same. for example, a moving red ball that has an elastic collion with a stationary blue ball will stop and the blue ball will move off at the same velocity as the red ball. in reality, such as in snooker, some energy is tranferred to a thermal store by sound, so the collision is not perfectly elastic.in an inelastic collion come energy is transferred to other stores. one example is when snooker balls colide, and energy is transferred by sound to a thermal store. another example is a collision after which the velocity of the combined objects is less than that of the original objects.
• explain that inertia is a measure of how difficult it is to change the velocity of an object and that the mass is defined as the ratio of force over acceleration


  it takes the resultant force to change the motion (the speed or direction) of an object. that means that if the resultant force is zero then the speed or direction of an object will not change. this law is really about the principle of intertia. the intertia of an object is a measure of how difficult it is to change its velocity. moving objects keep moving, and objects that are stationary do not move.
• apply newton's second law in calculations relating forces, masses and accelerations


  a leaping animal uses its backlegs to exert a force on itself. its motion changes because there is a resultant force. a resultant force can change the speed of an object of an object, change the direction of motion of an object and change both the speed and direction of motion of an object. if the speed or direction of motion of an object changes then it is accelerating.newtons second law states that the acceleration that the resultant force produces on an object depends on the size of the resultant force and the mass (inertia of the object). equation: force (n) = mass (kg) * acceleration (m/s^2)
• describe, using free body diagrams, examples of the special case where forces balance to produce a resultant force of zero (qualitative only)

  suppose that you draw a free body diagram for a car parked in a car park. and then you draw another free body diagram for a feather falling at a steady speed. in both cases, the resultant force is zero so the motion does not change. both objects are in equilibrium
• describe, using free body diagrams, examples where two or more forces lead to a resultant force on an object

  a leaping animal uses its back legs to exert a force on itself. it's motion changes because there is a resultant force. a resultant force can change the speed of an object of an object, change the direction of motion of an object and change both the speed and direction of motion of an object. if the speed or direction of motion of an object changes when it is accelerating.for objects on which a resultant force is acting, you can use free body diagrams. the international space station orbits the earth roughly every 90 minutes. this means that it is moving at a steady speed. however, its direction of motion is constantly changing
• describe examples of the forces acting on an isolated solid object or system

  when you jump out of a place you accelerate. your motion changes because there is a resultant force on you. the air exerts a force on you, but the earth exerts a larger force. as you accelerate the force of the air increases. eventually, the force of the air on you equals the force of the earth on you, and your motion no longer changes. you have reached terminal velocity. a parachute increases the force of the air to reduce your velocity. without air resistance, you would reach the speed of sound in about 30 seconds.when a rocket takes off there is a resultant force on it that produces a large acceleration. the burning fuel pushes exhaust gases out of the bottom of the rocket. the gases pushing on the rocket and the rocket pushing on the gases are another example of newtons third law. when the force of the gases on the rocket is bigger than the force of the earth on the rocket then the rocket will accelerate.
• use vector diagrams to illustrate resolution of forces, a net force (resultant force), and equilibrium situations


  when you have drawn your free body diagram, you can work out the resultant force, or net force. forces are vectors so you need to take account of their direction.you can work out which two forces at right angles add up to a particular force by resolving the force in two directions. if you draw the force and angle on a graph paper you can use a ruler to work out the componentssuppose that you draw a free body diagram for a car parked in a car park. the resultant force is zero so the motion does not change. the object is in an equilibrium.
• apply newton's first law to explain the motion of an object moving with uniform velocity and also an object where the speed and/or direction change

  when you are in the back of a car, going round a roundabout, or on a ride going in a circle at a fair or them park, it sometimes feels as if you are being flung sideways. what is actually happening is that you continue travelling in a straight line while the car turns. you do not change direction until the side of the car pushes on you. this is an example on newtons first law of motion which states "an object will continue ti stay at rest or move with with uniform velocity unless a force acts on it." it takes a resultant force to change the motion (the speed of direction) of an object.
• represent such forces as vectors

  a free body diagram is adiagram that shows the forces acting on a single object, such as you in bed, of a skier doing a ski jump. you usually represent the obkect as a simple dot or box and the forces acting on the objects by force arrows. you can use your diagram to predict or explain the motion of the object or to do calculations. when you have drawn your free body diagram, you can work out the resultant force, or net force. forces are vectors so you need to take account of their direction
• describe how such examples involve interactions between pairs of objects which produce a force on each object

  friction on a sliding box: the force of the box on the surface. the force of the surface on the box. the mechanism that produces it is that the atoms that make up the surface interact when rough surfaces slide over each other.drag on a falling object: the force of the falling leaf on the air. the force of the air on the falling leaf. the mechanism that produces it is that the particles of the liquid or gas collide with the object and the object pushes them away.normal force on a elephant: the force of the elephant on the ground. the force of the ground on the ground. the mechanism that produces it is that solid objects deform slightly when you exert a force on them. the bonds between the particles are compressed. upthrust on a floating boat: the force of the boat on the water. the force of the water on the boat. the mechanism that produces it is that gravity produces a pressure difference in a fluid. the pressure produces a net upwards forcetension on the cord of a bungee jumper: the force of the bungee jumper on the cord. the force of the cord on the bungee jumper. the mechanism that produces it is that solid objects deform slightly when you exert a force on them. the bonds between the particles are stretched
• recall examples of ways in which objects interact

  non-contact forces are when objects interact without being in contact with each other. examples are electrostatics, magnetism and gravity. these forces arise because charges, magnets and masses interact at a distance. electric charges and magnets both repel and attract but gravity only attractscontact force is when two objects physically touch each other. when you stand on a diving board, the board pushes up on you. solid surfaces can exert a force on objects that exert a force on them. examples are friction, air resistance and normal force