Please enable JavaScript.

Coggle requires JavaScript to display documents.

iGCSE Physics - Forces (Forces (Types of force (Gravitational (This is the…

- - - - Displacement is the minimum "as the crow flies" distance from a starting point to the finishing point. It is a vector quantity (has direction)
      - Distance is how far you travel between any two points by any route. It is a scalar quantity
      - Speed is how fast you go, the rate of change of distance.
        
        average speed = distance moved / time taken
        
        average speed = (initial speed + final speed) / 2
      - Velocity is rate of change of displacement
        
        It is a vector measurement.
        
        average velocity = change in displacement / change in time
    - - When the velocity of an object changes then we say it undergoes an acceleration
      - If the velocity of the object increases then the acceleration is in the SAME direction as the velocity
      - If the velocity of an object decreases then the acceleration is in the OPPOSITE direction as the velocity.
      - Acceleration is the rate of change of velocity. It is a vector
        
        acceleration = change in velocity / time
      - Acceleration is usually uniform, which means that the speed [velocity] is changing at a constant rate. This is shown by a straight line on a velocity time graph
  - - - Between A and B, the cyclist is cycling at a constant speed
        
        Since the gradient is a straight line the velocity is constant
        
        Between B and C the cyclist is stationary as there is no change in his position
        
        Between C and D, the cyclist is cycling at a constant speed again
      - The gradient of the graph gives us the average velocity
        
        The curve of a distance-time graph is determined by whether the velocity is positive or negative
        
        It is also determined by whether the velocity is constant, increasing or decreasing
      - We can work out the speed at any instant by measuring the gradient of the distance time graph
    - - Acceleration is the gradient of the velocity-time graph
      - Velocity time graph of a train
        
        Between O and A, the train has a positive velocity increasing in size. The train has an acceleration in the direction of the velocity (positive)
        
        Between A and B, the train travels at a constant velocity and has a constant acceleration
        
        Between B and C, the train slows down. The train has a positive velocity decreasing in size. The train has an acceleration in the opposite direction of the velocity (negative)
        
        Slowing down can also be called a deceleration
        
        The corresponding distance time graph is like this
        
        Between O and A, the gradient is positive and increasing in size
        
        Between A and B, the gradient is positive and constant
        
        Between B and C< the gradient is positive but decreasing in size.
        
        We can work out the speed at any instant by measuring the gradient of the distance time graph
      - Displacement is the area under the velocity-time graph
- - - - Speed up
      - Slow down
      - Change direction
      - Change shape
  - - - This is the attractive force exerted between bodies because of their masses
      - This force increases if either or both of the masses is increased and decreases if they are moved further apart
      - Weight is the gravitational force of the Earth on an object
    - - This is the repulsive force that stops two touching bodies moving into each other
      - The word 'normal' means that this force acts at 90degrees to the surfaces of the bodies. It is caused by repulsive molecular forces
    - - This is the force that opposes motion
      - The kinetic energy of the moving object is converted to heat energy by the force of friction
    - - This is the force that opposes the movement of objects through air
      - Drag is a more general term used for the opposition force in any gas or liquid
      - Objects are often streamlined to reduce this force
    - - This is the force experienced by objects when they are placed into a fluid (liquid or gas)
      - An object will float on a liquid if the upthrust force equals its weight
    - - Between magnets but also the force that allows electric motors to work
    - - Attractive and repulsive forces due to bodies being charged
  - - - distance
        
        Metres (m)
      - speed
        
        Metres per second (m/s)
      - work, energy
        
        Joules (J)
      - temperature
        
        Degrees Celsius, Kelvin (K)
      - voltage
        
        Volts (V)
      - frequency
        
        Hertz (Hz)
    - - displacement
        
        Metres (m)
      - velocity
        
        Metres per second (m/s)
      - acceleration
        
        Metres per second squared (m/s^2)
      - force
        
        Newtons (N)
      - momentum
        
        Newton seconds (Ns)
    - - A vector can be represented on paper by an arrow drawn to scale
      - The arrow indicates the direction and the length of the line is proportional to the magnitude
    - - Resultant force
        
        A number of forces acting on a body may be replaced by a single force which has the same effect on the body as the original forces all acting together
        
        When the original forces are drawn head-to-tail the overall force, called the resultant force from the tail of the first force to the head of the last
      - If the force vectors of 4N and 6N are in the same direction, they simply add together.
      - If the force vectors of 4N and 6N are in the opposite direction, we subtract. The resultant force becomes 2N
  - - - Weight is a force measured in Newtons - It is a force that the earth (or a planet) exerts on an object. It always acts towards the centre of the earth. It depends on gravity and on the planet
      - Mass is measured in kilograms - it is the amount of matter in an object. This always remains constant no matter what planet the object is on
    - - weight = mass x gravitational field strength
      - On the Earth g, the gravitational field strength is 9.81 N/kg. At GCSE this is quoted at 10 N/kg.
        
        1kg = 10N
      - The gravitational field strength is also the acceleration due to gravity. (10m/s^2)
        
        Gravity makes objects accelerate. All objects accelerate towards Earth at 10m/s^2 regardless of their mass.
        
        A canon-ball and a watermelon dropped together at the same time will fall together
  - - - This is also known as the law of Inertia (the property of a body that resists change in motion)
      - A car will maintain a constant speed if the drive force and the drag are balanced. The total (resultant) force is zero.
      - Balanced Forces
        
        When forces are balanced, they are of equal size, but opposite in direction. Note that this does NOT mean there is no force
        
        When forces are balanced, it means that...
        
        If an object is stationary, it will continue to be stationary. If you are sitting on a chair, your weight is balanced by an upwards force from the chair
        
        If an object is moving, it will move at constant speed. If you are pedalling a bicycle at a constant speed, the force you put into the pedals is balanced by the air resistance or friction.
        
        This explains why a car has a maximum speed. The engine has only a limited force it can provide. Eventually the air resistance (and the friction) balance out the force the engine can provide.
    - - Thus Resultant Force = mass x acceleration
        
        Force in N
        
        Mass in kg
        
        Acceleration in m/s
        
        Acceleration is always caused by a resultant force, the vector sum of all the forces, also known as the unbalanced force. The acceleration is always, without exception, in the same direction as the resultant force
      - Unbalanced forces
        
        If the forces are in the same direction, they add up to make a resultant force. In this picture the resultant force is shown in black.
        
        If the forces are in opposite directions they take away. The resultant is in the direction of the bigger of the two forces.
        
        Note in this case, the bigger force is from right to left, so the resultant force is from right to left.
        
        These forces are unbalanced: one is bigger than the other. Wherever there are unbalanced forces on an object, the object will move (accelerate) in the direction of the bigger force.
        
        If the object is stationary, it will accelerate. Therefore it will move
        
        If the object is moving at a constant speed, it will accelerate to a higher speed.
    - - Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation.
        
        Free Body diagram for a book on a table
      - Forces always act in pairs (sometimes called Newton III pairs). This is true whether the forces are in equilibrium, moving, stationary or acceleration.
        
        The following diagram shows the Newton III pairs for the free body diagram of the book
        
        It is important to remember that an object will only start moving or accelerate if the forces acting on it are unbalanced.
  - - - We have said that if there is an unbalanced force, then there is acceleration. Remember that acceleration can be negative as well as positive.
      - This means that a force applied to a moving object can make the object slow down instead of speeding up.
    - - If you drop a ball bearing through thick treacly (viscous) oil, you will see that the ball bearing falls slowly at a constant speed and doesn't change speed.
        
        This is because the weight downwards is balanced by the upwards drag force.
        
        This constant speed is called terminal speed. It is encountered whenever objects fall through fluids (liquids or gases)
      - Air is a fluid because it's a gas.
        
        So a feather falling through air will have a terminal speed of 10cm/s
        
        A sky diver falling through air has a terminal speed of about 60m/s (about 230km/h).
        
        If he hit the ground at that speed, he would be killed.
        
        To reduce the terminal speed he opens a parachute which increases the surface area (and the drag) which slows him down to about 5m/s
        
        Think about sky divers jumping from a plane
        
        Initially the velocity of the skydiver is 0 m/s and the weight of the skydiver causes her to accelerate down at 10 m/s^2. The weight is the resultant force on the skydiver
        
        As the skydiver speeds up, the upwards air friction (drag) increases, causing a reduction in the downward resultant force and thus the acceleration of the skydiver
        
        The skydiver is still increasing in downward velocity but not as rapidly as before
        
        The drag increases until it is equal in magnitude to the weight of the skydiver. Then the drag (upwards force) balances the downward force (weight).
        
        Therefore there is no acceleration and the velocity is constant
        
        This is terminal velocity.
        
        A speed time graph of a parachutist would look like this
  - - - If you are driving at 50km/h, it only needs a light force on the pedal to stop the car in 100m.
      - If you are driving at 100km/h it would need a much bigger braking force to stop the car within 100m
    - - There is a reaction time while you see the emergency, realise there is an emergency, and then put your foot on the brake. During this time, the car is still travelling at the original speed
      - Thinking distance will increase if
        
        The driver loses concentration (mobile, talking, tired...)
        
        The driver has drunk alcohol or taken drugs
        
        The car is driving too fast
        
        Braking distance will increase if
        
        The car is in poor condition, especially if the tires are worn down
        
        The road is wet, icy or greasy as this reduces friction
        
        The car is driving too fast
        
        Thinking distance + braking distance = stopping distance
  - - - moment = force x perpendicular distance
      - The units for moment are Newton metre (Nm)
    - - Moments have 2 possible directions, clockwise or anti-clockwise
        
        If the clockwise moment is bigger than the anticlockwise moment, then the object will turn clockwise
        
        If the anticlockwise moment is bigger than the clockwise moment, then the object will turn anticlockwise
        
        If the clockwise moment is equal to the anticlockwise moment, then the object will stay where it is
      - This leads to an important rule in Physics
        
        Principle of Moments: the system is in equilibrium if... total clockwise moment = total anticlockwise moment
        
        This means that the system is balanced
        
        F1d1 = F2d2
        
        Weight = mass x 10
        
        Garfield has considerably more weight than Odie - so for them to 'see-saw' Odie has to sit a bigger distance away from the pivot (fulcrum)
    - - When you're on a bridge your weigh produces an upward force at each support.
        
        If you are in the middle of the bridge, this upward force is the same at both supports
        
        However if you move to the left, you put more weight on the left-hand support, so the upward force is greater at the left-hand support
    - - In Physics, we find it a lot easier to think of objects as point masses. All objects have a point at which they balance, called the centre of mass.
        
        We think of all the mass as being concentrated at the centre of mass
      - The centre of mass is the point at which the weight of the object is said to act. The green arrow is the line of action of the force from the centre of mass. Force due to gravity on mass is weight
      - Note that it is called the centre of mass not centre of weight. This is because if the object were in space, it would still have a centre of mass, even though it was weightless. Sometimes the centre of mass is called the centre of gravity
        
        For any regular object (e.g. box, a cylinder, etc.), the centre of mass is in the very centre of the object
        
        The stability of a system is connected to its equilibrium state (the state where there are no net forces acting - all are balanced). There are three types of equilibrium that can be used to describe a system.
        
        If a system in stable equilibrium is disturbed slightly, then it tends to return to its original equilibrium state
        
        It usually oscillates around that state first but eventually settles down in exactly the same position as it started at.
        
        It does this because its centre of gravity is already at the lowest possible position
        
        An unstable system moves away from its original equilibrium state when disturbed. It is not easy to return it to the original equilibrium position.
        
        It wants to move away from the original position as soon as you disturb it.
        
        It does this because its centre of gravity is high up and when disturbed the centre of gravity continues to move to a lower position (a more stable one)
        
        A neutral system is one where the centre of gravity of the system is always as the same level. If you disturb it slightly it will therefore settle in the new position without a need to move further on or to move back
        
        Centre of mass of a Funnel
        
        The centre of gravity of the funnel is high up - it will be at the middle of the mass - and as most of the mass is in the funnel end it will be up above the middle of its height when it is standing like this.
        
        You only have to tip the funnel a little way for the line of action of the weight (the weight arrow) to fall outside the base and produce a turning moment. The funnel will therefore tip over easily.
        
        The centre of gravity has not changed but the line of action acts through a wider base
        
        For the funnel to tip over the line of action of the weight force arrow will have to form a clockwise moment. You have to push it a lot for that to happen
        
        Note that when the funnel is standing on the wide base area it has to be tipped a long way before its equilibrium position shifts.
        
        That is because the base is wide and the centre of gravity low
      - Centre of Mass and Stability
        
        An object is stable if it is difficult to knock over, if it will tend to return to its original position when disturbed
  - - - If we load a spring, we find that the extension (e) or stretch is proportional to the force (F). If we double the force, we double the stretch
    - - When you pull on an elastic band, it is easy to stretch initially, but becomes much more difficult as you stretch is.
        
        Force and extension are not proportional to each other
        
        Unlike the spring, it does not follow the same pattern when it is released
      - If you repeatedly stretch and release an elastic band, you will notice that it gets hot
    - - A metal wire obeys Hooke's law when the force is relatively low.
      - If a higher force is applied to the wire, the wire starts to become thinner at one point, and becomes easier to stretch
        
        This is the elastic limit (Y on the graph).
      - Beyond the wire's elastic limit, the wire is permanently deformed as it can no longer return to its original shape. A very small increase in force then causes the wire to break
  - - - The kinetic energy of the plane is so high that it is vaporised on impact, except for the wing tips
      - The brakes on your bike get hot when you have converted kinetic energy to heat. You can feel the force as a moving football hits you. But you cannot see or feel momentum
    - - collisions
      - explosions
      - Both of these are difficult to explain just by studying the energy changes. Momentum makes them easier to understand.
    - - Momentum is the product of mass and velocity, which means mass and velocity multiplied together
        
        momentum = mass x velocity
        
        The units are kilogram metres per second (kg m/s) [or Newton seconds (ns); they are the same]
        
        Momentum has a direction. The direction is very important. The momentum will ALWAYS be in the direction of the velocity
    - - When two objects come together, that is a collision; when two objects fly apart, we call that na explosion
      - Momentum is always conserved, provided no external forces act. This means that momentum before a collision or explosion = momentum after a collision or explosion
        
        We always assume that no external forces are acting
      - Explosion