Physics Unit 2

Physics Unit 2

1. Forces and their effects

   • Resultant forces
     • Whenever two objects interact the forces they exert are equal and opposite
       • hand pushing on table= table pushing back
     • Lots of forces acting on one point
       • replaced by single force doing everything together
         • resultant force
           • may cause a change in the state of object
   • Forces and motion
     • Acceleration
       • a=f/m
       • m/s squared
       • determined by resultant force (N) and mass (kg)
       • the rate of change of velocity per unit of time
       • a= v1-v2/t
         • final velocity- initial velocity/ seconds taken
     • velocity/time graphs
       • Velocity = An objects speed in a certain direction
       • the steeper the gradient the greater the acceleration
         • a downward slope means constant decceleration
       • a flat line = constant velocity
       • distance = area of graph shape
       • m/s
   • Forces and breaking
     • when a car travels at a steady speed the driving force is balanced by resistive forces
       • therefore the resultant force is 0
     • when the breaks are applied
       • work done by the friction between the brakes and the wheels reduce the kinetic energy of the vehicle
       • the temperature of the breaks increase
     • Stopping distances
       • a drivers reaction time can be influenced by
         • tiredness
         • drugs
         • alcohol
       • a vehicles braking distance can be affected by
         • adverse road/weather conditions
         • poor condition of the vehicle
   • Forces and terminal velocity
     • the faster an object falls the greater the friction
     • Three stages of falling
       • An object falls b/c gravity and weight. There is no air resistance. There is a resultant force downwards
         • It gains speed, air resistance increases, there is a resultant force downwards.
           • The objects weight is balanced by the air resistance, no resultant force, the object reaches terminal velocity
     • w=m x g
       • N=kg x N/kg
   • Forces and elasticity
     • forces may change the shape of an object
     • a force applied to an elastic object will result in the object stretching and storing elastic potential energy
     • if an object returns to its original shape elastic potential energy is stored in the object when work is done on it
     • f = k x e

2. The Kinetic energy of objects speeding up and slowing down

   • Forces and Energy
     • work is done when a force causes an object to move through a distance
       • Joules
       • Energy is transferred when work is done
     • W = F x d
     • power = work done in a given time
       • P = E/t
     • the change in gravitational potential energy is what happens when something is raised
       • Ep = m x g x h
     • kinetic energy = 1/2 x mass x speed
       • joules/kg/ meters per second
   • Momentum
     • Momentum is the tendency of an object to carry on moving in the same direction
     • Momentum = mass x velocity
     • kg m/s
     • momentum is conserved
       • if nothing else acts on it the momentum stays the same through collisions and explosions

3. Currents in electrical circuits

   • Static electricity
     • When insulating materials are rubbed against each other they become electrically charged
       • Negatively charged electrons are rubbed of one material and onto the other
         • equal amounts of opposite charge
       • this only works for insulators
     • when two charged objects are brought together they exert a force on each other
     • electrical charges move better through other substances
       • e.g. metal
   • Electrical circuits
     • Electric current is a flow of charge
       • The size of the electric current is the rate of flow of electric charge
         • I = Q/T
           • Current = charge/time
           • amps = coulombs/ seconds
     • potential difference is the work done per coulomb of charge that passes between two points in a circuit
       • V = W/Q
         • potential difference = work done/ charge in coulombs
     • Current / potential difference graphs show that as current changes so does potential difference
       • V= I x R

4. Using Mains Electricity safely and the power of electrical appliances

   • Household electricity
     • direct current
       • current that always passes in the same direction
         • batteries, cells
     • alternating current
       • always changing direction
       • mains electricity
         • 50 hertz
         • 230 V
     • Electrical cables
       • 2 or 3 inner wires
         • copper core
       • flexible plastic outer layer
     • Structure of a plug
     • If there is too much current
       • it is stopped by the fuse/ circuit breaker/ live wire
         • when there is too much current the fuse will melt and break the circuit
       • a residual circuit breaker will detect a difference in the current between the live and neutral wires
         • hey are faster than fuses
     • appliances with metal cases are usually earthed
       • the earth wire creates a safe route for the current to flow through
   • Current, charge and power
     • When an electrical charge flows through a resistor, the resistor gets hot
       • therefore energy is wasted
         • filament bulbs waste energy
     • The rate at which energy is transferred by an appliance is the power
       • power = current x potential difference
         • P = I x v
       • energy transferred = potential difference x coulombs
         • E = V x Q
       • power = energy/time
         • P=E/t

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