Yr 10 Physics March 2017

Formulae you need to know are in bold.

CONTENTS

1. MOTION GRAPHS
2. FORCES
3. WORK, ENERGY, AND POWER
4. MOMENTUM
5. STATIC ELECTRICITY
6. ELECTRIC CURRENT

1 Motion Graphs

• The slope or gradient of a distance-time graph represents speed.

• The velocity of a body is its speed in a given direction, velocity is a vector.

  NB other examples of vectors are force, acceleration, momentum.

• quantities with no specified direction are called scalars

  NB examples of scalars are energy, time, distance, temperature, etc

• Motion Equations you need to be familiar with:

• average velocity = total distance / total time,   v  =  d/t

• acceleration = change in velocity / time taken  

•  SUVAT EQUATIONS:   s = ½ (u + v) t,    a  =  (v - u) / t ,    v2 = u2 + 2 a s

• The slope(gradient) of a velocity-time graph = acceleration.

• The area under a velocity-time graph = distance travelled.

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2 Forces

• Force is a vector (like velocity, it has size and direction)

• Distance or speed are called  scalars, they have size but not direction.

• The faster a body moves through a fluid the greater

the opposing frictional force which acts on it.

• A body falling through a fluid will initially accelerate

due to gravity, eventually the resultant force on the body

will be zero, and it will fall at its terminal velocity.

• at terminal velocity Weight down = Friction up

• weight = mass Χ gravitational field strength,  W  = mg

(newton, N) (kilogram, kg) (newton/kilogram, N/kg)

• Whenever two bodies interact, the forces they exert

on each other are equal & opposite (Newton's Third Law)

• A number of forces acting on a body may be replaced by

a single force which has the same effect as the original set

of forces. The single force is called the resultant force (here in red):

• If the vectors to be combined are at right angles then

  the resultant is reprented by the diagonal of the rectangle  as shown below:


• eg. the resultant can be found by drawing the rectangle above to scale using say, 1cm = 1N,
   then measure the length of the diagonal and convert it to N.

• If the resultant force acting on a stationary body is zero,

it is either at rest, or moving at a steady speed.

• If the resultant force acting on a stationary body is not zero,

the body will accelerate in the direction of the resultant force.

• Resultant force = mass Χ acceleration,   OR,   F = ma

(newton, N) (kilogram, kg) (metre per second squared m / s2 )

• When a vehicle travels at a steady speed the frictional

forces balance the driving force (zero resultant force).

• Stopping distance = braking distance + thinking distance.

Typical Stopping Distances


30
MPH

The Highway Code by Select All

The Highway Code by Select All

 
 

9 metres     14 metres

= 23 metres
(75 feet)
or 6 car lengths


50
MPH

The Highway Code by Select All

The Highway Code by Select All

 
 

15 metres

38 metres

= 53 metres
(175 feet)
or 13 car lengths


70
MPH

The Highway Code by Select All

The Highway Code by Select All

The Highway Code by Select All

 

21 metres

75 metres

= 96 metres
(315 feet)
or 24 car lengths


 The Highway Code by Select AllThinking Distance

 The Highway Code by Select AllBraking Distance

 http://www.highway-code.com/images/wt.gifhttp://www.highway-code.com/images/wt.gifaverage car length = 4 metres

• A driver’s reaction time is affected by tiredness, age, drugs, or alcohol.

• A vehicle’s braking distance depends on the brakes, tyres, the road, and weather.

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3 Work, Energy, Power

• When a force causes a body to move through a distance,

 energy is transferred, and work is done.

• work done = force Χ distance moved in direction of force,   W  =  F x d

     (joule, J) (newton, N) (metre, m)

• Work done against frictional forces is mainly changed into heat.

• Squashed materials have elastic potential energy stored in them.

• The kinetic energy of a body depends on its mass and its speed.

      kinetic energy = ½  x  mass  x  v2 ,     KE =   ½  m v2

(joule, J) (kilogram, kg) (metre/second)2 , (m/s)2 )

• Gravitational Potential Energy GPE depends on height and weight:

    GPE  =  weight  x  height  ,   GPE  =  m g h

   (Joule J,  Newtons N,  metres m)

•  Power  =  work done /  time taken,   P  =  W / t

     P = Work / t   or  Energy / t ,  units are Watts

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4 Momentum

• momentum = mass Χ velocity ,   mom. = mv

(kilogram metre/second, kg m/s) (kilogram, kg) ( m/s)

• Momentum has both size and direction (another vector)

• When a force acts on a body a change in momentum occurs.

• Momentum is conserved in any collision/explosion,

provided no external forces act,

ie. momentum before collision = momentum after collision

 Top - two trolleys of same masses exploding apart, bottom - two trolleys of different masses exploding apart

• force = change in momentum / time taken for change,   F  =  mv - mu / t

we use this equation to explain why the force is large when the impact time is small

in a collision.

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5 Static electricity

• When materials are rubbed against each other they can

become electrically charged. Negatively charged electrons

are rubbed off one material onto the other.

• The material that gains electrons becomes negatively charged.

The material that loses electrons has an equal positive charge.

• Two charged bodies will exert a force on each other.

• Like charges repel, unlike charges attract.

• Electric charges move easily through metals (conductors), but not through insulators.

• Electric fields exist around charged objects (like magnetic fields around magnets)

 • Electric field direction is always + to –  

 • Electric field is strongest where field lines are closest together

 

• The rate of flow of electric charge is called the current.

• current in a wire is a flow of negatively charged electrons

   current  =  charge / time,  OR,   Q  =  I t

     (Amps)       (Coulombs)   (seconds)

• A charged body can be discharged by connecting it to earth

with a conductor. Charge then flows through the conductor.

The greater the charge on an isolated body the greater the potential

difference between the body and earth. If the pd is high enough a

spark may jump to earth.

• Electrostatic charges can be useful, eg insecticide sprayers.

• Electrostatic charge build up can be a nuisance, eg. lightning.

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6 Electric Current

• Current-potential difference graphs are used to show how

current through a component varies with pd across it.

A resistor               A filament lamp                  A diode

• The current through a resistor (at a constant temperature)

is proportional to the voltage across the resistor.

• Voltage = current Χ resistance,   V  =  I R

     (volt, V) (ampere, A) (ohm, Ω)

• The resistance of a filament lamp increases as the

 temperature of the filament increases.

• The current through a diode flows in one direction only.

The diode has a very high resistance in the reverse direction.

• The resistance of a light-dependent resistor (LDR)

 decreases as light intensity increases.

• The resistance of a thermistor decreases as the temperature increases.

• The current through a component depends on its resistance,

 the greater the resistance the smaller the current.

• The voltage from cells in series is the sum of the voltage of each cell.

• Rules for components connected in series like below:

 

 

 

 

− total resistance = sum of the resistance of each component

− there is the same current through each component

− the total voltage of the supply is shared between the components.

• Rules for components connected in parallel:

− voltage across each component is the same

− the total current through the whole circuit is the sum

 of the currents through the separate components.

•  in series circuits if 1 item fails, all items turn off, in parallel circuits

  other items still work, ALSO all items can be switched independently so

  parallel wiring for lighting is preferred.

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