GCSE Physics

• Speed is defined as the distance travelled per unit time.
• If the speed of something is changing, it is accelerating.
• The acceleration of free fall near to the Earth is constant.
• Average speed is calculated by dividing the total distance by the total time.
• Velocity is the speed in a given direction.
• Acceleration is the change in velocity per unit time.
• Acceleration is calculated by dividing the change in velocity by the time taken.
• The equation for acceleration is a = v - u/t.
• The equation for final speed is v2 = u2 + 2as.
• Distance is measured in metres (m), time in seconds (s), speed and velocity in metres per second (m/s), and acceleration in metres per second squared (m/s2).
• In a distance-time graph:

  The gradient is velocity

  • Negative gradient is returning back to the

  starting point

  • A horizontal line means it is stationary

  • If the distance is zero, it is back at the starting point

  • A curved line means that the velocity is changing,

  and it is accelerating.
• In a velocity-time graph:

  • The gradient is acceleration

  Negative gradient (i.e. negative acceleration)

  is deceleration

  • If the speed is zero, it is at rest

  • A horizontal line means constant speed

  • The area under the line is the distance travelled

  • A curved line means that the acceleration is

  changing.
• A vector has magnitude and direction

  A scalar has just a magnitude
• Scalars -

  Distance

  Speed

  Time

  Energy


  Vectors -

  Displacement

  Velocity

  Acceleration

  Force
• Forces can change the speed, shape or direction of a body and they are measured in Newtons (N).

  There are various different types of forces (e.g. gravitational, electrostatic).
• Friction is a force between two surfaces which impedes motion and results in heating. Air

  resistance is a form of friction.
• Newton’s first law states that an object has a constant velocity unless acted on by a resultant force.
• Newton’s second law states that force = mass × acceleration

  F = ma
• • Newton’s third law states that every action force has an equal and opposite reaction force.

  For example, the force of the Earth’s gravity on an object is equal and opposite to the force of the object’s gravity on the Earth.
• Mass is a measure of how much matter is in an object, measured in kilograms (kg). 


  Weight is a gravitational force (the effect of a gravitational field on a mass).
• weight = mass × gravitational field strength 


  W = mg
• The weight of an object acts through its centre of gravity.
• • Initially, there is no air resistance and the only force acting on it is weight
  • As it falls, it accelerates which increases its speed and hence air resistance
  • This causes the resultant force downwards to decrease
  • Therefore, the acceleration decreases, so it is not speeding up as quickly
  • Eventually they are equal and opposite and balance so there is no resultant force
  • So, there is no acceleration and the terminal velocity is reached
• Factors which increase the

  thinking distance include:

  • Greater speed
  • Slower reaction time due to alcohol, tiredness or distractions. Reaction time can also be increased by caffeine, which reduces the thinking distance.

  Factors which increase the stopping distance include:

  • Greater speed or mass
  • Poor road conditions (icy, wet) or car conditions (worn tires, worn brake pads)
• The distance travelled in the time between pressing the brakes and the vehicle coming to a stop is called the braking distance.
• The distance travelled in the time between the driving realising he needs to brake and actually pressing the brakes is called the thinking distance.
• The stopping distance is the sum of the thinking distance and braking distance.
• •Elastic deformation is when the object returns to its original shape when the load has been removed, an example being a spring being stretched under normal usage.
• Hooke’s law states that for a spring, F = kx

  where F is the force applied to the spring, k is the spring constant, and x is the extension.
• Linear (straight line) force-extension graph:

  • Elastic deformation following Hooke’s law

  o The point it stops being linear is called the

  limit of proportionality. From then on, it does not obey Hooke’s law.

  • Gradient is the spring constant, k

  Non-linear (curved line) force-extension graph:

  • Deformation not following Hooke’s law

  • After this region, it will fracture
• The moment of a force is a measure of its turning effect, measured in Newton metres (Nm).
• moment = force × perpendicular distance from the pivot

  moment = Fd
• An object is in equilibrium when the sum of clockwise moments equals the sum of anticlockwise moments (the principle of moments) and there is no resultant force.
• The momentum of an object is the product of its mass and velocity:

  • momentum = mass × velocity p = mv

  • It is measured in kilogram metres per second (kgm/s).
• The force exerted on an object is equal to its change in momentum over time:

  • force = change in momentum/time taken

  F =mv−mu/t
• Safety features in cars work by increasing the time taken for the people in the car to come to rest (i.e. there is the same change in momentum in a longer time, so the force is reduced). For example, a seatbelt achieves this by stretching.
• In a collision, the total momentum before is equal to the total momentum afterwards
• Current is measured in amperes and is the rate of flow of charge at a point in the circuit

  current is I=Q/t.

  In metals, current is due to a flow of electrons. Can be the flow of ions. Conventional current is the rate of flow of positive charge - this is the opposite direction to the flow of electrons because electrons are negatively charged. Current is conserved at a junction in a circuit because charge is always conserved.

  It is measured with an ammeter and connected in series
• Potential difference is the work done per unit charge in moving between two points, measured in volts.

  Measured with a voltmeter and is placed in parallel across a component.

  The higher the potential difference, the greater the current
• Resistance is the opposition of flow of charge (current)

  The resistance of a component is measured in ohms and is given by the potential difference across it divided by the current through it. The greater the resistance, the harder it is for current to flow through the component.

  In an ohmic conductor, the current is directly proportional to the voltage. In a non-ohmic conductor (like a filament lamp) the resistance changes as the voltage and current change.