gcse phy topic 1

• Speed


  Distance travelled per unit time
• Acceleration

  Change in velocity per unit time
• Velocity

  Speed in a given direction
• Units

  • Distance in metres (m), Time in seconds (s), Speed and Velocity in metres per second (m/s), Acceleration in metres per second squared (m/s^2)
• Distance-time graph

  Gradient is velocity, Negative gradient is returning to starting point, Horizontal line is stationary, Distance zero is back at starting point, Curved line is accelerating

  
• Velocity-time graph

  Gradient is acceleration, Negative gradient is deceleration, Speed zero is at rest, Horizontal line is constant speed, Area under line is distance travelled, Curved line is changing acceleration

  
• Vector


  Has magnitude and direction
• Scalar


  Has just a magnitude
• Examples of Scalars


  • Distance, Speed, Time, Energy

  
• Examples of Vectors

  • Displacement, Velocity, Acceleration, Force
• Forces


  • Can change speed, shape or direction of a body, Measured in Newtons (N)

  
• Friction

  Force between two surfaces which impedes motion and results in heating, Air resistance is a form of friction
• Finding resultant of forces

  Add if in same direction, Subtract if in opposite direction
• Newton's first law

  An object has a constant velocity unless acted on by a resultant force
• Newton's third law

  Every action force has an equal and opposite reaction force
• Mass


  Measure of how much matter is in an object, Measured in kilograms (kg)
• Weight

  Gravitational force, Effect of a gravitational field on a mass, Calculated as Mass x Gravitational field strength
• Gravitational field strength on Earth is 10N/kg
• Weight of an object acts through its centre of gravity
• Motion of a body falling in a uniform gravitational field

  Initially no air resistance, only weight force, Acceleration increases speed and air resistance, Resultant force decreases, Acceleration decreases, Terminal velocity reached when forces balance
• Thinking distance


  Distance travelled between driver realising need to brake and pressing brakes, Increased by greater speed, slower reaction time
• Braking distance


  Distance travelled between pressing brakes and vehicle stopping, Increased by greater speed, mass, poor road/car conditions
• Stopping distance

  Sum of thinking distance and braking distance
• Elastic deformation


  Object returns to original shape when load removed, Example: Stretching a spring

  
• Force-extension graph

  Linear graph follows Hooke's law, Curved graph does not follow Hooke's law, Limit of proportionality where it stops being linear
• Conservation of momentum

  Total momentum before a collision equals total momentum afterwards
• if the net force on an object is zero, then the object has zero acceleration (it remains stationary)
• Why falling objects reach a terminal velocity

  1. Object starts falling

  2. Weight is larger than air resistance

  3. Resultant force is downwards, causing acceleration

  4. As velocity increases, air resistance increases

  5. Air resistance equals weight, resultant force is zero

  6. Object reaches terminal velocity
• Terminal velocity


  Velocity remains constant, no longer accelerating or decelerating
• When an object first starts falling, its weight downwards is much larger than any air resistance upwards
• As the object accelerates and its velocity increases

  The air resistance will also increase
• When the air resistance equals the weight


  The resultant force will fall to zero
• Terminal velocity


  Velocity at which there is no resultant force acting on the object, so no acceleration
• If the object opens a parachute

  The surface area increases massively, air resistance increases, resultant force is upwards, object decelerates and reaches a new, lower terminal velocity
• The new terminal velocity with the parachute will be lower than the terminal velocity without the parachute, because the object has slowed down
• Different types of elasticity

  1. Explain

  2. Explain spring constant

  3. Explain Hooke's law

  4. Look at force extension graphs
• Applying force to an object

  Can cause it to compress, stretch or bend
• Other objects are less elastic so harder to notice change in shape
• To stay still, need to apply more than one force
• Deformation


  When an object changes shape