p2 - forces

• equation linking speed, distance and time

  distance (m) = speed (m/s) x time (s)
• how can speed be calculated fro a distance-time graph?

  the speed is equal to the gradient of the graph
• what must be done to calculate speed at a given time from a distance-time graph for an accelerating object?

  • drawing a tangent to the curve at the required time
  • calculating the gradient of the tangent
• equation for acceleration

  acceleration (m/s^2) = change in velocity (m/s) / time taken (s)
• how can distance travelled by an object be calculated from a velocity-time graph?

  it is equal to the area under the graph
• what is a scalar quantity?

  a quantity with only magnitude and no direction
• what is a vector quantity?

  a quantity with both magnitude and direction
• how can a vector quantity be drawn and what does this show?

  • as an arrow
  • the length of the arrow represents the magnitude
  • the arrow points in the associated direction
• examples of vector quantities

  • force
  • displacement
  • velocity
• examples of scalar quantities

  • temperature
  • time
  • mass
  • speed
  • distance
• how do you calculate average speed for non-uniform motion?


  average speed (m/s) = change in distance (m) / change in time (s)
• what piece of apparatus may be used to record the time taken for a very fast object to move a given distance?

  light gates
• what is a force?

  a push or pull acting on an object due to an interaction with another object
• two categories force can be split into:

  1. contact force (objects touching)
  2. non-contact force (objects separated)
• examples of contact force

  • friction
  • air resistance
  • tension
• examples of non-contact force

  • gravity
  • electrostaticity
  • magnetism
• what is the name given to the single force that is equivalent to all the other forces acting on a given object?

  resultant force
• newton's first lawon a stationary object

  if the resultant force on a stationary object is zero, the object will remain at rest
• newton's first law on a moving object

  if the resultant force on a moving object is zero, the object will remain at constant velocity
• if an object changes direction but remains at a constant speed, is there a resultant force?

  since there is a change in direction, it means there is a change in velocity so there must be a resultant force
• when does an object fall with terminal speed?

  • when the upwards forces (air resistance) and the downwards forces (weight) are equal to each other
  • no resultant force, so constant speed
• what happens to the magnitude of air resistance on a falling object when the velocity increases?

  as velocity increases, the force of air resistance on the object will also increase
• equation for newton's second law

  • resultant force (N) = mass (kg) x acceleration (m/s^2)
  • F = m x a
• newton's second law

  an object's acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass
• what is inertia?

  • the tendency of an object to continue in its state of rest or uniform motion
  • an object's resistance to motion
• what is intertial mass?

  • a measure of how difficult it is to change a given object's velocity
  • the ratio of force over acceleration
• newton's third law

  whenever two objects interact, the forces that they exert on each other are always equal and opposite
• equation used to calculate an object's momentum

  momentum (kg m/s) = mass (kg) x velocity (m/s)

  P = m x v
• unit used for momentum

  • kg m/s
  • kilogram metres per second
• in a closed system, what can be said about the momentum before and after a collision?

  the total momentum before is equal to the total momentum afterwards
• equation linking change in momentum, force and time

  • force x time = change in momentum
  • F x Δt = P
  • F x Δt = m x Δv
• explain how a seatbelt improves a passenger’s safety during a collision

  • the passenger must decelerate from the vehicle’s velocity at impact to zero, so they experience a force.
  • this force is equal to the rate of change of momentum.
  • seatbelts increase the time over which the force is applied, reducing the rate of change of momentum and therefore reducing the force felt by the passenger
• what quantity is equal to the force experienced in a collision?

  the rate of change of momentum
• what does it mean if a force is said to have done 'work'?

  the force causes an object to be displaced through a distance
• what is the equation used to calculate work done? give appropriate units.

  • work done (J) = force (N) x distance (m)
  • W = F x d
• under what circumstances is one joule of work done?

  when a force of one newton causes a displacement of one metre
• what is power?

  the rate of which work is done
• unit for power

  watt (W)
• what is one watt equal to?

  • one joule per second
  • 1 W = 1 J/s
• equation used to calculate power

  power (W) = work done (J) / time (s)