Forces and motion

• Does a distance quantity require a specific direction? I.e. is it a scalar or vector quantity?

  No specific direction is required so it is a scalar quantity
• if an object moves 3 metres to the left and then 3 metres back to its initial position, what is the object’s total displacement?

  • the object has zero displacement
  • displacement is a vector quantity so it also involves direction
  • the object starts and ends at the same point
• state a typical value for the speed of sound

  330 m/s
• what is a typical value for human walking speed?

  1/5 m/s
• what is a typical value for human running speed?

  3 m/s
• what is a typical value for human cycling speed?

  6 m/s
• state the equation linking distance, speed and time. give appropriate units.

  distance = speed x time

  • distance (m), speed (m/s), time (s)
• why can an object travelling at a constant speed in a circle not have a constant velocity?

  • speed is a scalar quantity
  • velocity is a vector quantity which means it can only be constant if the direction is constant
  • in circular motion, the direction is continuously changing
• how can speed be calculated from 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
• state the equation for the average acceleration of an object. give appropriate units.

  acceleration = (change in velocity) / (time taken)

  • acceleration (m/s squared), velocity (m/s), time (s)
• how can the distance travelled by an object be calculated from a velocity-time graph?

  it is equal to the area under the graph.
• give an approximate value for the acceleration of an object in free fall under gravity near the earth’s surface.

  9.8 m/s squared
• what can be said about the resultant force acting on an object when it is falling at terminal velocity?

  • the resultant force is zero
  • when at terminal velocity, the object is moving at a constant speed and so isn’t accelerating
• state Newton’s first law for a stationary object.


  if the resultant force on a stationary object is zero, the object will remain at rest
• state Newton’s first law for a moving object.

  if the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction) .
• What can be said about the braking forces and driving forces when a car is travelling at constant velocity?

  The braking forces are equal to the driving forces.
• If an object changes direction but remains at a constant speed, is there a resultant force?

  Since there is a change in direction, there is a change in velocity and so there must be a resultant force.
• What is inertia?

  The tendency of an object to continue in its state of rest or uniform motion.
• State the defining equation for Newton's Second Law.

  • Resultant force = Mass x Acceleration
  • F = ma
• State Newton's Second Law in words.

  An object's acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass.
• What is inertial mass?

  • A measure of how difficult it is to change a given object's velocity
  • The ratio of force over acceleration
• What is the symbol used to represent an approximate value?

  ~
• State Newton's Third Law.

  Whenever two objects interact, the forces that they exert on each other are always equal and opposite.
• What is the stopping distance of a vehicle equal to?

  The sum of thinking distance and braking distance.
• For a given braking distance, if the vehicle's speed is increased, what can be said about its stopping distance?

  The stopping distance is increased with an increase in speed.
• Give a typical range of values for human reaction time.

  0.2 seconds - 0.9 seconds
• Give three factors which can affect a driver's reaction time.

  1. Tiredness
  2. Drugs
  3. Alcohol
• Give two factors which may affect braking distance.

  1. Adverse (wet/icy) road conditions
  2. Poor tyre/brake conditions
• Describe the energy transfers that take place when a car applies its brakes.

  • Work is done by the friction force between the brakes and wheel
  • Kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs
• To stop a car in a given distance, if its velocity is increased, what must happen to the braking force applied?

  The braking force must also be increased.
• State two consequences of a vehicle undergoing very large decelerations.

  

  1. Kinetic energy converted to heat is very high causing brakes to overheat
  2. Loss of control of the vehicle