Topic 5 Forces

• What is a force?


  A force is a push or pull that can cause an object to move, stop, or change direction.
• What is a contact force?

  Its when objects are physically touching.

  E.g.friction, air resistance & tension
• What is a non-contact force?

  When objects are not touching.

  E.g. magnetism, electrostatic force & gravity
• What is a scalar quantity?

  Something that has magnitude only

  E.g. distance, speed, mass, energy & temperature
• What is a vector quantity?

  A quantity that has both magnitude and direction.

  E.g. displacement, velocity, acceleration, force & momentum
• What does no resultant force mean?

  No acceleration !
• How is weight calculated?

  Weight = mass x gravitational field strength

  • W = mg

  Weight is measured in newtons (n)

  Mass is measured in kilograms (kg)

  Gravitational field strength is measured in newtons per kilogram (N/kg)

  Earth has 9.8N/Kg & the Moon has 1.6N/Kg
• What is work done and how is it calculated?

  Work done means energy transferred by a force

  Work done = force x distance

  • W = Fd

  Work done is measured in joules (J)

  Force is measured in newtons (N)
• What does any object that deforms elastically obey?

  Hooke's Law
• What is Hooke's law?

  Force and extension are directly proportional
• How can force be calculated?

  Force = spring constant x extension

  • F = Ke

  Force is measured in newtons (N)

  Spring constant is measured in N/m

  Extension is measured in m
• How do we calculate elastic potential energy?

  E = 1/2 x k x e^2
• How do we calculate a moment?


  Moment = force x distance from pivot

  Moment is measured in Nm

  Force is measured in N

  Distance is measured in m
• What is the principle of moments?

  When the sum of (total) clockwise moments is equal to the sum of anticlockwise moments, a system is in equilibrium
• How can gears be used to increase moments?

  Gears can drive a larger gear with a smaller one to increase the moment
• Why does pressure in fluids occur?

  Pressure underwater occurs due to the weight of the column of water above an object exerting a force down on it.
• How do we calculate pressure in fluids?

  Pressure = height x density x gravitational field strength

  • P = hpg

  Pressure in Pa

  Height in m

  Density in kg/m^3

  g in N/kg
• How does pressure in a gas occur?

  Due to particles colliding with the walls of its container, exerting a force on them
• How can we decrease the volume in a gas?

  Add more gas or increase temperature which increases the frequent collisions
• What does the gradient of a distance/time graph give?

  Speed/velocity

  speed or velocity (m/s) = distance or displacement(m)/time(s)

  • v = d/t
• What does the gradient of a speed/velocity time give?

  Acceleration

  acceleration (m/s^2) = change in velocity(m/s)/time(s)

  • a = v/t
• What is Newton's equation of motion?

  v^2 - u^2 = 2as (Newton's third law)

  s = displacement (m)

  u = initial velocity (m/s)

  v = final velocity (m/s)

  a = acceleration (m/s^2)
• What are Newton's law of motion?

  First law: If no resultant force acts on an object, its motion will be constant (velocity will not change)

  Second law: An object will accelerate in response to a resultant force

  Third law: Whenever two object interact with each other, they exert equal and opposite forces on each other
• What is stopping distance and how is it calculated?

  The distance a vehicle travels from the moment the driver applies the brakes until the vehicle comes to a complete stop.

  Stopping distance = thinking distance + braking distance
• What is thinking distance affected by?

  • Speed
  • Distractions
  • Alcohol
  • Drugs
  • Tiredness
• What is braking distance affected by?

  • Speed
  • Road conditions
  • Weather conditions
  • Tyre and brake condition
• How is momentum calculated?

  Momentum = mass x velocity

  • Momentum = mv

  Momentum is measured in kgm/s

  Mass is measured in kg

  Velocity is measured in m/s

  In any collision, total momentum is conserved
• What is force?

  Another measure of the rate change of momentum

  F = ma = change in momentum/time
• How does time affect momentum?

  The longer the time over which momentum is lost or gained, the lower the force exerted
• What is the purpose of seat belts, air bags and crumple zones?

  They all increase collision time, so momentum is lost over a longer time