P2 2 Forces (continued)

Falling objects
1. The weight of an object is the force of gravity on it. As it is a force, weight is measured in newtons (N)
2. The mass of an object is the quantity of matter in it. Mass is measured in kilograms (kg)
3. Bathroom and kitchen scales are misleading because they claim to measure WEIGHT in the units of MASS
4. The force of gravity on an object is the gravitational field strength where the object is
  1. The unit of gravitational field strength is newtons per kilogram N/kg
  2. The gravitational field strength on Earth is approximately 10 N/kg
  3. The gravitational field strength on the Moon is approximately one sixth of that on Earth, so 1.6 N/kg
  4. weight (in newtons) = mass (in kilograms) x gravitational field strength (in newtons per kilogram)
    1. W = mg
5. Terminal velocity
  1. The terminal velocity of a falling object is the maximum velocity it reaches when falling in a fluid
    1. "Fluid" means a liquid or gas, including the air
    2. At terminal velocity, the forces on the object are balanced
      1. "Balanced" means equal in magnitude but opposite in direction
      2. Weight downwards equals the drag force (or air resistance) upwards
        Drag force increases with speed
Stretching and squashing
1. The extension of a spring is directly proportional to the force applied to it, provided the limit of proportionality is not exceeded
  1. When the limit of proportionality is exceeded, the spring is permanently deformed and will not return to its original length
  2. This is known as Hooke's Law
  3. Force applied (in newtons) = spring constant (in newtons per metre) x extension (in metres)
    1. Extension is the difference between the length of a spring and its original length
    2. The spring constant is the force per unit extension needed to stretch it
    3. F = ke

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