Unit 3: Dynamics

• If an object is in equilibrium, it means the object is not accelerating; it is either:

  • Stationary
  • Moving at a constant velocity
• Newton’s third law states that ‘every action force has an equal and opposite reaction force’
• Newton’s second law is F = ma, where F is resultant force (N), m is mass (kg), and a is acceleration in the direction of the resultant force (ms^-2)
• An object of greater mass will have a greater resistance to changes in its motion than an object of smaller mass because a = F/m. Therefore, the object of greater mass requires a larger force to cause the same acceleration on the object, in any direction
• Newton’s first law states that an object at rest or moving with constant velocity will stay that way unless a resultant force acts upon it
• Weight (N) is the product of mass (kg) and the acceleration due to gravity, g (ms^-2). W = mg
• Terminal velocity is reached when the air resistance and thrust force become balanced, resulting in a zero resultant force and zero acceleration, allowing the object to move at a constant maximum velocity
• In an elastic collision, the kinetic energy before is equal to the kinetic energy afterwards, while in an inelastic collision, some energy is lost to the surroundings
• An equation that can be used to calculate momentum is momentum = mass × velocity. The units of momentum are kgms^-1
• The principle of conservation of momentum states that in an isolated system, the total momentum is always conserved
• Linear momentum is always conserved, not only in elastic collisions
• The rate of change of momentum can also be described as the resultant force
• Impulse is the change in momentum, represented by F∆t = ∆(mv)
• The area underneath a force-time graph represents the impulse, which is the change in momentum
• The equation used for the principle of conservation of momentum in one-dimensional collisions is m1u1 + m2u2 = m1v1 + m2v2
• For a force of 18N acting on an object of 6kg, the object’s acceleration is 3ms^-2
• For an elastic collision, the relative speed of approach is equal to the relative speed of separation, not faster
• Most real-life collisions are inelastic
• Most real-life collisions are inelastic because some energy is almost always converted to other forms during a collision, resulting in kinetic energy not being conserved