Chapter 3: Dynamics

Base unit


Base unit



m (meter)






s (second)

Electric current


A (ampere)

Thermodynamic temperature


K (kelvin)

Amount of substance



Luminous intensity



Newton’s first law of motion

An object will remain at rest or continue to move with uniform velocity, unless it is acted upon by an external resultant force.

It explains:

  • If the forward force and backward force cancel out, the stationary object will remain stationary.
  • If backward force is greater than the forward force, a moving object will slow down.
  • If the forward force is greater than backward force, the stationary object will move forward.


Newton’s second law

For a body of constant mass, its acceleration is directly proportional to the resultant force applied to it.


Newton’s third law

When two bodies interact, the forces they exert on each other are equal in magnitude and opposite in direction (action and reaction are equal and opposite forces).


When you sit on a chair, your weight pushes down on the chair, the chair also pushes up against you.


When riding a bus

Newton’s 1st law

When you get on a bus and stand up. When the bus is stationary you feel no force, when the bus accelerates you feel a backward force. You want to stay where you are but the bus forces you to move. When the bus is at a constant speed you feel no forwards or backwards forces. The bus slows down and you feel a forward force.

Newton’s 2nd law

As more people get on the bus its mass increases. If the driving force of the bus’s engine is constant we can see that it takes longer for the bus to gain speed.

Newton’s 3rd law

As you stand on the bus you are pushing down on the floor with a force that is equal to your weight. If this was the only force acting you would begin to move through the floor of the bus. The floor is exerting the equal and opposite force towards you.

Terminal velocity

Terminal velocity is the maximum velocity of an object travelling through a fluid, where its resultant force is zero and depends on the weight and surface area of object.