DYNAMICS
NEWTON’S LAWS OF MOTION
Newton's laws of motion are three physical laws which provide relationships between the forces
acting on a body and the motion of the body.
The First Law: Force and Inertia
The Second Law: Force, Mass and Acceleration
The Third Law: Action and Reaction
Newton’s first law
Every body continues in a state of rest or uniform motion in a straight line unless a net (external)
force acts on it.
Newton’s First Law is also called the Law of Inertia
Inertia: the tendency of an object to resist changes in its state of motion
The First Law states that all objects have inertia. The more mass an object has, the more inertia it
has (and the harder it is to change its motion). Inertia is a term used to measure the ability of an
object to resist a change in its state of motion.
Newton's first law of motion declares that a force is not needed to keep an object in Motion.
Newton’s Second Law
The rate of change of momentum of a body is directly proportional to the net force acting on the
body, and the momentum change takes place in the direction of the net force.
F=mxa
Force is directly proportional to mass and acceleration. The force F that appears in the second
law is the net force.
Mass will resist changes in motion
Mass: is a measure of the amount of matter in a body, & is the property of a body which resists
change in motion.
Weight: is the force of gravitational attraction (exerted by the Earth) on a body.
Newton’s Third Law
When object X exerts a force on object Y, object Y exerts a force of the same type that is equal in
magnitude and opposite in direction on object X. The two forces ALWAYS act on different
objects and they form an action-reaction pair.
LINEAR MOMENTUM AND ITS CONSERVATION
FORCE AS RATE OF CHANGE OF MOMENTUM
PRINCIPLE OF CONSERVATION OF MOMENTUM
NB: For a perfectly elastic collision between two bodies, the relative speed of approach is equal
to the relative speech of separation.
NB: whilst the momentum of a system is always conserved in interactions between bodies, some
change in kinetic energy usually takes place.
In inelastic collisions, total energy is conserved but Kinetic Energy may be converted into
other forms of energy such as sound and heat energy.
Case 1:
Case 2:
Case 3: