Download Force and Motion

Force and Motion
(A) Basic Ideas about forces






Force is something that can change the motion (velocity) of an object. It is due
to the interaction between objects. Therefore, it exists in pairs.
Force is a vector quantity. It has both magnitude and direction. Its SI unit is
newton (N)
Force can be measured by a spring balance.
Forces can be classified into contact and noncontact forces. Examples of
contact forces include tension,normal reaction, friction. Examples of noncontact
forces include gravitational force, electrostatic force and magnetic force.
The free-body diagram of an object is used to show all the forces acting on the
object. It is very useful when we are solving problems involving forces.
To specify a force clearly, we have to state clearly what object the force is acting
on and what object the force is given by. For example, the weight of an object is
the gravitational force acting on the object by the earth.
(B) Inertia and Newton's first law
Inertia of a body is the tendency to maintain its state of rest or uniform motion.
Mass of an object is a measure of its inertia. The larger the mass of an object, the
larger is its inertia. The SI unit of mass is kilogram (kg)
Newton's first law:
Every object remains in the state of rest or uniform motion unless acted on by a net
force.
(C) Net force and motion: Newton's second law
Newton's second law:
The acceleration of an object is directly proportional to, and in the same direction as,
the net force / unbalanced force acting on it, and inversely proportional to the mass
of the object, i.e.
1
for the same F
a  F for the same m, and a 
m
F
Therefore, we have a 
or F  ma.
m
When mass is measured in kg, acceleration is measured in m s-2 and force is
measured in N, we have F = ma
(D) Daily life examples of force





Tension is the force in a string. Its direction depends on what it acts on.
Normal reaction is the force acting on an object by the surface in contact with
the object. It is always perpendicular to the surface.
Friction arises whenever an object slides or tends to slide over another object. It
always opposes or prevents motion.
Fluid friction, e.g. air resistance, is the friction that arises when an object moves
through a fluid (gas or liquid). An object falling in a fluid will reach its terminal
speed when its weight is balanced by fluid friction.
The weight of an object is the gravitational force acting on it by the Earth. Its SI
unit is newton (N). It varies from place to place.
1
Force and Motion
(E) Mass and Weight
The mass of an object is the measure of its inertia. It is the same wherever the object
is. It is a scalar. Its SI unit is kg.
The weight of an object is the gravitational force on the object by the earth. It
depends on where the object is. It is a vector. Its SI unit is N.
The weight W (in N) of an object with mass m (in kg) is given by
W = mg,
where g is the acceleration due to gravity (m s-2)
Measuring instruments such as beam balance, spring balance and weighing scale
can be used to measure mass. But in fact, they do not measure mass. For example,
spring balance indeed measures tension and weighing scale indeed measures
normal reaction.
(F) Addition and resolution of forces
Force is a vector quantity. It has both magnitude and direction.
Two or more forces can be added to find the resultant using the'tip-to-tail' method
(Fig a) or the parallelogram of forces method (Fig b). Only forces acting on the same
object can be added.
Tip-to-tail method
Parallelogram of force method
A force can be resolved into its components along any two perpendicular directions
as shown below.
Motions along two perpendicular directions are independent of each other.
(G) Action and reaction: Newton's third law
Newton's third law:
Forces always occur in pairs. If A exerts a force (action) on B, B exerts an equal but
opposite force (reaction) on A.The two forces exist simultaneously.
An action-and-reaction pair should
•
always have equal magnitude,
•
be opposite in direction, and
•
act on different interacting objects.
2
Force and Motion