Force and Motion
Force: Force can be defined as a push or a pull that changes or tends to change
the state of rest or uniform motion of an object or changes the direction or shape of an object. It
causes objects to accelerate or add to their overall pressure. In simple terms, it is a push or a
pull on an object that takes place when two objects interact. It is the basic cause of motion from
rest.
Unit of Measurement
It is measured in ‘Newton’ (N). Newton measures the force needed to move,
accelerate or speed up objects. The basic formula for it is, F = m.a, where, ‘m’
stands for the mass in kilograms and ‘a’ stands for acceleration in m/sec2.
Systems and External World
Systems: It is the object or objects of interest. Everything around the
system with which the system can interact with is called the external world.
In Figure 2, the book is the system. Your hand, Earth, String and the table are parts
of the external world that interact with the book.
Contact Forces:
A Contact force as the name suggests acts on a point of an object by direct
contact. Forces that act between the objects are in contact with each other are
called Contact Forces. These forces are produced and experienced when a body
comes in contact with another body. The Contact may be Continuous or sometimes
momentary. The former case is known as Continuous force and the latter case is
known as impulse force.
Field Forces:
Is a map of the force felt over a particular area of space. To explain this better,
we should first describe a field. While it might sound mysterious, a field, in physics,
is really just a map of a particular quantity over an area of space.
Free-body diagram:
Free-body diagrams are diagrams used to show the relative magnitude and
direction of all forces acting upon an object in a given situation. A free-body
diagram is a special example of the vector diagrams that were discussed in an
earlier unit. These diagrams will be used throughout our study of physics. The size
of the arrow in a free-body diagram reflects the magnitude of the force. The
direction of the arrow shows the direction that the force is acting. Each force arrow
in the diagram is labeled to indicate the exact type of force. It is generally
customary in a free-body diagram to represent the object by a box and to draw the
force arrow from the center of the box outward in the direction that the force is
acting.
Net Force:
Is defined as the sum of all the forces acting on an object. The equation below
is the sum of N forces acting on an object.
Magnitude and Equation
According to Newton's Second Law, when an object accelerates, then
there must be net force acting on it. Conversely, if a net force acts on an object,
that object will accelerate. The magnitude of the net force acting on an object is
equal to the mass of the object multiplied by the acceleration of the object as
shown in the formula below.
If the net force acting on an object is zero, then the object is not
accelerating and is in a state that we call equilibrium. When an object is in
equilibrium, then two things can be true: either the object is not moving at all, or
the object is moving with a constant speed.
Newton's 2nd Law:
The acceleration of an object as produced by a net force is directly
proportional to the magnitude of the net force, in the same direction as the net force,
and inversely proportional to the mass of the object.
Newton's 1st Law:
States that an object will remain at rest or in uniform motion in a straight
line unless acted upon by an external force
Inertia:
Is the resistance an object has to a change in its state of motion
Equilibrium:
When all the forces that act upon an object are balanced, then the object is said
to be in a state of equilibrium.
Weight:
Gravitational force of attraction on an object, caused by the presence of a
massive second object, such as the Earth or Moon. Weight is a consequence of the
universal law of gravitation.
Gravitational Field:
Is the force field that exists in the space around every mass or group of masses. This
field extends out in all directions, but the magnitude of the gravitational force decreases as the
distance from the object increases. It is measured in units of force per mass, usually newton's per
kilogram (N/kg). A gravitational field is a type of force field and is analogous to electric and
magnetic fields for electrically charged particles and magnets, respectively.
Apparent Weight:
It is the force an object experiences as a result of all the forces acting on it, giving the
object acceleration.
Weightlessness does not mean that an object’s weight is actually zero; rather, it means that
there are no contact forces pushing up on the object, and the object’s apparent weight is zero.
Drag Force:
Is the force exerted by a fluid on the object moving through the fluid. This force is
dependent on the motion of the object, the properties of the object, and the properties of the
fluid that the object is moving through.
Terminal Velocity:
As the object falls, the force of gravity initially causes it to continuously speed up as
predicted by Isaac Newton. As it gets faster and faster, the air drag force increases until
eventually, the air drag force is exactly equal to the force of gravity, and there is no net force
acting on the object. If these two forces are exactly balanced, the object will no longer speed up
or slow down but will continue falling at a constant velocity, called the terminal velocity.
Interaction Pair:
Are two forces that are in opposite directions and have equal magnitude.
Newton's 3rd Law:
All forces in the universe occur in equal but oppositely directed pairs. There are no
isolated forces; for every external force that acts on an object there is a force of equal magnitude
but opposite direction which acts back on the object which exerted that external force. In the case
of internal forces, a force on one part of a system will be countered by a reaction force on another
part of the system so that an isolated system cannot by any means exert a net force on the
system as a whole. A system cannot "bootstrap" itself into motion with purely internal forces - to
achieve a net force and acceleration, it must interact with an object external to itself.
Tension:
The tension force is the force that is transmitted through a string, rope, cable or wire when it
is pulled tight by forces acting from opposite ends. The tension force is directed along the length
of the wire and pulls equally on the objects on the opposite ends of the wire.
The normal force
Is the support force exerted upon an object that is in contact with another stable object.
For example, if a book is resting upon a surface, then the surface is exerting an upward force
upon the book in order to support the weight of the book. On occasions, a normal force is exerted
horizontally between two objects that are in contact with each other. For instance, if a person
leans against a wall, the wall pushes horizontally on the person.
Vectors in two dimensions
Adding Vectors
If you were adding together two vectors at right angles, vector A pointing north and vector B pointing east,
you could use the Pythagorean theorem to find the magnitude of the resultant, R.
Vectors Component:
Which are vectors parallel to the x-axis and another parallel to the y-axis. This can always be
done and the following vector equation is always true
.
This process of breaking a vector into its components is sometimes called vector resolution.