Download Definitions Topic 2

Definitions
Topic 2
1. Distance is a scalar ( magnitude only) quantity and is the total distance
of the path taken and depends on the path taken. Displacement is vector
quantity ( direction and magnitude) and is a measure of the net distance
traveled and does not depend on the path taken.
Scalar quantities only measure magnitude. For example: You are going
40 km/ h.
Vector quantities have both magnitude (“how much” or “how big”) and
direction .
2. Speed is a scalar quantity and equal to total distance over time : v = s
t
3. Velocity is a vector quantity and equals total displacement over time : v
= Δs
Δt
4. Free Fall
5. Newton’s Laws of Motion
NEWTON’S FIRST LAW OF MOTION
An object at rest tends to stay at rest and an object in motion tends to remain in motion
with constant velocity, unless acted on by an unbalanced force (a non-zero net force).
OR
If the net force acting on a body is equal to zero, the body will move with constant
velocity and zero acceleration.
NEWTON’S SECOND LAW
The acceleration of an object is directly proportional to the net external
force acting on it and is inversely proportional to its mass. The direction of
acceleration is in the same direction as the net force acting on the object
NEWTON’S THIRD LAW
Whenever one object exerts a force on a second object, the second object exerts an
equal and opposite force on the first.
OR
If body A exerts a force on body B (an “action”) then body B exerts an equal and opposite
force on body A (a “reaction”). These two forces have the same magnitude but opposite
direction.
6. Translational equilibrium occurs when a body is not accelerating. That is,
the net force acting on the body is zero.
7. Inertia : The tendency of a body to maintain its state of rest or in uniform
motion in a straight line is called inertia. Hence Newton’s First Law is often called
the law of inertia.
8. The contact force that acts perpendicular to a common surface of contact is
usually referred to as the Normal Force (‘normal’ means perpendicular) and
labeled F or
FN
in diagrams.
9. Change in momentum based on force and time is called impulse. Impulse
changes momentum in much the same way that force changes velocity causing
acceleration.
Formulas - Units
Linear momentum ( p) is defined as the product of an object’s mass and its
velocity.
p  mv
(Linear momentum defined)
10. ELASTIC COLLISIONS
Net momentum before collision = Net momentum after collision
Total Kinetic Energy is conserved.
Law of Conservation of Momentum and Collisions
The total momentum of an isolated system of bodies remains constant (i.e. when
no external forces act on a system, the total momentum of the system stays the
same).
pbefore  p after
11. Inelastic Collisions
In an inelastic collision KE is conserved. The law of conservation of energy still
holds true.
Completely Inelastic Collision ( v
 v A  vB )
A completely inelastic collision is a collision where the two bodies stick and move
together as one after the collision. e.g. a bullet imbedding itself in wood.
mAu A  mBuB  mA  mB v
(Completely inelastic collision)
NOTE: FORMULA NOT IN DATA
12. Work in physics is given a very specific meaning to describe what is
accomplished by the action of a force when it acts on an object as the object
moves through a distance.
Work is defined to be the product of displacement and the component of the
force parallel to the displacement.
When a constant force acts in the same direction as displacement the work done
by the force is:
Work = Force x Displacement
W  Fs
13. Work due to gravity is independent of path followed = mgh
Work done BY gravity on an object that is moving horizontally ( by another force),
is zero because the angle at which gravity acts is completely vertical with no
horizontal component i e perpendicular to the displacement, ( 90 0 ) .
14. Energy is one of the most important concepts in science. By definition ,
energy is the ability to do work. Kinetic energy is the energy of motion. The
work – energy theorem relates the work done on an object to the kinetic
energy ( K) of that object:
The net work done on an object by all forces acting on it is equal to the
change in kinetic energy of the object.
W = ΔEk CHANGE in Ek
( unit: Joules J = N m)
15. Kinetic Energy
* Brainpop:
Kinetic Energy
Kinetic energy is the energy associated with a body in motion.
Kinetic Energy is:
1 2 p2
E  mv 
2
2m
( E k Kinetic Energy defined)
Note:
16. Potential Energy (Stored Energy)
*
Potential Energy is energy associated with the position of a system (not its
motion).
There are three types of potential energy:
Gravitational potential energy (PE of a diver is converted into KE as she falls).
Elastic potential energy (energy stored in the diving board as the diver jumps on
it or energy stored in a compressed spring).
Electrical potential energy (covered later).
17. Gravitational Potential Energy
Gravitational potential energy is defined as the energy a body has because of its
height relative to a given point.
EP = mgh ( mgy )
(Gravitational potential energy defined)
Gravitational Potential Energy Difference
Experimentally we can only quantify changes in potential energy. Gravitational
potential energy difference is defined as the work that must be done by an
external force to move an object through a vertical displacement Δh. The work
done by the external force is stored as potential energy.
Δ EP = mg Δh (Gravitational potential energy difference)
Conservation of Energy is one of the most important laws in Physics.:
Energy can not be created or destroyed; it may be transformed from one
form into another, but the total amount of energy of a system never changes.
18. The work- energy theorem applies to changes in kinetic and potential
energy that can also be used to explain changes in thermal, mechanical ,
electrical and nuclear energy as well. It is important to understand how
energy changes or transforms form one form to another or from one location
to another.
Drawing:
Conservation of Mechanical Energy is:
E  Ek  Ep = Constant
(Mechanical energy is conserved when only gravity does
work)
For example, when a ball is thrown vertically Ek is converted to Ep; and on the
way down Ep is converted back to Ek. But E is always constant (provided only
gravity does work i.e. no air resistance
19. Power
* Brainpop: Power
Definition of work has no reference to time but it is often necessary to know how
quickly work can be done. Power is defined as the rate at which work is
performed.
Power = work (or energy)  time
P=W
t
Note:
Power is a scalar quantity.
20. Efficiency
Efficiency is defined as the amount of useful work performed per amount of
available energy.
Efficiency 
Usefuloutput
Efficiency = Power output
Totalinput
21. The centripetal acceleration ( ac ) of an object in uniform circular
motion is NOT in the same direction as the tangential velocity vector. If it were,
the object would accelerate and motion would not be uniform. Therefore,
centripetal acceleration is a good example of how you can cause acceleration
just by changing direction without changing speed.
As a matter of fact, the centripetal acceleration is directed towards the center.
This centripetal force ( Fc causing the acceleration) causes the velocity vector to
continuously change direction thereby maintaining uniform circular motion. The
centripetal force is always perpendicular to the direction of motion and does no
work. Therefore, there is no change in kinetic energy and no change in speed
(energy-work theorem) but a change in velocity.
If there were no centripetal force the object would move in a straight line.
22. Newton’s Universal Law of Gravitation
Every point mass of matter in the universe attracts
every other point mass with a force that is proportional
to the product of the masses of the two particles and
inversely proportional to the square of the distance
between them.
Translating this into an equation, we have:
Fg 
GMm
r2
(Newton’s Universal Law of Gravitation)

Point mass assumption: If the separation between
the two objects is large compared to their radii we
can treat spherical objects as point masses particles with all their mass concentrated at the
center (Figure 12-2 Young and Freedman 2000 p.359) and r =
distance between the two centers of the
spheres.

Gravitational forces always act along a line joining
the two particles (Figure 12-1 Young and freedman 2000; 359).

Even when the masses of the two particles are different, the two
interacting forces have equal magnitude (and form an action-reaction pair
Newton’s 3rd Law).
Don’t confuse g with G. ^ CP DVD - Jolly’s Method of measuring of G

23. Gravitational field strength ( g) and gravitational force (F) definition :
Physicists wondered how a mass knows the presence of another mass nearby
that will attract it. They developed the idea of a gravitational field. A mass M is
said to create a gravitational field in the space around it. This means that when
another mass ( m) is placed at some point near M it feels the gravitational field.
The gravitational field strength ( g) at a certain point is the force per unit
mass ( F/m) experienced by a small point mass m…. at that point :
g= F
m
=
g = GM
R2