Transcript Work and Kinetic Energy (III-NEWTON)

Work
 In
Physical science, work is the use of
force to move an object some distance.
In scientific terms, you do work only
when you exert on force on an object
and move it.
 Work
is done on an object only if the
object moves in the same direction as
the force.
Calculating Work
 You
can calculate the work a force does if you
know the size of the force applied to an objects
and the distance over which the force acts. The
distance involved is the distance the object
moved in the direction of that force. The
calculation for work is shown in the following
formula:
Work = force . distance
W = Fd
Example:
How much work does the force of
gravity do when 25 N object falls a
distance of 3.5 m?
W = Fd
= (25 N) (3.5m)
W = 87.5 J

If a force is exerted perpendicular to the motion,
no work is done. What if a force is exerted at
some other angle to the motion?
o The magnitude of the component of the
force F acting in the direction of motion is
found by multiplying the force F by the
cosine of the angle between F and the
direction of motion.
W = F(coso)d
= Fd coso
Example:
A sailor pulls a boat along a dock using a rope
at an angle of 70o with the horizontal. How
much work is done by the sailor if he exerts a
force of 240 N on the rope and pulls the boat
30.0 m?
W = Fd coso
= (240N)(30.0m)(cos70o)
= (240N)(30.0m)(0.63)
W= 4536 J
 Work
done against gravity can be found using
the equation Work = Force . height or W = Fh.
Since F = mg, we can use the equation W = mgh
( where m = mass, g = gravity, or 9.8 m/s² and h
= height).
Ex:
You lift a 2 kg book and put it on a shelf 3 meters
high. How much work did you do?
W = mgh
= (2kg)(9.8 m/s²)( 3m)
W= 58.8 J
Work Transfers Energy
 Energy
is the ability of a person or an object to
do work or to cause a change. When you do
work on an object, some of your energy is
transferred to the object. You can think of work
as the transfer of energy. In fact, both work and
energy are measured in the same unit, the
joule. If you put energy into an object, then you
do work on that object.
Kinetic Energy
 Is
a form of energy
that moving objects
have.
 The energy of
motion.
Forms of kinetic energy
 Vibrational
Energy: It is the energy due to
Vibrational motion.
 Rotational Energy: It is the energy due to
rotational motion.
 Translational Energy: It is the energy due to
motion from one location to another.
 Relativistic energy: It is the energy of objects
operating at very high speed.
Calculating Kinetic Energy
 Kinetic
energy can be calculated
using the following formula:
Kinetic Energy = mass . velocity2
2
KE = ½ mv2
 If
you double the mass of an object,
you double its kinetic energy.
 If you double the object’s velocity,
its kinetic energy is four times
greater.

Here’s a question not many ask: why is the
velocity squared in the kinetic energy equation,
E = ½mv2. Why should the energy depend on
the square of the velocity?
To find out, let us look at how the first
equation is derived:
 We start with the constant acceleration
equation, 2ad = vf2 - vi2
 Then substitute a = F/m into that 2Fd/m = vf2 vi2
 If we let the initial velocity equal zero, and
define work as force through a distance, we get
W = E = Fd = ½ mvf2
 Not
all objects start at rest. They may already
have kinetic energy when additional work is
done on them. If we define initial kinetic
energy, Kei, and final kinetic energy, Kef, as the
energies the object has before and after work
is done, we can write
W = AKE
or
W= KEf - KEi
 This equation represents the work energy
theorem. It can be stated: The net work done
on an object is equal to its change in kinetic
energy.
Example: A shot putter heaves a 7.26 kg shot with final
velocity of 7.50 m/s.
a. What is the kinetic energy of the shot?
b. The shot was initially at rest. How much work was
done on it to give it this kinetic energy?
Solution:
a) KE = ½ mv2
= ½(7.26 kg)(7.50m/s)2
= 204 kg.m2/s2
KE = 204 J
b) Work done is equal to change in the kinetic energy.
W = KEf - KEi
= 204 J – 0 J
W = 204 J
Reporters:
PELIN, Kaina Angelique
PAQUEO, Caedmon Allan
MOSENDE, Kristina Libra
CALIAO, Lyeziel
MAKINANO, Clementine
JORTA, Myrine
III - NEWTON