Download PS Energy and Work

ENERGY AND WORK
HONORS IPC
Introduction
When a force acts upon an object to causing it to move (a distance), it is said that work was done on
the object. In order for a force to qualify as having done work on an object, there must be a
displacement and the force must cause the displacement.
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Energy is necessary to supply the force to do work. In fact, the definition of energy is “the ability to do
work.” Energy can be classified a two distinct types; potential and kinetic. Potential energy is
stored energy. There are three (3) distinct types of potential energy; chemical potential energy,
gravitational potential energy and elastic potential energy. Kinetic energy is the energy resulting in
the motion of an object.
Gravitational Potential Energy exists whenever an object has position within a field force (i.e.
gravity). The gravitational potential energy of an object can be calculated by applying the following
expression (relationship):
Ug = mgh
Where;
Ug = gravitational potential energy (j)
m = mass of the object (kg)
g = acceleration due to gravity (9.8 m/s2)
h = height above the reference point (m)
Elastic Potential Energy exists when a force is applied to an object that results in its elastic
deformation. In other words, when an object is stretched or compressed. Consider that work is
required to elastically deform an object. The work is derived from applying a force over a distance as
shown with the following expression:
W = Fd
Where;
W = work done on the object (j)
F = force applied to the object (N)
d = distance that the force is applied to the object (m)
The amount of work necessary to elastically deform an object is equivalent to the energy stored in that
object. There is an equivalent relationship between work done by a net force acting on an object and
the energy of the object. Consider that energy associated with the work done by a net force does not
disappear after the net force is removed (or becomes zero); it is transformed into the kinetic (or
potential) energy of the object. This relationship is defined by the Work-Energy Theorem.
∆W = ∆E
The principle of Conservation of Energy asserts that, in a closed system, energy can be transformed
from one form to another and that the total amount of energy is conserved. When an object changes
its position (with respect to height), potential energy is converted to Kinetic Energy, which exists
whenever an object is in motion. The kinetic energy of an object can be determined with the following
expression:
K = ½ mv2
Where;
K = kinetic energy (j)
m = mass of the object (kg)
v = velocity (m/s)
Explore
A simple pulley is a machine that can be used to redirect a force acting on an object. As shown below,
measuring an applied force over a distance to lift an object can be easily done with this apparatus.
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Determination of work
1. Set up a pulley, an object and spring scale as shown above.
2. Apply a force by pulling down on the spring scale.
3. Record the force applied and the distance that the object moved.
4. Repeat steps 1-3 with different objects.
Data Table 1 – Determination of Work
Object
Applied Force (N)
Distance (m)
Work (J)
1
2
3
4
Calculations – Show your calculations (with units!!) below.
Determination of Gravitational Potential Energy
1. Determine the mass of the objects from above.
2. Use the mass of the objects, gravitational acceleration and the height (distance) to determine
the gravitational potential energy of the objects.
Object
1
Data Table 2 – Determination of Gravitational Potential Energy
Mass of Object (kg) Grav.Accel. (m/s2)
Height (m)
Grav. Pot. E (J)
2
3
4
Calculations – Show your calculations (with units!!) below.
Practice
Identify the type of energy described below:
1. Energy in the food you at last night. ___________________________________
2. A slingshot with the band pulled back. ____________________________________
3. A bowling ball rolling down a lane. _____________________________________
4. A diver preparing to dive off a platform. ________________________________
5. Gasoline in the tank of a car. _______________________________________
6. A skydiver falling toward the ground ____________________________________
7. A 55.2 kg shopping cart is pushed for 13.7 meters with a force of 550.0 N. How much work is done
to the cart?
8. The image below shows a spring being compressed by a force. Use the information provided to
calculate the amount of energy stored in the spring. What type of potential energy is it?
0.75m
0.25m
12.0 Newtons
9. A large chunk of ice with mass 15000 grams falls from a roof 8.6 meters above the ground. What is
the potential energy of the ice when it was on the roof?
10. Determine the Kinetic energy of a 2502kg roller coaster, which is moving at a velocity of 20.0 m/s.
11. What is the velocity of a 55.7kg woman running with a kinetic energy of 412.7 J?
12. A 35.0 kg girl slides down a frictionless water slide that is 2.50m high. How much potential energy
does the girl have? If all the potential energy is converted to kinetic energy, what is the girl’s kinetic
energy at the bottom of the slide? What will her velocity be at the bottom?
13. A vehicle with a mass of 1376kg runs out of gas as it reaches the base of a hill. If the car is
travelling at 7.75 m/s and no more energy is added, how high up the hill will it coast before
stopping? Hint:Remember conservation of energy.