Chapter 4Mr. Kingscience 7
Chapter 4Mr. KingScience 7
WHAT IS ENERGY ???
- Energy is the ______to do ______
Question:
- Can work be done without using energy????
- Can you use energy and not do work at the same time???
EXPLORE ACTIVITY (P. 118)
What it means to do “work” in the science world:
People use the word work in many different ways. You say you are doing work when you are studying for a test. A lifeguard will say he is at work when he is sitting in his tower watching the swimmers. A television anchor person will say she is at work when she is on TV reporting the news.
In science, the word work has a special meaning. Work is done ONLYwhen a force moves an object. When you push, lift or throw an object, you are doing work. You are using your strength to act on a object, causing it to move a certain distance. A force acting through any distance is work. You are doing work whenever you move an object from one place to another.
Work is done on an object when force No work is done on an object
and motion are in the same direction when force and motion are in
Opposite directions
This person is using an upward forceThis person is using force to hold
to move the box up. the books up but is moving forward.
- Work is defined as the transfer of energy through both
Force and Motion; the force MUST be in the same direction
as the motion.
Work is NOT done every time a force is applied. Remember that in
order for work to be done, you must move an object through some distance. If you push or try to lift something and it doesn’t move, you have not done any work even if you feel exhausted!!! Soooo…….
Look at this person pushing against the wall.
Is this person doing work? Why or why not?
EX: This ant looks like it is doing work!! It ISN’T !!
Remember that to do work you have to be moving an object through some distance in the same direction…
Why isn’t this ant doing work???
EX: This womanIS doing work! Why ?
Ex: The quarter back IS doing work…..why?
EX: Is this skier doing work? Why or why not?
EX: Is this person doing work? Why or why not?
WORK : measured in Joules
A Joule is the work done when
the force of 1 Newton moves an
object a distance of 1 meter.
FORCE : measured in Newtons
DISTANCE: measured in meters
EX: W = F ● D = 20 N ● 15 m = 300 J
Practice Problems
EX: How much work is done if 30 N of force is exerted over 5 m ?
Formula: W = F ● d
Substitution: = 30 N ● 5 m
Answer: 150 J
EX: How much work is done if 850 N of force is used to move an
Object 4 m?
Formula:W = F ● d
Substitution: = 850 N ● 4 m
Answer: = 3400 J
EX: If 20 J of work is done using 4 N of force, how far did the
object move?
Formula:W = F ● d
Substitution:20 J = 4 N ● d
20 J = 4 N ● d
4 4
Solution: 5 = d
Answer: It moved 5 m
20 = d
4
5 = d
EX: If 150 joules of work was done moving an object 25 m, how
much force was used?
Formula:W = F ● d
Substitution:150 J = F ● 25 m
150 J = F ● 25 m
25 25
Solution: 6 = F
Answer: 6 N of force
- This is STORED energy
- There are two types: Gravitational and Elastic
A) Gravitational Potential Energy
- Defined as the work done in lifting an object against
the force of gravity.
- The higher an object is lifted the more potential energy
it gains. (p. 124-125)
Where does the boy have the most potential energy??
How could you give the skier more potential energy???
B) Elastic Potential Energy
- Defined as energy stored when work stretches or twists an object.
How is the compressed spring storing energy?
How is the stretched string storing energy?
What is being stretched?
How do you gain more potential energy?
UNITS (Labels) Joules = Newtons ● meters
EX:
30 m
What is the potential energy of a car sitting at the top of this roller coaster is its weight is 7500 N?
Formula:P.E. = Weight ● Height
Substitution: = 7500 N ● 30 M
Answer: = 225, 000 J
Practice:
1.
If the weight of the ball at the end of the
pendulum is 20N and the height of the ball
at point C is 0.5 m, what is its potential
energy?
Formula:
Substitution:
Answer:
2.
If the skier weighs 150 N and goes up
to the top of a ski hill that is 60 feet
high, what is the skier’s potential
energy at point A?
Formula:
Substitution:
Answer:
- Defined as energy in motion. (Examples p. 121-122)
- The amount of Kinetic Energy possessed by an object can never be greater than the amount of work done on an object.
Units: E = Kinetic energy measured in Joules (J)
m = mass of object in kilograms (kg)
v² = velocity of object ()
What is the kinetic energy of a
baseball that has a mass of 0.15 kg and is traveling at 20 m/s?
E = mv²
2
= (0.15)(20)² (remember PEMDAS, E before M)
2
= (0.15)(400) = 60 = 30 J
2 2
EX: Determine which object has more Kinetic Energy: object A
that weighs 10 kg moving at 20 m/s or object B, a 20 kg object
moving at 10 m/s?
Object AObject B
E = mv²E = mv²
2 2
= (10)(20)² = (20)(10)²
22
= (10)(400) = (20)(100)
22
= 4000 = 2000
22
= 2000 J = 1000 J
Try: Determine which has more K.E. :
A 230 kg hammerhead shark moving at 500 m/s
Or
A 200 kg blue shark moving at 600 m/s ?
OR
Potential Energy and Kinetic energy - In class worksheet
Answer each question; show formula, substitution and labeled answer.
1. A person weighs 500 N and they climb to the top of a no9umtain 100 m
straight up. What is the person’s potential energy at the top of the
mountain?
2. In physical education class, a student weighing 400 N climbs a rope 5 m
high. How much potential energy will the person have ay the to of the rope?
3. A skier weighing 120 N has a measured potential energy of 1800 Joules.
What is the skier’s height?
4. What is the kinetic energy of a chipmunk who’s mass is 2.5 kg and is
traveling at a velocity of 10 m/s?
5. What is the kinetic energy of an object with a mass of 7 kg traveling at a
velocity of 20 m/s² .
- This is a type of energy felt as heat.
( See pages 132 – 133)
- At the ball’s highest point , before it swings down, it has
Potential Energy
- As it swings downward the energy switches forms, it changes from potential energy (stored) to kinetic energy (in motion).
It loses potential energy and gains kinetic energy.
- As it swings upward, the energy switches back from kinetic energy ( motion) back to potential energy (stored)
It loses kinetic energy and gains potential energy.