Download Work and Energy Work: Occurs as a force is applied over a distance

Unit 10: Work and Energy
Work: Occurs as a force is applied over a distance.
*It is a vector. (Has a number and direction)
*If there is no movement, there is no work on that object.
* When the object moves, it must move in the same direction as the force for there to be work.
Work = Force (Distance)
W = F(d)
Units: Joule (J) = 1 kg(m/s2)
Ex:
1.)
Carly pushes a cart with a force of 2N for a distance of 5m. What work is done on the cart
W=?
F=2N
D=5m
2.)
Jake pushes a car with a force of 125N. The car moves and 25J of work is done. How far did the car
move?
W=25J
F=125N
d=?
Power: the rate at which work is done.
Power = Work/Time
P = W/t
Units: Watts (w) = J/s
*Doing work in a faster time requires more power.
*Doing more work in the same amount of time will increase the power.
Ex:
1.)
Travis does 500J of work when he lifts a box. If it takes him 20s to lift the box, how much power
did he use?
P=?
W=500J
t=20s.
2.)
Tim uses a power of 400W when he pushes a stool for 2s. How much work did he do?
P=400W
W=?
t=2s
3.)
Kaley lifts a box with 80J. If her power was 40w, how much time did it take her?
P=40w
W=80N
t=?
1.)
2.)
3.)
Describe/define work in your own words.
Describe/define power in your own words.
Give one way that you can increase the amount of power being used (hint: think about the equation
P = W/t)
Simple Machines
Machines: A device that changes a force.
*They make work easier to do.
*They can change the size of the force needed, the direction of a force, or the distance over which a
force acts
Force Ex.: Changing a tire
Increase Distance Ex.: Using oars to row a boat
Change direction Ex.: Flagpole; pull down on the rope and the flag goes up.
-
The work done by a machine is ALWAYS less than the work done on a machine by a person.
Why? Friction, lost as heat, air resistance.
Input Force (2N): the force that a person does on a machine.
Input Distance (25m): the distance that a person exerts the force.
Work Input (50J): the work the person does on the object.
Output Force (1N): the force that the machine does.
Output distance (10m): the distance that the machine exerts that force.
Work Output (10J): the work that the machine does.
6 Kinds of Simple Machines
1.)
2.)
3.)
4.)
Levers: a rigid arm that rotates around a fixed point (called a fulcrum).
o 1st Class Lever: See Fig. 1 on board.
 Ex: seesaw, crowbar, claw hammer
nd
o 2 Class Lever: See Fig. 2 on board.
 Ex: hinged door, wheel barrow.
o 3rd Class Lever: See Fig. 3 on board.
 Ex: baseball bat, golf club.
Wheel and Axle: Has two different cylinders with different radii.
 Ex: Steering wheel, wheel barrow, screwdriver.
Inclined Plane: A slanted surface along which a force moves an object to a different elevation.
 Ex: Wheel chair ramp.
Wedge: a V-shaped object whose sides are two identical planes sloped towards each other.
*You direct a force downward, but the wedge directs that force outward.
- Ex: axe, nail, knife.
5.)
6.)
Screw: an inclined plane wrapped around a cylinder.
 Ex: screw, jar, spiral staircase
Pulley: has a rope that fits into a groove on a wheel.
 Ex: flag pole, clothes line.
*If you are using only one pulley, the mechanical advantage is 1. This only changes the
direction that the force is moving.
*So you want to use more than one pulley!
- Fixed pulley: the wheel is attached to a fixed location.
*Only changes direction, but not the size of the force.
-Movable pulley: the wheel is attached to the object being moved.
*Reduce input force.
-Pulley system: combination of fixed and moveable pulleys.
*Reduce input force.
-Compound Machine: A machine that combines more than one simple machine.
Ex: car, bicycle, wheel barrow
Mechanical Advantage
Mechanical advantage: the number of times that a machine increases an input force.
*If this number is over 1, the force gets multiplied.
*If this number is less than 1, the distance is increased.
Equations:
Mechanical Advantage = Output Force/Input Force
Mechanical Advantage =Input Distance/Output Distance
*Distance formula is not perfect because it does not include things like friction.
Ex:
1.)
2.)
3.)
Sarah was running up an inclined plane that is 6.0m long. At the end of the ramp, she was 1.5m
higher. What is the mechanical advantage of the ramp?
MA =?
ID=6.0m
OD=1.5m
Megan pulls on the handle of a claw hammer with a force of 15N. If the hammer has a mechanical
advantage of 5.2, how much force is exerted by the claw?
MA=5.2
OF=?
IF=15N
Matthew uses a ramp to push a wheelchair to the door. If the mechanical advantage of the ramp is
1.1 and the output force was 90N, what force did Matthew use?
MA=1.1
OF=90N
IF=?
Efficiency: the percentage of work input that becomes work output.
*No machine is 100% efficient!
- Why? Heat loss, friction, air resistance.
How can you make a machine more efficient?
1.)
2.)
3.)
Add a lubricant (oil) because it reduces friction.
Reduce Air resistance by changing its shape
Change the type of friction (sliding friction to rolling friction).
MA Practice Problems:
1.)
2.)
3.)
4.)
5.)
6.)
7.)
8.)
9.)
10.)
11.)
Sam has a flat tire and is using a jack with a mechanical advantage of 3.4. If she exerts 60N of force
on the jack, what force is exerted on the car?
Nick is using a ramp to move a sofa. If the mechanical advantage of the ramp is 0.68 and the height
needed to be moved is 2.5m, what is the distance of the ramp?
Steve is moving into a new house. He uses 50N of force to move the box when using a pulley
system. The box should have taken 80N of force in order to move it. What is the mechanical
advantage of the pulley system?
Chris uses a crowbar to open a locked box. If the MA of the crowbar is 3.9 and Chris’s input
distance is 0.67m, what is the output distance of the crowbar?
Danny is chopping wood and the axe has a mechanical advantage of 1.5. If the force exerted on the
wood is 140N, what force did Danny exert on the axe?
Missy swings a golf club a distance of 1.5m and the end of the golf club moves 4m. What is the MA
of the golf club?
The power steering in an automobile has a mechanical advantage of roughly 75. If the input force
on the steering wheel is 49 N, what is the output force that turns the car’s front wheels?
Suppose you need to remove a nail from a board by using a claw hammer. What is the input
distance for a claw hammer if the output distance is 2.0 cm and the mechanical advantage is 5.5?
The mechanical advantage of an automobile’s wheel and axle is 0.0893. If the wheel’s output force
is 2220 N, what is the input force that turns the axle?
A wedge with a mechanical advantage of 0.78 is used to raise a house corner from its foundation. If
the output force is 7500 N, what is the input force?
Suppose the screw has a mechanical advantage of 12.5. If the screw is turned several times, so that
the input distance is 15.7 m, how much has water been lifted upward by the screw (output distance)?
Writing Assignment:
Write one paragraph that describes how you use 3 simple machines in your daily/weekly activities. Have two
sentences for each simple machine, one identifying the machine and one describing the way in which you use it.
Energy and its Forms
Energy: the ability to do work.
*Energy is transferred by a force moving an object over a distance.
*Work is a transfer of energy.
*Units: Joules (J) = kgm2/s2
Kinetic Energy: the energy of motion.
*This depends on the object’s mass and speed.
*Kinetic energy = ½ (m)(v2)
*Changing the velocity impacts the kinetic energy more than just changing the mass
Ex:
1.)
Ted throws a ball with a speed of 12 m/s. If the ball has a kinetic energy of 216 J, what is the mass
of the ball?
KE=216
m=?
v=12 m/s
Potential Energy: the energy that is stored as a result of position or shape.
There are 2 kinds of potential energy:
1.)
Elastic Potential Energy: the potential energy of an object that is stretched or compressed.
Ex: slinky, rubber-band, spring
2.)
Gravitational Potential Energy: potential energy that depends on the object’s height.
*PE= (m)(g)(h)
Potential energy= (mass)(gravity)(height)
*For an object, changing the position will impact the potential energy the most.
*The acceleration due to gravity makes a difference in the potential energy too. So, my potential
energy would be different on the moon than it is here.
Ex:
1.)
2.)
3.)
Abby is diving at the Olympics. She has a mass of 50 kg and it is 10m high. What is her potential
energy?
PE=?
m=50kg
g=9.8m/s2
h=10m
Emily is climbing a rock wall and has a potential energy of 6953.1J. If her height is 12.9m, what is
her mass?
PE=6953.1 J
m=?
g=9.8 m/s2
h=12.9m
DJ is on a pogo stick that is 1.5m high on Jupiter’s surface. If his mass is 64kg and his potential
energy is 2491.2J, what is the acceleration due to gravity on Jupiter?
PE=2491.2J
m=64kg
g=?
h=1.5
*All forms of energy can be classified as kinetic energy, potential energy, or energy stored in a field.
Mechanical Energy: the kinetic energy and the potential energy of an object.
*KE + PE
(KE & PE) Thermal energy: total potential and kinetic energy of all the particles in an object.
(Experienced as heat)
*The more the molecules/particles move, the higher the thermal energy.
(PE)Chemical Energy: Energy stored in chemical bonds.
Ex: Food
(Field)Electrical Energy: the energy associated with electric charges.
Ex: electricity, lightning, batteries
(Field)Electromagnetic Energy: a form of energy that travels through space in the form of waves.
Ex: sunlight
(PE)Nuclear Energy: the energy stored in the nucleus of an atom.
Energy Conversions
Energy can be converted from one form to another.
Energy Conversions: the changing of energy from one form to another.
Law of Conservation of Energy:
Energy cannot be created or destroyed. It can only transfer from one form to another.
*Most common one is kinetic energy to potential energy or vice versa.