Chapter 9 test
A 44 kg cheetah runs at a constant speed of 31 m/s. What is its KE?
21142 J
A 65kg person climbs a ladder straight up 35m high. What is the persons GPE at the top of the ladder? Use g = 9.8 m/s^2
22295 J
From the previous question (How much work was done to push a 10 N book 5 meters?) How much power was used if it took 2 s?
25 W
What is the Effort Force by a 6-m lever with fulcrum 2m from object on a 240 N Fr?
80
Gravity=
9.8 m/s² can be rounded to 10m/s²
Work (W)
Energy used to move an object with a given displacement
Watts can also be written as
Joule / sec
Gravitational potential energy units
Joules (J)
Unit for Work
Joules (J)
Units for Kinetic Energy
Joules (J)
Kinetic Energy symbol
KE
Equation for conservation of Energy
KE + PE = Constant 1/2 × m × v² + g × m × h = Constant
Equation for Kinetic energy
KE = ½ × mass × (velocity)² ½ × m × (v)²
This shows the transformation of energy when a skate boarder goes down a ramp.
KE → GPE
This shows the transformation of energy when a skate boarder goes up a ramp
KE → GPE
Net Force (Fnet)
The sum of all forces acting on an object (N)
Mechanical Energy
The sum of potential and kinetic energies
Compound machine
Two or more simple machines (Bicycle)
A bird flying horizontally at 550m high and has 2802 J of GPE. What is its mass?
0.52 kg
From the previous question (Two people each applied 100N of force to move a crate 3m. Whats the work done?) How much power would be used if it took them 60s?
10 W
What is the effort force of a 16 cm diameter wheel and 4 cm radius axle on a 200 N Fr?
100
A mechanical jack does 5350 J of work to lift a car 0.50m in 50s. What is the power output by the jack?
107 W
Work symbol
W
An apple with a weight of 1 N falls 1 m straight down. How much work is done by he gravitational force?
W = 1 Joule
A crane uses an average force of 5200 N to lift an I-beam 25 m, straight up. What is the work done by the crane?
W = 130,000 Joules
The brakes on a bicycle apply 125 N of frictional force to the wheels as the bike travels 14.0 m to come to a stop. How much work is done by the brakes on the wheels?
W = 1750 Joules
This equation triangle is used to calculate Work given Force and Distance
W = F *s
Average power equation triangle
W on top and P on left and t on right
Units for Power
Watts (W)
KE = 0 when this is true
When v = 0
Kinetic Energy equation for velocity
v = √2KE/m
KE = PE for a falling object when this is true
when h is at h/2
A girl with a weight of 560 N walks up some stairs that go up vertically 5.5m a. Calculate her power output if she climbs the stairs in 12.6 sec. b. Calculate her power output if she clim the stairs in 10.5 sec.
a. 244.4 W b. 293.3 W
Units for displacement
m
height units
m
Kinetic energy equation for mass
m = 2KE/ v²
Gravitational Potential energy equation for mass
m = PE ÷ (g × h)
velocity units
m/s
time symbol
t
A rower uses 120 W to do 3600 J of work on the oars. How long does it take him?
t = 30 s
Fe is the
"Effort force" (input force)
Fr is the
"Resistive Force" (Output force)
At ground what does potential energy equal?
0
Two people each applied 100N of force to move a crate 2m. Friction is 50N. What is the Fnet?
150 N
A 1200 kg car going at 18m/s. Find its KE.
194400J
A 2kg box on a shelf has 49J of GPE. How high off the ground is it?
2.5 m
From the previous question (Two people each applied 100N of force to move a crate 2m. Friction is 50N. What is the Fnet?) what is the work done on the crate?
300J
What is the mechanical advantage of an inclined plane 16m long and 4m high?
4
A 1200- kg car at the top of a 42 m hill. Calculate GPE
493920 J
A ball rolls down a frictionless half-pipe from a 4m height. How high does it go on the other side?
4m
How much work was done to push a 10 N book 5 meters?
50 J
From the previous question (Two people each applied 100N of force to move a crate 2m. Friction is 50N. What is the Fnet?) (what is the work done on the crate?)) what power was used if it took them 60s?
5W
A rower does 3600 J of work on the oars in 60.0 s. What is his power output?
60 W
Using a pulley system a crew does 2,000 J of work to lift a box that required 1200 J of work. What is the efficiency of the pulley system?
60%
Two people each applied 100N of force to move a crate 3m. Whats the work done?
600 J
A 2kg box falls off a shelf and hits the ground with 49J of KE. How fast was it going?
7 m/s
A bowling ball travels horizontally at 2 m/s and has 16J of KE. What is its mass?
8 kg
Efficiency
A ratio of output work/input work Fout/Fin
Equation for Average Power
Average Power = Work / Time P = W / t
This means to preserve or keep constant
Conservation
A ball rolls down a frictionless half pipe from a 2m height, and goes 2m on the other side due to
Conservation of Energy
Equation for simple machines
Depends on which machine
Simple machines
Does work by motion in 1 direction or axis
Input Force
Effort force applied to machine (N) Fe
Kinetic Energy (KE)
Energy of an object due to mass and velocity
Gravitational (GPE)
Energy stored by position usually height+mass+gravity
Net force symbol
Fnet
Energy Transformations
For mechanical energy KE↔PE
Equation triangle to calculate effort force of simple machines.
Fr on top Fe on left and MA on right
This is the force that opposes motion
Friction
Gravitational Potential Energy symbol
GPE
What are the two types of mechanical energy
GPE, KE
Gravitational Potential Energy equation
GPE= m × g × h mass × gravity × height
Examples of Potential Energy
Gravitational (due to "g"), Elastic (Springs, rubber bands)
Displacement (s)
Has distance and direction, object is moved by Fnet (m)
Mechanical advantage (MA)
How much machine multiplies effort force (Fin)
This is multiplied times your effort force by a simple machine
MA
MA equation for screw
MA = 2πR/ P
MA equation for incline plane
MA = L / h
MA equation for wedge
MA = L / t
MA equation for pulley
MA = N
MA equation for wheel and axle
MA = R / r
MA equation for Lever
MA= Le / Lr
This is the sum of KE + PE
ME
Net force units
N
Joules can also be written as
N × m
Conservation of Energy
Neither created nor destroyed, only transforms
Do you do work holding a bag of groceries? If so, on what axis?
No work because there is no displacement
Do you do work carrying a suitcase 5m in a straight line?
No work done because there is not force applied.
Is a scissor a simple machine?
No, it is a compound machine. It contains a wedge and a lever.
Do you do work applying a 100-N force to a car but, its not moving? Why?
No, there is no displacement
Do you do work balancing a book on your head? Why?
No, there is no displacement
Do you do work carrying a 2 N book bag 5m across the floor? Why?
No, there is no force on the bag
Average Power symbol
P
Potential energy symbol
PE
(A 2 kg ball sits on top of a 10m pole. What is PE and KE) The ball falls off the pole what is PE and KE at 5 m?) What is PE and KE just as the ball hits the ground?
PE: 0J, KE: 200J
(A 2 kg ball sits on top of a 10m pole. What is PE and KE) The ball falls off the pole what is PE and KE at 5 m?
PE: 100J, KE: 100 J
A 2 kg ball sits on top of a 10m pole. What is PE and KE?
PE: 200 J, KE: 0j
This is the name of the rate at which work is done.
Power
Average Power (P)
Rate at which work is done over a time period.
Output force
Resulting force generated by machine (N) Fe
This is a compound machine using a wedge and a lever
Scissors
Potential energy (PE)
Stored in an object by position
Energy
The amount and ability to do work in Joules (J)
This is accomplished by adding a net force to an object to move it a certain distance.
Work
Equation for Work
Work = Force × Displacement W= F × s
An object moving in the direction of Fnet has ____________________
Work down on it
Do you do work opening a car door? On what axis?
Yes, on X axis
Do you do work pushing a 200N sled 5-m across the ground? Why?
Yes, on the X axis
Do you do work pushing a door open? If so, on what axis?
Yes, on the X axis
Do you do work climbing up a 5-m ladder? Why?
Yes, on the Y axis
Do you do work typing on a keyboard? If so, on what axis?
Yes, on the Y axis
Do you do work walking up a flight of stairs? If so, on what axis?
Yes, on the Y axis
Do you do work lifting up a book bag? If so, on what axis?
Yes, on the Y axis.
What are the 6 different simple machines?
lever, wedge, inclined plane, screw, wheel and axle, pulley.
Gravitational Potential energy equation for height
h = PE ÷ (m × g)
mass units
kg
Displacement symbol
s
A crane uses an average force of 5200 N to lift and does 130,000 J to lift an I-beam, straight up. How far does the crane lift the beam?
s = 25m
time unit
seconds