Work, Energy, and Momentum Flashcards
Energy definition
- Energy is a property or characteristic of a system to do work
- Energy measured in Joules J=kg*m2/s2
Kinetic Energy definition
Energy of motion
any obj that is moving has KE
K=½mv2
Potential energy definition
different types:
- gravitational
- electrostatic
- elastic (ie. compressed spring)
Gravitational Potential Energy
U=mgh
- NOTE: U directly proportional to all three variables
- used when close to earth’s surface
Total Mechanical Energy
E= U + K
Ef=Ei
KEi + Ui = KEf + Uf
1st Law of Thermodynamics
Energy is never created or destroyed, merely transfered from one system to another
Fill out this table:
- non conservative forces= make richer or poorer
- conservative forces=move money from one account to another
How can you determine if a force is conservative based on the net work needed to move a particle on a round trip?
- if the net work done to move a particle in any round trip path is zero= conservative
- if the net work done to move a particle b/w 2 points is the same regardless of path taken= conservative
Definition of work
- A process by which energy is transferred from one form to another (scalar) (J)
- if the force and the distance applied are in the same direction, work is positive
- If the force and the distance applied are in opposite directions, work is negative
Work = ?
W=FdcosØ
- where ø is the angle b/w the force and displacement vectors
- therefore only forces parallel or antiparallel to d vector do work b/c cos 0 = 1, cos 90= zero
- cos #<90=positive and cos 90-180=negative
- classic example is that no work is done by your arms when you carry a bucket of water for a mile b/c you’re lifting bucket vertically while its motion is horizontal
Power = ?
P=Work/time
- measured in Watts (J/s)
- rate at which energy is transferred from one system to another
Work Energy Theorem = ?
Wnet= ∆KE = Kf- Ki
Wcons. force= ∆KE = -∆PE
NOTE: ∆ means final-initial
-∆ means initial-final
How much work is done by the force of gravity on a satellite which moves in a circular orbit?
Zero. Because uniform cicular motion = constant velocity= no net work
What is the work done by gravity as a ball rises? As the ball falls?
As the ball rises, is losing speed so work is -
As the ball falls is gaining speed so work is +
(workair resistance is always - )
gravitational potential energy (far from earth) = ?
U= (-GMm)/r
NOTE: negative sign in there to keep relationship that as r increases, U decreases
Which is a greater potential energy -10J or -100J?
-10J b/c it is scalar (in vector world -100J would have been bigger)
Pulleys
The distance of pulling increases by the same factor that the effort decreases
Stationary pulleys: how much force and how far do you have to pull to move 100N box 1m?
- If the weight of the box is 100 N, you have to pull with a force of 100 N
- For every 1 meter you pull, the box goes up 1 meter
One Moving Pulley: how much force and how far do you have to pull to move 100N box 1m?
- Force needed to pull is halved because strings on both side of the pulley contribute equally. Therefore you supply 50 N (which is transmitted to the right-hand rope) while the left-hand rope contributes the other 50 N
- Because effort here is halved, the distance required to pull the box is doubled
Two moving pulleys: how much force and how far do you have to pull to move 100N box 1m
- Counting the ropes reveal that when we tug on one rope, it gets transmitted to a system where 4 ropes pull on the load. Thus, you can pull the 100 N box with only 25 N.
- However, for every 4 m you pull, the box only goes up 1 m.
momentum = ?
p=mv
(vector) (kg*m/s)
* for more than two objects, total momentum is vector sum of indiv momentums
Inertia def.
tendancy of objets to resist changes in their motion and momentum
Impulse = ?
I=F∆t=∆p=m(vf-vi)
(vector) (kg*m/s)
where ∆t is time of collision
- Definition: force applied to an object over time causes a ∆ objcet’s momentum=impulse
- if impulse happens over a longer period of time, the force decreases (ie. car safety measures)
elastic collisions
both momentum and KE are conserved
inellastic collisions
momentum is conserved, KE is not
(∆KE lost is amount of energy released from system ie as light, heat, sound)
totally inellastic collisions
momentum is conservced, KE is not
(∆KE lost is amount of energy released from system ie as light, heat, sound)
area under graph = impulse= ∆p = m∆v
area of triangle= 1/2 bh=500=(10kg)∆v
∆v=50 m/s
ANSWER: B
Mechanical advantage = ?
Mechanical advantage = Fout/Fin
Two ropes are holding up a 100N block, what is the tension in each rope?
T1 + T2 = mg
therefore tension in each is 1/2 the block
i.e. a lone moving pulley
Efficiency
Efficiency= Wout/Win
=(load*load distance)/(effort*effortdistance)
often given as percentage
Center of mass
the point within an object that follows a parabolic path of flight
PE of a spring
PE= ½ kx2
where x= dist from equilib position
