(Phys110) Exam 3 Flashcards
what is the definition of work and what are its units?
> energy transferred to or from a system by an applied force
Joules (J)
what is theta in the work formula?
the angle between the force and displacement vector
how do you know if work is positive?
when the force and the direction of motion point in the same direction
how do you know if work is negative?
when the force and direction of motion point in opposite directions
what is Wnet and how do you solve for it?
> the summation of the work done by each force
draw FBD, label forces, find work done by each force using the work formula, be careful to label positive and negative force, and add all of those works up
what is kinetic energy?
the energy of motion (moving)
what is potential energy?
stored energy associated with position (when an object changes position)
What is the work-kinetic energy theorem?
> Wnet = delta-K
summation of work = change in kinetic energy (Kf - Ki)
how do you solve for work?
> draw FBD to identify forces
make sure to label whether work is positive or negative
use the work formula to find the work done by each force
what are the units for energy?
Joules (J)
what are the units for force?
newtons (N)
what is thermal energy? (delta-Eth)
the work of kinetic friction
what does delta-s stand for in the equation for spring potential energy?
how much the spring compresses/stretches
what force is Hooke’s Law for?
spring force (Fsp)
what does delta-r stand for in delta-Eth?
the length of the rough surface
what is an example of a dot product?
vector A x vector B = AxBx + AyBy
a cannon, tilted upwards, fires a cannonball from the top of a tower and the ball lands on the ground. what is the energy transformation for this situation?
Ug > K: ball starts at some height (Ug) and then falls/moves downward (K)
Your tutor places her ipad on a frictionless table. She pushes the ipad against a spring, compressing it, then releases the ipad. what is the energy transformation for this situation?
Us > K: ipad touches spring and compresses it (spring potential) and then the spring pushes the ipad away (kinetic)
A PC screen falls on top of a spring, which compresses the spring. what is the energy transformation for this situation?
Ug > Us: PC starts at some position (gravitational potential) then falls on top of the spring, compressing it (spring potential)
A snot-nose kid goes down a slide and comes to a stop at the bottom. what is the energy transformation for this situation?
Ug > Eth: kid starts at some position(gravitational potential) then slides/moves down (kinetic) and comes to a stop at the bottom (work of friction)
A random telephone pole slides down a smooth hill, then across a horizontal rough surface, then hits a horizontal anchored spring.
Ug > Eth + Us: pole starts at some position (gravitational potential), hits a rough surface that causes a slow down (work of friction), then hits a spring and compresses it (spring potential)
what does delta-r stand for in the work formula?
displacement/total distance traveled
When solving for work, what should you label on your FBD?
Forces, x-axis, y-axis, angles, and the direction of motion.
If a problem asks you “how far does the object slide on the slope”, what are you solving for?
delta-s (generic direction because the object is on a slope)
If a problem asks you “how far does the object slide across the horizontal surface”, what are you solving for?
delta-x (horizontal surface implies moving along the x-axis in the x-direction)
what kind of position is used in the gravitational potential energy formula?
y-axis/y-direction position ONLY
can you put a horizontal distance/position into the Ug formula?
NO - you can only use vertical/y-axis/y-direction positions/distances in the Ug formula
when is final spring potential energy zero?
if the spring is not compressed
when is initial gravitational potential energy zero?
if the object starts at the zero point
what is the zero point in energy problems
up to you, but normally it’s the lowest part of the picture
when is final gravitational potential energy zero?
if the object ends up at the zero point
when is initial gravitational potential energy NOT zero?
if the object does not start at the zero point - if it starts at some position
when is final gravitational potential energy NOT zero?
if the object does not start at the zero point - if the object starts at some height that’s not the zero point
when is initial kinetic energy zero?
when the object is not moving initially (if the problem does not make it clear that the object is not moving initially, assume that it is initially at rest)
when is initial kinetic energy NOT zero?
when the object is moving initially
when is final kinetic energy zero?
when the object stops in the end
when is final kinetic energy NOT zero?
when the object is still moving in the end
how do you find impulse when given a Force vs. Time graph?
find the area under the curve - find geometrical shape(s) in the curve, figure out what area formula fits the shape(s), solve, and if there’s more than one area add all the areas up to get impulse
> remember that if the area is under the time axis, that area will be negative
what is the variable for impulse?
J
what is the variable for momentum?
P
do negative and positive signs matter when solving for momentum?
yesss - the velocities in the momentum equation are vectors, therefore positive and negative signs matter
what is the conservation of momentum ?
The total momentum of an isolated system is constant. Interactions within the system do not change the system’s total momentum - it’s for objects that interact with each other and not their environment (collisions)
> Pf = Pi
when can you use the conservation of momentum?
when there is a collision
how many terms need to be included in the momentum conservation?
depends on how many objects are interacting. for example, if three objects are interacting, there will be three terms (m1v1 + m2v2 + m3v3)
what are the units for impulse ?
Ns
what is an inelastic collision?
- when momentum is conserved and energy is not
- when objects collide and stick together
-an explosion counts as an inelastic collision because the object is stuck together at first then breaks apart
how can you solve a problem that has an inelastic collision in it?
conservation of momentum
> Pf = Pi
what is an elastic collision?
- both energy and momentum are conserved
- after colliding, objects don’t stick together
in an elastic collision, what happens to velocity when both masses are equal to each other? (mass 1 hits mass 2 and mass 2 initially at rest)
after the first object collides with the second, the first stops moving (V1f=0) and the other object is pushed away, having the same initial velocity of the first block (V2f=V1i)
in an elastic collision, what happens to velocity when mass 1 is larger than mass 2? (mass 1 hits mass 2 and mass 2 initially at rest)
- after the heavier mass 1 collides with lighter mass 2, mass 1 will go about the same speed as it did before it hit the block, therefore initial and final velocities are about the same (V1f=V1i)
- after the heavier mass 1 collides with the lighter mass2, mass 2 will go 2x the speed of mass 1 (V2f=V1i)
in an elastic collision, what happens to velocity when mass 2 is larger than mass 1? (mass 1 hits mass 2 and mass 2 initially at rest)
- after the lighter mass 1 hits the heavier mass 2, the lighter mass rebounds at the same speed it had before but negative because it’s traveling in the opposite direction (V1f= -V1i)
- after the lighter mass 1 hits the heavier mass 2, mass 2 barely moves so it doesn’t have any velocity (V2f=0)
what is one requirement in elastic collisions?
one of the objects needs to be at rest
in an elastic collision, if one of the objects is not at rest, what do you do?
> make it’s velocity zero (i.e. if the object has a velocity of +3m/s, subtract 3 from it to make it zero and subtract 3 from the other object’s velocity too)
plug into the elastic equation to solve for the final velocities of both objects
after solving, add/subtract the number you did in step 1 (i.e. add 3 onto the final velocities because you initially subtracted)
when dealing with a collision, how do you solve?
> write the objects that are colliding with each other
write Pi (before collision) and see if any terms are zero
write Pf (after the collision) and see if any terms are zero
equate Pi and Pf (Pi=Pf)
a mosquito and a truck have a head-on collision. which one has a larger change of momentum?
A. mosquito
B. truck
C. they have the same change of momentum
D. can’t say without knowing their initial velocities
C. they have the same change of momentum
> just like N3L
starting from rest, a marble first rolls down as steeper hill, then down a less steep hill of the same height. for which is it going faster at the bottom?
A. faster at the bottom of the steeper hill
B. faster at the bottom of the less steep hill
C. same speed at the bottom of both hills
D. can’t say without knowing the mass of the marble
C. same speed at the bottom of both hills
> energy transformation is the same for both (Ug>K)
> they both fall the same height
> a longer ramp does not mean it’ll take longer to get to the bottom but time does not affect anything here
