Exam 2--141 Flashcards
What force is work concerned with?
Force parallel
Work
the amount of energy a force can pump into (or take out) of a system
Kinetic energy
the energy of motion
generally, Wnet = ∆KE
Potential energy
stored energy / energy associated with position
shortcut to figuring out the amount of energy a force can pump into (or take out) of a system
How can we say that a collision is an isolated system and apply conservation of momentum?
1) the collision forces between the two objects we make internal forces by lumping the two objects together in a two object system
2) the friction force we artificially cancel out by making the ∆t so small that friction does not have time to act
How can we say that Bing on the skateboard with the fire extinguisher is isolated?
Friction is not that significant and normal force cancels out gravity
Conservation of momentum and Bing on the skateboard
some mass of air gets a big velocity in the negative direction from the fire extinguisher
so, Bing has to have a velocity forward to maintain the same momentum
What types of forces can do work?
Only forces parallel to the objects motion can do work
What types of forces can create torque?
Forces perpendicular to the object can twist the object
Is energy a vector?
No
Energy does not encode direction
Can simply add more energy or take away energy
What types of forces have potential energy?
conservative forces
Conservative forces
only the initial and final positions matter, not the path taken
no energy is lost by taking a longer path and correcting it (unlike something like friction)
Spring force
a contact force
linear force (force gets bigger as spring is stretched)
conservative force
What does the spring constant correspond to?
K corresponds to the stiffness of the spring
large K means a more stiff spring
Pendulum and energy
at bottom, all PE goes to KE energy
at top, all PE gravitational energy
Going from rest to rest? Like raising a book
Kinetic energy is 0
Allows Work to correspond to the change in potential energy
Power
the rate at which work is done
Unit: J/s (Watt)
Different between impulse and work
Work is force acting through a distance which produces a change in kinetic energy
Impulse is force acting through time which produces a change in momentum
Similarity between impulse and work
Both are ways of saying that forces change velocities
Why is bouncy ball better at knocking over a block than deadball?
Bouncy ball has velocity before and after the impact which creates a greater change in momentum (due to direction as well)
Since momentum is related to force through impulse, this means that the bouncy ball creates a larger force on the block than the deadball
When does conservation of momentum hold true?
an isolated system
Elastic collisions
total initial KE equals total final KE
ex: Newton’s cradle, billard balls
Are there ever any truly elastic collisions?
No
some energy is always lost
Can elastic collisions have 0 final KE?
No
Final KE equals initial KE and we need some initial KE to make the collision happen
Inelastic collisions
Some kinetic energy is lost
Initial KE is greater than final KE
losses energy to sound, heat, and especially permanent deformation
Permanent deformation
a dent in an object
collision forces doing work
changes KEf
Perfectly inelastic collisions
max amount of KE is lost
two objects have fused into one
don’t lose all KE energy because need some final velocity to balance momentum
Can you have KE=0 in perfectly inelastic collisions?
Yes
Let’s say two clay balls are thrown at each other and then they collide and stop
What does tangential velocity refer to?
the change in arc length over time
Two dots are positioned on the outer edge of a circle and the inner edge of a circle. Which moves faster?
In tangential terms, the one one the outer edge moves faster
In angular terms, both dots move at the same speed (both cover the same amount of angle)
Centripetal acceleration
points towards the center of a circle
due to velocity changing direction not magnitude
caused by familiar forces
always have centripetal acceleration when moving in a circle
Different between centripetal and angular acceleration
angular acceleration is an angular quantity whereas centripetal acceleration is a linear quantity (m/s2)
angular acceleration just refers to if angular velocity is slowing down or speeding up
can be rotating without an angular acceleration (magnitude not changing)
Orbit
a gravitational pull is the force that causes a centripetal acceleration
Universal gravitation
any mass in the universe attracts any other mass
r in universal gravitation
center to center distance between two objects
Kepler’s 3rd Law
square of orbital period is proportional to the cube of the orbital radius
Period
time for one complete revolution
Kepler’s 1st law
orbits are ellipses with the sun at a focus point
Keplers 2nd law
equal areas are swept out by the planet in equal times
this fits conservation of energy
Top of loop-da-loop
normal force and gravity act downwards
set normal force equal to zero to solve for the minimum speed needed to make it around the loop
What is force equivalent to in rotational motion?
torque
Torque
a twist that causes an angular acceleration
torques come from known forces
Why do you get more torque further from the hinge?
bigger distance that the force can act through
What is mass equivalent to in rotational motion?
Inertia
Why does a hoop roll slower than a disk?
Hoop has all the mass in a large radius to get around
this leads to greater resistance / rotational inertia
Arc length velocity compared to translational velocity
both are measured in m/s
both are the same magnitude
arc length velocity is being laid down along the bath (one revolution lays down one circumference of arc length)
therefore, translational velocity equals arc length velocity
Why is KE not zero at the apex of projectile motion?
there is still kinetic energy in the x direction
Is momentum conserved in inelastic collisions? What about perfectly elastic?
Yes
Momentum is conserved in both
What do you have to account for when using gravitational potential energy?
Gravity works on the object’s center of mass
only very specific cases, like a rod standing up, do we take this into consideration
A ball rolls down a ramp. What happens to kinetic and potential energy?
Kinetic energy varies with velocity and velocity increases constantly as the ball accelerates. Therefore, the square of the increasing velocity gives the kinetic energy graph a concave up parabola shape
Potential energy varies with height and gets steeper and steeper as the ball loses height more quickly. Therefore, potential energy graph has a concave down curve
What happens to total energy (KE+PE) if there is friction?
Some of the total energy is lost to the outside system
Mechanical energy is not conserved
Does center of gravity mean that masses are equal on both sides?
No
Center of gravity accounts for both mass and radius
So, something with a larger radius would have less mass on a given side
If something moves at a constant speed, what is the net work?
The net work is zero
There was no change in velocity which means there was no acceleration/net force, and W=F*x, so this would mean that net work is zero
Can still be doing work though when the potential energy changes (like moving books off ground)
Why does conservation of momentum not apply to free fall?
the net force on the system does not equal 0
gravity is unbalanced in free fall and therefore, this is not an isolated system
If you double angular acceleration do you double your final angular speed?
Yes
