Class 3 Flashcards
What is the equation for work? What is really going here?
W = Fd cos(theta)
Fcos(theta) is looking for the component of force parallel to d
What is the unit of work?
N x m = J
1 Newton-meter or a Joule
What kind of work is kinetic friction?
Negative work because friction always acts against motion
What kind of work is centripetal force?
Zero because the centripetal force is ALWAYS perpendicular to the instantaneous velocity.
What kind of work is the normal force?
Usually zero, because the normal force is perpendicular to motion if the object is not moving.
One example where it does do work is on an elevator. Displacement is up/down not left/right!
What kind of force is gravity? What does this mean?
Gravity is a conservative force, meaning work done by gravity only depends on the initial and final heights of the object, not the path the object follows.
What is the equation for calculating work done by gravity?
Wg = mgh
What is the equation for power?
P = Watts/time
What is power?
The rate at which work is done
What is the equation for power for a constant force parallel to a constant velocity?
P = Fv
What is the equation for kinetic energy?
KE = 1/2mv^2
Total work done on something is equal to what?
It’s change in kinetic energy
Change in potential energy = ?
mg(delta)h
KEi + PEi = ?
KEf + PEf
What are the two ways you can calculate mechanical advantage?
MA = F resistance/F effort
or MA = d effort/d resistance
Does mechanical advantage ever change the amount of work you have to do?
No because you can’t create energy from using a ramp, tool, etc. You gain an advantage by not needing as much FORCE.
Mechanical advantage will always be ______ than one.
Greater
Is momentum a vector or scalar?
Vector
What is the equation for momentum?
p = mv
mass x velocity
What is the equation for change in momentum?
(delta)p = Favg x (delta)t
note that delta p is sometimes called “J”
If you throw a ball at a padded wall versus a hard wall, what happens to momentum, time, average force, etc.
Change in momentum WILL NOT change, but the time to change momentum INCREASES.
Since change in momentum = Favg/delta t, increasing the time, decreases the average force!
This is why we use airbags.
Is momentum conserved in [free] collisions conserved?
YES! Momentum before collision will be the same as the momentum after the collision.
What is the difference between perfectly inelastic and perfectly elastic collisions?
A collision is perfectly INELASTIC if objects STICK TOGETHER after the collision.
Do inelastic or elastic collisions result in the greatest loss of kinetic energy?
Inelastic
