2. Energy

Question 1

Formula for KE vs gravitational PE vs elastic PE vs total mechanical energy (when sum of KE and PE are constant) vs work-energy theorem

Answer 1

1/2mv^2 vs mgh vs 1/2kx^2 vs U + K vs W = KE final - KE initial

Question 2

What’s the relationship b/w KE and speed? What about vel?

Answer 2

when speed doubles, KE quadruples. KE has nothing to do w/ vel, it’s the same regardless of direction

Question 3

What’s the relationship b/w PE and mass and height?

Answer 3

direct relationship by same factor

Question 4

Be careful how height is described at ground level when solving PE

Answer 4

Height can be zero on top of a cliff if the problem specifies it like that

Question 5

Formula for work vs power

Answer 5

W = Fdisplacementcos(theta) where theta = angle b/w Force vector and displacement vector; W = Pressure*deltaVol (or whatever area under the curve) vs W/t = deltaE/t, P = Fv where v=vel and F=force

Question 6

Units for energy/work vs power

Answer 6

J vs J/s

Question 7

Hooke’s Law. How to find freq from spring

Answer 7

F = -kx FOR SPRINGS; Higher k —> stiffer spring
f = 1/2pi(sqrt(k/m)) where m is molar mass, k is spring constant

Question 8

Is energy a vector or scalar?

Answer 8

scalar

Question 9

Is work being done by the system in an isovolumetric vs isobaric process?

Answer 9

No vs yes, find area under the curve to get Work

Question 10

Is work being done when piston expands vs compresses?

Answer 10

Yes —> W>0 vs no —> W<0

Question 11

How to find momentum. Will momentum be conserved in elastic or inelastic collisions?

Answer 11

p=mv. Momentum is always conserved unless masses aren’t constant or when an outside force is introduced

Question 12

Is kinetic energy conserved or not conserved in elastic and in elastic collisions?

Answer 12

Conserved in elastic, not conserved in inelastic (in which case KE can turn to heat or sound)

Question 13

What are conservative forces and examples? What about non conservative forces?

Answer 13

Forces that are independent from path and don’t dissipate energy, hence net energy = 0 (ex: gravity and electrostatic)
Forces that are path dependent and dissipate into energy like heat or thermal energy, hence some energy is lost (ex: friction, air resistance, viscosity); they’re not taken into account for mechanical energy eqn

Question 14

What’s the eqn to solve for work of nonconservative forces?

Answer 14

W_nonconservative = deltaE = deltaU + deltaK

DeltaU can be zero if object started and landed at same height
Negative W means that energy is dissipated from system

Question 15

Work-energy theorem

Answer 15

Net W = deltaK = KEfinal - KEinitial

Question 16

In UCM, how does displacement vector relate to force vector and work?

Answer 16

Displacement vector and force vector are perpendicular to each other —> no work is done in UCM

Question 17

Mechanical advantage and efficiency are both ratios. What quantities are used for these ratios?

Answer 17

Mech advantage compares forces and efficiency compares work

Question 18

How to find impulse. Is it a vector or scalar?

Answer 18

J = deltap (change in momentum) or F*deltat where F = force and deltat = change in time. Vector