SOME ENERGY/WORK QUESTIONS (Energy2Probs)

1) What are some different units for energy?

*See notes*

2) What are the types of energy and how are they measured?

*See notes*

3) What are the types of mechanical energy?

*Gravitational Potential,
Elastic Potential, Kinetic.*

4) Describe the energy changes in a jack in a box ( a bobble head on a spring with friction). How would this look on a graph?

5) Which type of mechanical energy does NOT depend on position?

*Kinetic Energy depends only
on mass and speed.*

6) Give a typical amount of Calories of mechanical energy likely to be used by a typical teenager per week. How would this be calculated?

*Anywhere from 12,000 Cal to
30,000 Calories.*

*Energy =Kinetic plus GPE plus
Work or add up all activities.*

7) What is the work done if I lift a 20 kg object 3 meters, then carry it at a constant speed horizontally for 5 meters?

*Work is Force times Dis in
direction of motion. Force of Gravity=Weight= 20*9.8=196N*

*Work = 196N*3 m= 588 Joules.
No work is done by carrying it horizontally.*

8) What is the purpose of a simple machine?

*To multiply force (or
sometimes to multiply time/distance). Work in usually equals work out.*

9) Explain why all simple machines are really either levers or ramps:

*Ramps: Planes, Screws, Wedges*

*Levers: :levers, wheel/axle,
pulley, gear*

10) A parachutist with a mass of
40.0 kg jumps out of an airplane at an altitude of

5000 m. After the parachute deploys, the parachutist lands with a
velocity of 6.00

m/s. Using the
work–kinetic energy theorem, find the energy that was lost to air
resistance

during this jump.

*GPE
+ KE = GPE + KE*

*Mgh
+ ½ mv ^{2}= mgh + ½ mv^{2}*

*40*9.8*5000
+ 0 = 0 + ½ (40)(6) ^{2}*

*1,960,000
Joules start, 720 Joules end, lost 1959280 Joules!*

11) A lever with an effort arm
of 4 cm, and a resistance arm of 8 cm is attached to a 2 string pulley holding
a 4 kg mass. If it takes 60 Newtons of force to push on the lever to lift the
mass then find:

Work In: Work
Out: Ideal
mech Advantage: Actual Mech
Advantage: Efficiency

* Lever IMA= 4/8=.5, Pulley IMA = 2:1,
machine IMA = 1:1*

*Actual Mech Advant=
Fout/Fin = (4)*9.8/ 60 = 39.2/60 = .6533*

*Efficiency = AMA/IMA=
.6533/1 = 65.33 %*

12) A force of 1250 N is needed
to move a crate weighing 3270 N up a ramp that is 4.55 m

long. If the elevated end of
the ramp is 0.750 m high, what is the efficiency of the ramp?

*Efficiency=WorkOut/WorkIn
= (3270*.75)/(1250*4.55) = 2452.5 J / 5687.5 J = 43.1%*

13) A spring is pulled back from rest 20 cm with 40 N of force. If it hits a 3 kg mass, what is its starting elastic energy? How fast is it going when it leaves the spring? (no friction). How high does it go?

*X=.2 m, F = 40N k = F/x=200
N/m, EPE = 1/2Kx ^{2}= ½*(200)*(.2)^{2}= 4 Joules*

*4 Joules = KE = 1/2mv ^{2}=
½*(3)*v^{2}, v = sqrt(2*4/3) = 1.633 m/s = v*

*4 Joules = GPE = 4 = mgh =
3*9.8*h, h = 4/(3*9.8)= 4/29.4=.136 m=h*

14) ) A still person with a mass of 156 kilogram catches a 9 kilogram ball going 30 meters/second, which causes him to roll down a hill (starting at 1.636 m/s) . What is the total energy after the collision? If they are on the top of a 2 meter tall hill, what is the velocity at the bottom of the hill?

*Before collision E= GPEball+
KE ball+GPE person = 156*9.8*2+9*9.8*2+1/2*9*30 ^{2}*

* =
3057.6 +176.4 + 4050=7284 Joules*

*AFTER collision E = GPE + KE
= 165*9.8*2 + ½ * 165* 1.636 ^{2}=3234J+220.8=*

*3454.8 Joules*

*At bottom E= 3454.8J =
KE=1/2mv ^{2}= ½*165*v^{2},*

* V=
sqrt (2*3454.8/165)=6.47 m/s*