Solving Problems with Energy Conservation

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Solving Problems with
Energy Conservation
Conservation of Mechanical Energy
 KE + PE = 0
(Conservative forces ONLY!! )
or
E = KE + PE = Constant
• For elastic (Spring) PE: PEelastic = (½)kx2
KE1 + PE1 = KE2+ PE2
 (½)m(v1)2 + (½)k(x1)2 = (½)m(v2)2 +(½)k(x2)2
x1 = Initial compressed (or stretched) length
x2 = Final compressed (or stretched) length
v1 = Initial velocity, v2 = Final velocity
Example: Toy Dart Gun
Mechanical Energy Conservation
E=
A dart, mass m = 0.1 kg is pressed
against the spring of a dart gun.
The spring (constant k = 250 N/m)
is compressed a distance x1 = 0.06
m & released. The dart detaches
from the spring it when reaches its
natural length (x = 0). Calculate the
speed v2 it has at that point.
(½)m(v1)2 + (½)k(x1)2 = (½)m(v2)2 + (½)k(x2)2
0 + ( ½ )(250)(0.06)2 =
(½)(0.1)(v2)2 + 0
Gives: v2 = 3 m/s
Example: Pole Vault
Estimate the kinetic energy
& the speed required for a
70-kg pole vaulter to just
pass over a bar 5.0 m high.
Assume the vaulter’s
center of mass is initially
0.90 m off the ground &
reaches its maximum
height at the level of the
bar itself.
Example:
Two Kinds of PE
v1 = 0
m = 2.6 kg, h = 0.55 m
Y = 0.15 m, k = ?
v2 = ?
A 2 step problem:
Step 1: (a)  (b)
(½)m(v1)2 + mgy1 = (½)m(v2)2 + mgy2
v1 = 0, y1 = h = 0.55 m, y2 = 0
v3 = 0
Gives: v2 = 3.28 m/s
Step 2: (b)  (c) (both gravity & spring PE)
 (½)m(v2)2 + (½)k(y2)2 + mgy2 =
(½)m(v3)2 + (½)k(y3)2 + mgy3
y3 = Y = 0.15m, y2 = 0
 (½)m(v2)2 = (½)kY2 – mg
Solve & get k = 1590 N/m
ALTERNATE SOLUTION: (a)  (c) skipping (b)
Example: Bungee Jump
m = 75 kg, k = 50 N/m, y2 = 0
v1 = 0, v2 = 0, y1 = h = 15m + y
y =?
Mechanical Energy
Conservation
with both gravity & spring (elastic) PE
 (½)m(v1)2 + mgy1
= (½)m(v2)2 + mgy2 + (½)k(y)2
= 0 + mg(15+y)
= 0 + 0 + (½)k(y)2
Quadratic Equation for y:
Solve & get y = 40 m & -11 m
(throw away negative value)




Δy =
+ 15m




(a)  (c) directly!!
Other forms of energy; Energy Conservation
In any process, total energy is neither
created nor destroyed.
• Energy can be transformed from one form to
another & from one body to another, but the total
amount is constant.
 Law of Conservation of Energy
• Again: Not exactly the same as the Principle of
Conservation of Mechanical Energy, which holds for
conservative forces only! This is a general Law!!
• Forms of energy besides mechanical:
– Heat (conversion of heat to mech. energy & visa-versa)
– Chemical, electrical, nuclear, ..
The total energy is neither decreased
nor increased in any process.
• Energy can be transformed from one form to
another & from one body to another, but the
total amount remains constant
 Law of Conservation of Energy
• Again: Not exactly the same as the Principle of
Conservation of Mechanical Energy, which
holds for conservative forces only! This is a
general Law!!
Problems with Friction
• We had, in general:
WNC = KE + PE
WNC = Work done by all non-conservative forces
KE = Change in KE
PE = Change in PE (conservative forces only)
• Friction is a non-conservative force! So, if
friction is present, we have (WNC  Wfr)
Wfr = Work done by friction
Moving through a distance d, friction force
Ffr does work Wfr = - Ffrd
When friction is present, we have:
Wfr = -Ffrd = KE + PE = KE2 – KE1 + PE2 – PE1
Also now, KE + PE  Constant!
Instead,
or:
KE1 + PE1+ Wfr = KE2+ PE2
KE1 + PE1 - Ffrd = KE2+ PE2
• For gravitational PE:
(½)m(v1)2 + mgy1 = (½)m(v2)2 + mgy2 + Ffrd
• For elastic or spring PE:
(½)m(v1)2 + (½)k(x1)2 = (½)m(v2)2 + (½)k(x2)2 + Ffrd
Example: Roller Coaster with Friction
A roller-coaster car, mass
m = 1000 kg, reaches a
vertical height of only
y = 25 m on the second
hill before coming to a
momentary stop. It travels
a total distance d = 400 m.
Calculate the work done by friction (the thermal energy
produced) & calculate the average friction force on the car.
m = 1000 kg, d = 400 m, y1 = 40 m, y2= 25 m, v1= y2 = 0, Ffr = ?
(½)m(v1)2 + mgy1 = (½)m(v2)2 + mgy2 + Ffrd
 Ffr= 370 N
Power
Power  Rate at which work is done or rate at
which energy is transformed:
• Average Power:
P = (Work)/(Time) = (Energy)/(time)
• Instantaneous power:
SI units: Joule/Second = Watt (W) 1 W = 1J/s
British units: Horsepower (hp). 1hp = 746 W
A side note:
“Kilowatt-Hours” (from your power bill). Energy!
1 KWh = (103 Watt)  (3600 s) = 3.6  106 W s = 3.6  106 J
Example: Stair Climbing Power
A 60-kg jogger runs up a long flight of
stairs in 4.0 s. The vertical height of the
stairs is 4.5 m.
a. Estimate the jogger’s power
ss output in watts and horsepower.
b. How much energy did this
ss require?
• Average Power
• Its often convenient to write power in terms of force
& speed. For example, for a force F & displacement d in
the same direction, we know that the work done is:

W=Fd
So
F (d/t) = F v = average power
v  Average speed of the object
Example: Power needs of a car
Calculate the power required for a 1400-kg car to do the following:
a. Climb a 10° hill (steep!) at a steady 80 km/h
b. Accelerate on a level road from 90 to 110 km/h in 6.0 s
Assume that the average retarding force on the car is FR = 700 N.
a. ∑Fx = 0
F – FR – mgsinθ = 0
F = FR + mgsinθ
P = Fv
l
b. Now, θ = 0
∑Fx = ma
F – F R= 0
v = v0 + at
P = Fv
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