Download Physics Homework: Elastic and Inelastic Collisions, Power and more Lecture notes Physics in PDF only on Docsity!
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- Homework #10
- Online and written homework assignments due tomorrow
- When using momentum conservation to solve a collision problem, remember that some velocities (and thus momenta) can be negative!
- Girl on a trampoline problem: consider girl grabbing box like a collision
- Monday schedule will be observed today:
- Lecture at 9:05 am
- Early office hours (2:00 to 3:30 pm)
- SI session @ LRC (7:15 to 8:30 pm)
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Elastic vs. inelastic collisions
(Perfectly) Elastic collision: When mechanical energy is conserved
(Perfectly) Inelastic collision: When colliding objects “stick” together after the collision Some of the mechanical energy is transformed into thermal energy
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Elastic collisions
Perfectly Elastic collision: here we have ( v2x ) i = 0 Momentum conservation along x axis :^ ^
m 1 ( v 1 x ) i = m 1 ( v 1 x ) f + m 2 ( v 2 x ) f
Mechanical energy conservation ( E (^) th = 0) : 1
2 m 1 ( v^1 x ) i
2
= 12 m 1 ( v 1 x ) f
2
+ 12 m 2 ( v 2 x ) f
2
Cannot determine the final velocities even if the initial velocities are known, but…
Momentum conservation + energy conservation yield:
(^ v 1 x ) f =^
m 1 m 2 m 1 + m 2
(^ v 1 x ) i
( v 2 x ) f =^
2 m 1 m 1 + m 2
(^ v 1 x ) i
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Elastic vs. inelastic collisions
Note:
Momentum conservation applies to both elastic and
inelastic collisions if there is no external force
(or this force can be neglected during the short time
over which a collision occurs)
DEMOS: gliders on air track
Newtons cradle
double ball drop
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Power question
P =
W
t
Car Mass (g) Speed (m/s) Time(s) A 100 3 2 B 200 2 2 C 200 2 3 D 300 2 3 E 300 1 4
PRS
Treat engine as applying a force external to the system (= car), thereby doing work on the system (transferring energy to the system)
K + U = W
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Muscle power problem
A 58 kg female athlete raised a 78 kg bar by a vertical distance of 1.5 m in 2.4 s. What is the power output of the weightlifter?
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Muscle power problem
A 58 kg female athlete raised a 78 kg bar by a vertical distance of 1.5 m in 2.4 s. What is the power output of the weightlifter?
System: bar + earth Environment: weightlifter does work on the system Know : m = 78 kg h = 1.5 m t = 2.4 s Find : P =?
Energy conservation : K + U = W 0 + (^) ( U (^) f U (^) i ) = W 0 + (^) ( mgh (^0) ) = W W = (78 kg ) (9.8 m / s^2 ) (1.5 m ) = 1150 J Power output :
P = W t
= 1150 J 2.4 s
= 480 W
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Racing track question
Two identical balls roll down separate tracks. They are released at the same time from the highest point. Which of the two balls will first arrive at the end of its track? Neglect friction.
A. Ball rolling down track A (in front) will arrive first. B. Ball rolling down track B (in back) will arrive first. C. The two balls will arrive at the same time.
PRS
A
B
