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Solution

For this problem, note that v 2 = 0 size 12{v rSub { size 8{2} } =0} {} and use conservation of momentum. Thus,

p 1 = p 1 + p 2 size 12{p rSub { size 8{1} } =p' rSub { size 8{1} } +p' rSub { size 8{2} } } {}

or

m 1 v 1 = m 1 v 1 + m 2 v 2 . size 12{m rSub { size 8{1} } v rSub { size 8{1} } =m rSub { size 8{1} } { {v}} sup { ' } rSub { size 8{1} } +m rSub { size 8{2} } { {v}} sup { ' } rSub { size 8{2} } } {}

Using conservation of internal kinetic energy and that v 2 = 0 size 12{v rSub { size 8{2} } =0} {} ,

1 2 m 1 v 1 2 = 1 2 m 1 v 1 2 + 1 2 m 2 v 2 2 . size 12{ { {1} over {2} } m rSub { size 8{1} } v rSub { size 8{1} rSup { size 8{2} } } = { {1} over {2} } m rSub { size 8{1} } v"" lSub { size 8{1} } ' rSup { size 8{2} } + { {1} over {2} } m rSub { size 8{2} } v rSub { size 8{2} } ' rSup { size 8{2} } } {}

Solving the first equation (momentum equation) for v 2 size 12{ { {v}} sup { ' } rSub { size 8{2} } } {} , we obtain

v 2 = m 1 m 2 v 1 v 1 . size 12{ { {v}} sup { ' } rSub { size 8{2} } = { {m rSub { size 8{1} } } over {m rSub { size 8{2} } } } left (v rSub { size 8{1} } - { {v}} sup { ' } rSub { size 8{1} } right )} {}

Substituting this expression into the second equation (internal kinetic energy equation) eliminates the variable v 2 size 12{ { {v}} sup { ' } rSub { size 8{2} } } {} , leaving only v 1 size 12{ { {v}} sup { ' } rSub { size 8{1} } } {} as an unknown (the algebra is left as an exercise for the reader). There are two solutions to any quadratic equation; in this example, they are

v 1 = 4 . 00 m/s size 12{ { {v}} sup { ' } rSub { size 8{1} } =4 "." "00"`"m/s"} {}

and

v 1 = 3 . 00 m/s . size 12{ { {v}} sup { ' } rSub { size 8{1} } = - 3 "." "00"" m/s"} {}

As noted when quadratic equations were encountered in earlier chapters, both solutions may or may not be meaningful. In this case, the first solution is the same as the initial condition. The first solution thus represents the situation before the collision and is discarded. The second solution ( v 1 = 3 . 00 m/s ) size 12{ \( { {v}} sup { ' } rSub { size 8{1} } = - 3 "." "00"`"m/s" \) } {} is negative, meaning that the first object bounces backward. When this negative value of v 1 size 12{ { {v}} sup { ' } rSub { size 8{1} } } {} is used to find the velocity of the second object after the collision, we get

v 2 = m 1 m 2 v 1 v 1 = 0 . 500 kg 3 . 50 kg 4 . 00 3 . 00 m/s size 12{ { {v}} sup { ' } rSub { size 8{2} } = { {m rSub { size 8{1} } } over {m rSub { size 8{2} } } } left (v rSub { size 8{1} } - { {v}} sup { ' } rSub { size 8{1} } right )= { {0 "." "500"`"kg"} over {3 "." "50"`"kg"} } left [4 "." "00" - left ( - 3 "." "00" right ) right ]`"m/s"} {}

or

v 2 = 1 . 00 m/s . size 12{ { {v}} sup { ' } rSub { size 8{2} } =1 "." "00"`"m/s"} {}

Discussion

The result of this example is intuitively reasonable. A small object strikes a larger one at rest and bounces backward. The larger one is knocked forward, but with a low speed. (This is like a compact car bouncing backward off a full-size SUV that is initially at rest.) As a check, try calculating the internal kinetic energy before and after the collision. You will see that the internal kinetic energy is unchanged at 4.00 J. Also check the total momentum before and after the collision; you will find it, too, is unchanged.

The equations for conservation of momentum and internal kinetic energy as written above can be used to describe any one-dimensional elastic collision of two objects. These equations can be extended to more objects if needed.

Making connections: take-home investigation—ice cubes and elastic collision

Find a few ice cubes which are about the same size and a smooth kitchen tabletop or a table with a glass top. Place the ice cubes on the surface several centimeters away from each other. Flick one ice cube toward a stationary ice cube and observe the path and velocities of the ice cubes after the collision. Try to avoid edge-on collisions and collisions with rotating ice cubes. Have you created approximately elastic collisions? Explain the speeds and directions of the ice cubes using momentum.

Phet explorations: collision lab

Investigate collisions on an air hockey table. Set up your own experiments: vary the number of discs, masses and initial conditions. Is momentum conserved? Is kinetic energy conserved? Vary the elasticity and see what happens.

Collision Lab

Section summary

  • An elastic collision is one that conserves internal kinetic energy.
  • Conservation of kinetic energy and momentum together allow the final velocities to be calculated in terms of initial velocities and masses in one dimensional two-body collisions.

Conceptual questions

What is an elastic collision?

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Problems&Exercises

Two identical objects (such as billiard balls) have a one-dimensional collision in which one is initially motionless. After the collision, the moving object is stationary and the other moves with the same speed as the other originally had. Show that both momentum and kinetic energy are conserved.

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Professional Application

Two manned satellites approach one another at a relative speed of 0.250 m/s, intending to dock. The first has a mass of 4 . 00 × 10 3 kg size 12{4 "." "00" times "10" rSup { size 8{3} } " kg"} {} , and the second a mass of 7 . 50 × 10 3 kg size 12{7 "." "50" times "10" rSup { size 8{3} } " kg"} {} . If the two satellites collide elastically rather than dock, what is their final relative velocity?

0.250 m/s

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A 70.0-kg ice hockey goalie, originally at rest, catches a 0.150-kg hockey puck slapped at him at a velocity of 35.0 m/s. Suppose the goalie and the ice puck have an elastic collision and the puck is reflected back in the direction from which it came. What would their final velocities be in this case?

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Questions & Answers

A ball is thrown vertically upwards so that its max height is 100m...at the same instant another ball is dropped frm 100m above the ground...where will the two balls meet and why?
Blessing Reply
50m
wenhe
why?
Blessing
plz calculate the answer...
Blessing
😟
Blessing
75m
S.M
how?😨
Blessing
...
Blessing
....
Blessing
because they approach each other at different speeds, but same acceleration, so you can calculate its height above ground.
S.M
Give an example (but not one from the text) of a device used to measure time and identify what change in that device indicates a change in time.
David Reply
hour glass, pendulum clock, atomic clock?
S.M
tnks
David
A heart pumping blood would indicate a change in time as its volume or pressure changed. The ratio of displacement or change in configuration between any 2 systems can indicate time.
Khashon
how did they solve for "t" after getting 67.6=.5(Voy + 0)t
Martin Reply
Find the following for path D in [link] : (a) The distance traveled. (b) The magnitude of the displacement from start to finish. (c) The displacement from start to finish.
David Reply
the topic is kinematics
David
can i get notes of solid state physics
Lohitha
just check the chpt. 13 kinetic theory of matter it's there
David
is acceleration a fundamental unit.
David Reply
no it is derived
Abdul
no
Nisha
K thanks
David
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Emmanuel
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Gift
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David
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Villaflor Reply
how about a conceptual framework can you simplify for me? needed please
Villaflor
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Afzal
I think they are constantly moving
Villaflor
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S.M
not possible to fix electron position in space,
S.M
Physics
Beatriz
yes of course Villa flor
David
equations of kinematics for constant acceleration
Sagcurse Reply
A bottle full of water weighs 45g when full of mercury,it weighs 360g.if the empty bottle weighs 20g.calculate the relative density of mercury and the density of mercury....pls I need help
Lila Reply
well You know the density of water is 1000kg/m^3.And formula for density is density=mass/volume Then we must calculate volume of bottle and mass of mercury: Volume of bottle is (45-20)/1000000=1/40000 mass of mercury is:(360-20)/1000 kg density of mercury:(340/1000):1/50000=(340•40000):1000=13600
Sobirjon
the latter is true
Sobirjon
100g of water is mixed with 60g of a liquid of relative density 1.2.assuming no changes in volume occurred,find the average relative density of the mixture...take density of water as 1g/cm3 and density of liquid 1.2g/cm3
Lila
plz hu can explain Heisenberg's uncertainty principle
Emmanuel Reply
who can help me with my problem about acceleration?
Vann Reply
ok
Nicholas
how to solve this... a car is heading north then smoothly made a westward turn during the travel the speed of the car remains constant at 1.5km/h what is the acceleration of the car? the total travel time of the car as it smoothly changed its direction is 15 minutes
Vann
i think the acceleration is 0 since the car does not change its speed unless there are other conditions
Ben
yes I have to agree, the key phrase is, "the speed of the car remains constant...," all other information is not needed to conclude that acceleration remains at 0 during the entire time
Luis
who can help me with a relative density question
Lila
1cm3 sample of tin lead alloy has mass 8.5g.the relative density of tin is 7.3 and that of lead is 11.3.calculate the percentage by weight of tin in the alloy. assuming that there is no change of volume when the metals formed the alloy
Lila
morning, what will happen to the volume of an ice block when heat is added from -200°c to 0°c... Will it volume increase or decrease?
adefenwa Reply
no
Emmanuel
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Kate
BPED..is my course.
Kate
No
Emmanuel
I think it is neither decreases nor increases ,it remains in the same volume because of its crystal structure
Sobirjon
100g of water is mixed with 60g of a liquid of relative density 1.2.assuming no changes in volume occurred,find the average relative density of the mixture. take density of water as 1g/cm3 and density of liquid as 1.2g/cm3
Lila
Sorry what does it means"no changes in volume occured"?
Sobirjon
volume can be the amount of space occupied by an object. But when an object does not change in shape it will still occupy the same space. Thats why the volume will still remain the same
Ben
Most soilds expand when heated but if it changes state at 0C it will have less volume. Ice floats because it is less dense ie a larger mass per unit volume.
Richard
how to calculate velocity
Okwethu Reply
v=d/t
Emeka
his about the speed?
Villaflor
how about speed
Villaflor
v=d/t
Nisha
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Mine is good. How about you?
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akinmeji
Hello
Mishael
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Jerry
hi
Sakhi
hi
H.C
so, what is going on here
akinmeji
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Ajayi
good morning ppl
ABDUL
If someone has not studied Mathematics enough yet, should theu study it first then study Phusics or Study Basics of Physics whilst srudying Math as well?
Riaz Reply
whether u studied maths or not, it is advisable to start from d basics cuz it is essential to know dem
Nuru
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Badmus
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Thomas
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Thomas
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Thomas
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Anand
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Nuru
A hydrometer of mass 0.15kg and uniform cross sectional area of 0.0025m2 displaced in water of density 1000kg/m3.what depth will the hydrometer sink
Lila
16.66 meters?
Darshik
16.71m2
aways
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aways
the mass is stretched a distance of 8cm and held what is the potential energy? quick answer
aways
Practice Key Terms 2

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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