College Physics

# 8.4Elastic Collisions in One Dimension

College Physics8.4 Elastic Collisions in One Dimension

Let us consider various types of two-object collisions. These collisions are the easiest to analyze, and they illustrate many of the physical principles involved in collisions. The conservation of momentum principle is very useful here, and it can be used whenever the net external force on a system is zero.

We start with the elastic collision of two objects moving along the same line—a one-dimensional problem. An elastic collision is one that also conserves internal kinetic energy. Internal kinetic energy is the sum of the kinetic energies of the objects in the system. Figure 8.6 illustrates an elastic collision in which internal kinetic energy and momentum are conserved.

Truly elastic collisions can only be achieved with subatomic particles, such as electrons striking nuclei. Macroscopic collisions can be very nearly, but not quite, elastic—some kinetic energy is always converted into other forms of energy such as heat transfer due to friction and sound. One macroscopic collision that is nearly elastic is that of two steel blocks on ice. Another nearly elastic collision is that between two carts with spring bumpers on an air track. Icy surfaces and air tracks are nearly frictionless, more readily allowing nearly elastic collisions on them.

### Elastic Collision

An elastic collision is one that conserves internal kinetic energy.

### Internal Kinetic Energy

Internal kinetic energy is the sum of the kinetic energies of the objects in the system.

Figure 8.6 An elastic one-dimensional two-object collision. Momentum and internal kinetic energy are conserved.

Now, to solve problems involving one-dimensional elastic collisions between two objects we can use the equations for conservation of momentum and conservation of internal kinetic energy. First, the equation for conservation of momentum for two objects in a one-dimensional collision is

$p 1 + p 2 = p′ 1 + p′ 2 F net = 0 p 1 + p 2 = p′ 1 + p′ 2 F net = 0 size 12{ left (F rSub { size 8{"net"} } =0 right )} {}$
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or

$m1v1+m2v2=m1v′1+m2v′2Fnet=0,m1v1+m2v2=m1v′1+m2v′2Fnet=0, size 12{ left (F rSub { size 8{"net"} } =0 right )} {}$
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where the primes (') indicate values after the collision. By definition, an elastic collision conserves internal kinetic energy, and so the sum of kinetic energies before the collision equals the sum after the collision. Thus,

$12m1v12+ 12m2 v22 = 12 m1 v′12 + 12 m2 v′22 (two-object elastic collision)12m1v12+ 12m2 v22 = 12 m1 v′12 + 12 m2 v′22 (two-object elastic collision)$
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expresses the equation for conservation of internal kinetic energy in a one-dimensional collision.

### Example 8.4

#### Calculating Velocities Following an Elastic Collision

Calculate the velocities of two objects following an elastic collision, given that

$m 1 = 0 . 500 kg, m 2 = 3 . 50 kg, v 1 = 4 . 00 m/s, and v 2 = 0 . m 1 = 0 . 500 kg, m 2 = 3 . 50 kg, v 1 = 4 . 00 m/s, and v 2 = 0 . size 12{m rSub { size 8{1} } =0 "." "500"" kg, "m rSub { size 8{2} } =3 "." "50"" kg, "v rSub { size 8{1} } =4 "." "00"" m/s, and "v rSub { size 8{2} } =0 "." } {}$
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#### Strategy and Concept

First, visualize what the initial conditions mean—a small object strikes a larger object that is initially at rest. This situation is slightly simpler than the situation shown in Figure 8.6 where both objects are initially moving. We are asked to find two unknowns (the final velocities $v′1v′1$ and $v′2v′2 size 12{v rSub { size 8{2} } '} {}$). To find two unknowns, we must use two independent equations. Because this collision is elastic, we can use the above two equations. Both can be simplified by the fact that object 2 is initially at rest, and thus $v2=0v2=0 size 12{v rSub { size 8{2} } =0} {}$. Once we simplify these equations, we combine them algebraically to solve for the unknowns.

#### Solution

For this problem, note that $v2=0 v2=0 size 12{v rSub { size 8{2} } =0} {}$ and use conservation of momentum. Thus,

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

$m1v1=m1v′1+m2v′2.m1v1=m1v′1+m2v′2. size 12{m rSub { size 8{1} } v rSub { size 8{1} } =m rSub { size 8{1} } { {v}} sup { ' } rSub { size 8{1} } +m rSub { size 8{2} } { {v}} sup { ' } rSub { size 8{2} } } {}$
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Using conservation of internal kinetic energy and that $v2=0v2=0 size 12{v rSub { size 8{2} } =0} {}$,

$1 2 m 1 v 1 2 = 1 2 m 1 v′ 1 2 + 1 2 m 2 v′ 2 2 . 1 2 m 1 v 1 2 = 1 2 m 1 v′ 1 2 + 1 2 m 2 v′ 2 2 . size 12{ { {1} over {2} } m rSub { size 8{1} } v rSub { size 8{1} rSup { size 8{2} } } = { {1} over {2} } m rSub { size 8{1} } v"" lSub { size 8{1} } ' rSup { size 8{2} } + { {1} over {2} } m rSub { size 8{2} } v rSub { size 8{2} } ' rSup { size 8{2} } } {}$
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Solving the first equation (momentum equation) for $v′2v′2 size 12{ { {v}} sup { ' } rSub { size 8{2} } } {}$, we obtain

$v′2 = m 1 m 2 v 1 − v′1 . v′2 = m 1 m 2 v 1 − v′1 . size 12{ { {v}} sup { ' } rSub { size 8{2} } = { {m rSub { size 8{1} } } over {m rSub { size 8{2} } } } left (v rSub { size 8{1} } - { {v}} sup { ' } rSub { size 8{1} } right )} {}$
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Substituting this expression into the second equation (internal kinetic energy equation) eliminates the variable $v′2v′2 size 12{ { {v}} sup { ' } rSub { size 8{2} } } {}$, leaving only $v′1v′1 size 12{ { {v}} sup { ' } rSub { size 8{1} } } {}$ 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

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

$v′1=−3.00 m/s.v′1=−3.00 m/s. size 12{ { {v}} sup { ' } rSub { size 8{1} } = - 3 "." "00"" m/s"} {}$
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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)(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 $v′1v′1 size 12{ { {v}} sup { ' } rSub { size 8{1} } } {}$ is used to find the velocity of the second object after the collision, we get

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

$v′2=1.00 m/s.v′2=1.00 m/s. size 12{ { {v}} sup { ' } rSub { size 8{2} } =1 "." "00""m/s"} {}$
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#### 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.

Figure 8.7