This activity allows you to observe
momentum conservation for a collision between a big fish
and a little fish. Before the collision (a.k.a. lunch), the little
fish is in motion and the big fish is at rest. After the collision, the
two fish move together at the same speed. One can reason
proportionally to predict the post-collision speed.

Fish Catch

Begin Activity

20 g

80 g

15 g

75 g

25 g

You may view and analyze nine different situations. The situations
differ in terms of the relative mass of the two fish. Click on an icon
to see the animation and accompanying analysis.

60 g

45 g

125 g

100 g

60

A little fish with a mass of 20 grams and a velocity of 80 cm/s is
eaten by a big fish that has a mass of 100 grams and is initially at
rest. The two fish move with the same speed after the collision (i.e.,
after lunch).

20

0.0

1600

--

###

80.0

Repeat Animation

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Before lunch, only the little fish has momentum. The before-lunch
momentum is the result of 20 grams in motion. After lunch, the big fish
and little fish move with the same velocity. A total of 80 grams is in
motion. The amount of mass in motion has increased by a factor of 4.
For total momentum to be conserved, the velocity with which this mass
moves decreases by a factor of 4 - from 80.0 cm/s to 20.0 cm/s.

1200

900

45

20.0

A little fish with a mass of 15 grams and a velocity of 80 cm/s is
eaten by a big fish that has a mass of 45 grams and is initially at
rest. The two fish move with the same speed after the collision (i.e.,
after lunch).

Before lunch, only the little fish has momentum. The before-lunch
momentum is the result of 15 grams in motion. After lunch, the big fish
and little fish move with the same velocity. A total of 60 grams is in
motion. The amount of mass in motion has increased by a factor of 4.
For total momentum to be conserved, the velocity with which this mass
moves decreases by a factor of 4 - from 80.0 cm/s to 20.0 cm/s.

15

300

16.0

960

Before lunch, only the little fish has momentum. The before-lunch
momentum is the result of 15 grams in motion. After lunch, the big fish
and little fish move with the same velocity. A total of 75 grams is in
motion. The amount of mass in motion has increased by a factor of 5.
For total momentum to be conserved, the velocity with which this mass
moves decreases by a factor of 5 - from 80.0 cm/s to 16.0 cm/s.

240

A little fish with a mass of 15 grams and a velocity of 80 cm/s is
eaten by a big fish that has a mass of 60 grams and is initially at
rest. The two fish move with the same speed after the collision (i.e.,
after lunch).

13.3

1000

A little fish with a mass of 15 grams and a velocity of 80 cm/s is
eaten by a big fish that has a mass of 75 grams and is initially at
rest. The two fish move with the same speed after the collision (i.e.,
after lunch).

75

200

Before lunch, only the little fish has momentum. The before-lunch
momentum is the result of 15 grams in motion. After lunch, the big fish
and little fish move with the same velocity. A total of 90 grams is in
motion. The amount of mass in motion has increased by a factor of 6.
For total momentum to be conserved, the velocity with which this mass
moves decreases by a factor of 6 - from 80.0 cm/s to 13.3 cm/s.

A little fish with a mass of 20 grams and a velocity of 80 cm/s is
eaten by a big fish that has a mass of 60 grams and is initially at
rest. The two fish move with the same speed after the collision (i.e.,
after lunch).

400

Before lunch, only the little fish has momentum. The before-lunch
momentum is the result of 20 grams in motion. After lunch, the big fish
and little fish move with the same velocity. A total of 100 grams is in
motion. The amount of mass in motion has increased by a factor of 5.
For total momentum to be conserved, the velocity with which this mass
moves decreases by a factor of 5 - from 80.0 cm/s to 16.0 cm/s.

80

320

A little fish with a mass of 20 grams and a velocity of 80 cm/s is
eaten by a big fish that has a mass of 80 grams and is initially at
rest. The two fish move with the same speed after the collision (i.e.,
after lunch).

1280

100

Before lunch, only the little fish has momentum. The before-lunch
momentum is the result of 20 grams in motion. After lunch, the big fish
and little fish move with the same velocity. A total of 120 grams is in
motion. The amount of mass in motion has increased by a factor of 6.
For total momentum to be conserved, the velocity with which this mass
moves decreases by a factor of 6 - from 80.0 cm/s to 13.3 cm/s.

1333

267

2000

1500

Before lunch, only the little fish has momentum. The before-lunch
momentum is the result of 25 grams in motion. After lunch, the big fish
and little fish move with the same velocity. A total of 100 grams is in
motion. The amount of mass in motion has increased by a factor of 4.
For total momentum to be conserved, the velocity with which this mass
moves decreases by a factor of 4 - from 80.0 cm/s to 20.0 cm/s.

500

A little fish with a mass of 25 grams and a velocity of 80 cm/s is
eaten by a big fish that has a mass of 75 grams and is initially at
rest. The two fish move with the same speed after the collision (i.e.,
after lunch).

25

A little fish with a mass of 25 grams and a velocity of 80 cm/s is
eaten by a big fish that has a mass of 100 grams and is initially at
rest. The two fish move with the same speed after the collision (i.e.,
after lunch).

Before lunch, only the little fish has momentum. The before-lunch
momentum is the result of 25 grams in motion. After lunch, the big fish
and little fish move with the same velocity. A total of 125 grams is in
motion. The amount of mass in motion has increased by a factor of 5.
For total momentum to be conserved, the velocity with which this mass
moves decreases by a factor of 5 - from 80.0 cm/s to 16.0 cm/s.

125

Before lunch, only the little fish has momentum. The before-lunch
momentum is the result of 25 grams in motion. After lunch, the big fish
and little fish move with the same velocity. A total of 150 grams is in
motion. The amount of mass in motion has increased by a factor of 6.
For total momentum to be conserved, the velocity with which this mass
moves decreases by a factor of 6 - from 80.0 cm/s to 13.3 cm/s.

A little fish with a mass of 25 grams and a velocity of 80 cm/s is
eaten by a big fish that has a mass of 125 grams and is initially at
rest. The two fish move with the same speed after the collision (i.e.,
after lunch).

1667

333