MORE MOMENTUM
Try These Problems: Use pages 56-61 in your textbook if you don’t understand momentum.
1. What are the units of momentum? What are the units of force?
kg ∙ m/s, kg ∙ m/s2
2. Find the momentum of the following objects all traveling at 20 m/sec.
cannonball withmass = 20 kg
20 kg x 20 m/s = 400 kg ∙ m/s
car with mass = 10,000 kg
104 kg x 20 m/s = 2 x 105 kg ∙ m/s
asteroid with mass =106 kg
106 kg x 20 m/s = 2 x 107 kg ∙ m/s
3. A car (5 kg, 0 m/sec) is hit by B car (15 kg,10 m/sec). What is the total momentum
before and after the collision?
The momentum is the same before and after the collision.
(5 kg x 0 m/s) + (15 kg x 10 m/s) = 0 kg ∙ m/s + 150 kg ∙ m/s = 150 kg ∙ m/s
4. In the problem above, what is the speed of car B if car A moves off at 5 m/sec?
The total momentum is 150 kg ∙ m/s. Subtract the new momentum of car A
(5 kg x 5 m/s) from the total momentum and divide out the mass of Car B to find its
speed. 150 kg ∙ m/s – 25 kg ∙ m/s = 125 kg ∙ m/s (125 kg ∙ m/s)/15 kg = 8.33 m/s
5. A train car (25 kg, 5 m/sec) couples with a train car (10 kg, 10 m/sec). What will the
new speed be of the coupled cars?
Total momentum is (25 kg x 5 m/s) + (10 kg x 10 m/s) = 125 kg ∙ m/s + 100 kg ∙ m/s
The total momentum = 225 kg ∙ m/s
Divide out the new coupled mass (25 kg + 10 kg) from the total momentum to find the
new speed of the coupled cars.
(225 kg ∙ m/s)/ (25 kg + 10 kg) = (225 kg ∙ m/s)/35 kg = 6.42 m/s
6. Which has more momentum, a comet with mass1020 kg moving at 0 m/sec, or
a fly with mass 10-9 kg moving 0.001 m/sec?
comet = 1020 kg x 0 m/s = 0 kg ∙ m/s
fly = 10-9 kg x 10-3 m/s = 10-12 kg ∙ m/s
The fly has more momentum, but just barely!
7. Write the formula for force. What are the units of force? Write the formula for
momentum. What are the units of momentum?
Force = mass x acceleration; kg ∙ m/s2 = 1 Newton (N)
Momentum = mass x speed (or more precisely, velocity), kg ∙ m/s
8. If a comet travels at a constant speed and therefore has no force, why does it do such
damage when it hits the earth?
All objects that hit something suddenly decelerate, which is a form of acceleration, so
they have force when they suddenly slow down upon impact. F = ma