Practice 6D: | 1 | 2 | 3 | 4 | Go up
Momentum and Impulse - by Stephanie Culnane, 2001

1.  A 63.0 kg astronaut is on a spacewalk when the tether line to the shuttle breaks.  The astronaut is able to throw a 10.0 kg oxygen tank in a direction away from the shuttle with a speed of 12 m/s, propelling the astronaut back to the shuttle.  Assuming that the astronaut starts from rest, find the final speed of the astronaut after throwing the tank.  

        Here's what you know, astronaut's m = 63.0 kg, tanks  = 10.0 kg, tank's v = 12.0 m/s.  Use the formula mv = mv.  Plug in (10.0 kg)

        (12 m/s)   = (63.0 kg)(v).  astronaut's v = 1.9 m/s.

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2.  An 85.0 kg fisherman jumps from a dock into a 135.0 kg rowboat at rest on the west side of the dock. If the velocity of the fisherman is 4.30 m/s to the west as he leaves the dock, what is the final velocity of the fisherman and the boat?

Here's what you know, fisherman's m = 85.0 kg, boat's m = 135.0 kg, fisherman's v = 4.30 m/s.  Use the formula mv = mv.  Plug in (85.0 kg)(4.30 m/s) = (85.0 kg + 135.0 kg)(v).  v = 1.66 m/s.

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3.  Each croquet ball in a set has mass of 0.50 kg.  The green ball, traveling at 12.0 m/s, strikes the blue ball, which is at rest.  Assuming that the balls slide on a frictionless surface and all collisions are head-on, fine the final speed of the blue ball in each of the following situations:

            a.  The green ball stops moving after it strikes the blue ball.

              b.  The green ball continues moving after the collision at 2.4 m/s in the same direction.

              c.  The green ball continues moving after the collision at 0.3 m/s in the same direction. 

            Here's what you know m = 0.50 kg, green ball's v = 12.0 m/s.

            a.  Use the formula mv = mv.  Plug in (0.50 kg)(12.0 m/s) = (0.50 kg)(v).  v = 12.0 m/s.

            b.  Use the formula mv = mv + mv.  Plug in (0.50 kg)(12.0 m/s) = (0.50 kg)(2.4 m/v) + (0.50 kg)(v).  v = 9.6 m/s.

            c.  Use the formula mv = mv + mv.  Plug in (0.50 kg)(12.0 m/s) = (0.50 kg)(0.3 m/s) + (0.50 kg)(v).  v = 11.7 m/s

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4.  A boy on a 2.0 kg skateboard initially at rest tosses an 8.0 kg jug of water in the forward direction.  If the jug has a speed of 3.0 m/s relative to the ground and the boy and skateboard move in the opposite direction at 0.60 m/s, find the boy's mass.


The total momentum before and after is zero, so if the jug of water has a momentum of (p = mv) (8.0 kg)(3.0 m/s) = 24 kgm/s forward, which means the skateboard and boy must have gained an equal but opposite momentum backward.

So if p = mv, and p = 24 kgm/s backwards, and v = .60 m/s:

24 kgm/s = m(.60 m/s) 

then the boy and his board must have a total mass of 40 kg, which means the boy has a mass of 38 kg, if the board has a mass of 2 kg.

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