Understanding Momentum and Impulse

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AP Physics 1 › Understanding Momentum and Impulse

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Two astronauts, Ann and Bob, conduct a collision experiment in a weightless, frictionless environment. Initially Ann moves to the right with a momentum of $p_{Ai}=30 \frac{kg*m}{s}$ , and Bob is initially at rest. In the collision, the two astronauts push on each other so that Ann's final momentum is $p_{Af}=-5\frac{kg*m}{s}$ to the left. What is Bob's final momentum?

$35\frac{kg*m}{s}$ to the right

$25\frac{kg*m}{s}$ to the right

$5\frac{kg*m}{s}$ to the right

$5\frac{kg*m}{s}$ to the left

$25\frac{kg*m}{s}$ to the left

Explanation

Apply conservation of momentum before and after the collision.

$p_{Ai}+p_{Bi}=p_{Af}+p_{Bf}$ .

Taking left to be the negative direction, and noting that Bob's initial momentum is 0 since he is at rest, we can use the provided information to see that $30 \frac{kg*m}{s} = -5 \frac{kg*m}{s} + p_{Bf}$ .

Solving for $p_{Bf}$ , we get $p_{Bf}=35 \frac{kg*m}{s}$ . Since this answer is positive, Bob's momentum is in the positive direction (to the right).

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