HW7: Momentum
Due: 11:59pm on Wednesday, April 11, 2018 *EXT*
You will receive no credit for items you complete after the assignment is due. Grading Policy
Momentum in an Explosion
A giant "egg" explodes as part of a fireworks display. The egg is at rest before the explosion, and after the explosion, it
breaks into two pieces, with piece B moving in the positive x direction. The masses of both pieces are indicated in , shown
traveling in opposite directions.
Part A
What is the magnitude of the momentum |pA,i | of piece A before the explosion?
Express your answer numerically in kilogram meters per second.
Hint 1. Initial momentum
The momentum of any object is determined by the product of the object's mass and velocity. The egg is initially at
rest. Use this to find the initial momentum.
ANSWER:
|p A,i | = 0 kg ⋅ m/s
Correct
Similarly, piece B has zero momentum before the collision. The total momentum of the "egg," the sum of the
two individual momenta, is also zero.
Part B
During the explosion, is the magnitude of the force of piece A on piece B greater than, less than, or equal to the
magnitude of the force of piece B on piece A?
, Hint 1. Forces in an explosion
The forces specified in this problem must obey Newton's third law, which states that every action has an equal-
magnitude and oppositely-directed reaction.
ANSWER:
greater than
less than
equal to
cannot be determined
Correct
Part C
The component of the momentum of piece B, p Bx,f , is measured to be +500 kg ⋅ m/s after the explosion. Find the
component of the momentum p Ax,f of piece A after the explosion.
Enter your answer numerically in kilogram meters per second.
Hint 1. Conservation of momentum
The law of conservation of momentum states that the total momentum in an isolated system of objects must
remain constant, regardless of the interactions (or collisions) between the objects. Thus, the total momentum of
the two pieces of the egg after the explosion must be equal to the total momentum of the two pieces of the egg
before the explosion.
ANSWER:
p Ax,f = -500 kg ⋅ m/s
Correct
Problem 8.05
The speed of the fastest-pitched baseball was 42.0 m/s , and the ball's mass was 145 g .
Part A
What was the magnitude of the momentum of this ball?
, ANSWER:
p = 6.09 kg ⋅ m/s
Correct
Part B
How many joules of kinetic energy did this ball have?
ANSWER:
K = 128 J
Correct
Part C
How fast would a 50.0 gram ball have to travel to have the same amount of kinetic energy?
ANSWER:
v = 71.5 m/s
Correct
Part D
How fast would a 50.0 gram ball have to travel to have the same amount of momentum?
ANSWER:
v = 122 m/s
Correct
Problem 8.09: Recoil Speed of the Earth
In principle, any time someone jumps up, the earth moves in the opposite direction.
Part A
To see why we are unaware of this motion, calculate the recoil speed of the earth when a 81.0 kg person jumps upward
at a speed of 2.10 m/s . Consult Appendix E in the textbook as needed.
Due: 11:59pm on Wednesday, April 11, 2018 *EXT*
You will receive no credit for items you complete after the assignment is due. Grading Policy
Momentum in an Explosion
A giant "egg" explodes as part of a fireworks display. The egg is at rest before the explosion, and after the explosion, it
breaks into two pieces, with piece B moving in the positive x direction. The masses of both pieces are indicated in , shown
traveling in opposite directions.
Part A
What is the magnitude of the momentum |pA,i | of piece A before the explosion?
Express your answer numerically in kilogram meters per second.
Hint 1. Initial momentum
The momentum of any object is determined by the product of the object's mass and velocity. The egg is initially at
rest. Use this to find the initial momentum.
ANSWER:
|p A,i | = 0 kg ⋅ m/s
Correct
Similarly, piece B has zero momentum before the collision. The total momentum of the "egg," the sum of the
two individual momenta, is also zero.
Part B
During the explosion, is the magnitude of the force of piece A on piece B greater than, less than, or equal to the
magnitude of the force of piece B on piece A?
, Hint 1. Forces in an explosion
The forces specified in this problem must obey Newton's third law, which states that every action has an equal-
magnitude and oppositely-directed reaction.
ANSWER:
greater than
less than
equal to
cannot be determined
Correct
Part C
The component of the momentum of piece B, p Bx,f , is measured to be +500 kg ⋅ m/s after the explosion. Find the
component of the momentum p Ax,f of piece A after the explosion.
Enter your answer numerically in kilogram meters per second.
Hint 1. Conservation of momentum
The law of conservation of momentum states that the total momentum in an isolated system of objects must
remain constant, regardless of the interactions (or collisions) between the objects. Thus, the total momentum of
the two pieces of the egg after the explosion must be equal to the total momentum of the two pieces of the egg
before the explosion.
ANSWER:
p Ax,f = -500 kg ⋅ m/s
Correct
Problem 8.05
The speed of the fastest-pitched baseball was 42.0 m/s , and the ball's mass was 145 g .
Part A
What was the magnitude of the momentum of this ball?
, ANSWER:
p = 6.09 kg ⋅ m/s
Correct
Part B
How many joules of kinetic energy did this ball have?
ANSWER:
K = 128 J
Correct
Part C
How fast would a 50.0 gram ball have to travel to have the same amount of kinetic energy?
ANSWER:
v = 71.5 m/s
Correct
Part D
How fast would a 50.0 gram ball have to travel to have the same amount of momentum?
ANSWER:
v = 122 m/s
Correct
Problem 8.09: Recoil Speed of the Earth
In principle, any time someone jumps up, the earth moves in the opposite direction.
Part A
To see why we are unaware of this motion, calculate the recoil speed of the earth when a 81.0 kg person jumps upward
at a speed of 2.10 m/s . Consult Appendix E in the textbook as needed.
