course Phy 201
022. `query 22
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Question: `qQuery gen phy 7.19 95 kg fullback 4 m/s east stopped in .75 s by tackler due west
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Your solution:
Supposing that East is the positive direction, original magnitude and direction of the momentum of the fullback is:
p = m * v1
= 115kg (4m/s)
= 380 kg m/s.
Because velocity is in the positive direction of x, the momentum is also in the positive x direction, which we had determined to be in the East.
Magnitude and direction of the impulse exerted on the fullback will be:
impulse = change in momentum or
impulse = pFinal - pInitial
= 0 kg m/s - 380 kg m/s
= -380 kg m/s.
We see that Impulse is negative so the direction is in the negative x direction, which we know will be West.
Impulse = Fave * `dt
Fave = impulse / `dt.
Therefore, Fave exerted on the fullback is:
Fave = 'dp / 'dt
= -380 kg m/s /(.75s)
= -506 N
This is in the negative direction of x, which will be West.
Force that will be exerted on the tackler is = and opposite to the force exerted on the fullback. The force on the tackler will be + 506 N.
The positive force is consistent with the fact that the tackler's momentum change is positive (meaning that it began in the west).
The impulse on the tackler will be to the East.
Confidence assessment rating:
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Given Solution:
`a** We'll take East to be the positive direction.
The original magnitude and direction of the momentum of the fullback is
p = m * v1 = 115kg (4m/s) = 380 kg m/s. Since velocity is in the positive x direction the momentum is in the positive x direction, i.e., East.
The magnitude and direction of the impulse exerted on the fullback will therefore be
impulse = change in momentum or
impulse = pFinal - pInitial = 0 kg m/s - 380 kg m/s = -380 kg m/s.
Impulse is negative so the direction is in the negative x direction, i.e., West.
Impulse = Fave * `dt so Fave = impulse / `dt. Thus the average force exerted on the fullback is
Fave = 'dp / 'dt = -380 kg m/s /(.75s) = -506 N
The direction is in the negative x direction, i.e., West.
The force exerted on the tackler is equal and opposite to the force exerted on the fullback. The force on the tackler is therefore + 506 N.
The positive force is consistent with the fact that the tackler's momentum change in positive (starts with negative, i.e., Westward, momentum and ends up with momentum 0).
The impulse on the tackler is to the East. **
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Self-critique (if necessary):
Self-critique Rating:
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