query assignment 22

course phy201

Хdn~ҨҳXɷeassignment #022

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Physics I

10-30-2007

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12:06:03

Query gen phy 7.19 95 kg fullback 4 m/s east stopped in .75 s by tackler due west

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RESPONSE -->

Original momentum of fullback is m*v or 95Kg*4m/s=380Kg*m*s

Impulse is the change in momentum and since the fullback's momentum went from 380 Kg*m*s to zero, the dP is -380 Kg*m*s and this is the impulse exerted on the fullback

Since momentum is a conserved quantity, the impulse exerted on the tackler must be the inverse of the impluse exerted on the fullback or +380 Kg*m*s

The fullback accelerated at a rate of -5.3 m/s and Fnet=m/a so 95Kg/-5.3m/s = -17.9N Fnet

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12:29:13

** We'll take East to be the positive direction.

The origional 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 iimpulse on the tackler is to the East. **

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RESPONSE -->

I did this right up until the point of calculating Fnet. I first calculated the rate of acceleration of the fullback as -5.3 m/s^2 and then divided this into the 95 Kg mass and got 17.9 N.

What is the meaning of my answer? Why am I looking at this wrong?

Acceleration is in m/s^2, not m/s.

Fnet = m a, not m / a. The unit of work is the kg m/s^2, or Newtons. kg / (m/s^2) would be kg s^2 / m, which is not a named unit.

Otherwise your reasoning would work. 95 kg * 5.3 m/s^2 = 500 N approx., in agreement with the given solution.

However when dealing with impulse-momentum you don't want to get down to the level of acceleration. The result can be calculated directly by impulse-momentum without reference to acceleration.

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Your work looks good. See my notes. Let me know if you have any questions. &#