assign_13_query

course PHY 121

eYƻ|assignment #013

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Physics I

06-24-2006

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18:45:40

prin phy and gen phy problem 4.02 net force 265 N on bike and rider accelerates at 2.30 m/s^2, mass of bike and rider

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RESPONSE -->

m = f/a

265N/2.30 = 115.22m

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18:45:53

A force Fnet acting on mass m results in acceleration a, where a = Fnet / m. We are given Fnet and a, so we can solve the equation to find m.

Multiplying both sides by m we get

a * m = Fnet / m * m so

a * m = Fnet. Dividing both sides of this equation by a we have

m = Fnet / a = 265 N / (2.30 m/s^2) = 115 (kg m/s^2) / (m/s^2) = 115 kg.

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18:48:39

prin phy and gen phy problem 4.07 force to accelerate 7 g pellet to 125 m/s in .7 m barrel

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RESPONSE -->

m = 7g

a = 125m/s^2

125 m/s is not 125 m/s^2; the given quantity is the final velocity, not the acceleration.

ds = .7m

7 * 125 = 875

You have the right idea, to multiply force mass and acceleration to get force.

It is true that 7 kg * 125 m/s^2 = 875 kg m/s^2 = 875 Newtons, but the mass is 7 grams rather than 7 kg, and the 125 m/s is not an acceleration.

You need to find the acceleration from the given information, then use the correct unit calculations when you find the force.

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18:49:28

** The initial velocity of the bullet is zero and the final velocity is 175 m/s. If we assume uniform acceleration (not necessarily the case but not a bad first approximation) the average velocity is (0 + 125 m/s) / 2 = 62.5 m/s and the time required for the trip down the barrel is .7 m / (62.5 m/s) = .011 sec, approx..

Acceleration is therefore rate of velocity change = `dv / `dt = (125 m/s - 0 m/s) / (.11 sec) = 11000 m/s^2, approx..

The force on the bullet is therefore F = m a = .007 kg * 11000 m/s^2 = 77 N approx. **

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RESPONSE -->

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See my note(s) and let me know if you have questions.

Keep working on units.