1. The speed of a bullet is measured to be 640 m/s as the bullet emerges from a

Question

1. The speed of a bullet is measured to be 640 m/s as the bullet emerges from a barrel of length 1.20 m Assuming the initial speed of the bullet is zero and a constant that the bullet spends in the barrel after it is fired. positive acceleration in the barrel, find the time 2. (Discussion) Please explain why we assume the initial speed of the bullet is zero together with a constant positive acceleration in the barrel in Pre-lab exercise problem 1 above. 3. (Discussion) Can we solve the Pre-lab exercise problem 1 if we assume a constant negative acceleration in the barrel (without assuming initial speed of the bullet to be zero)? 4. Suppose a rocket ship in deep space moves with constant acceleration equal to 9.8 m/s, which gives the illusion of normal gravity during the flight. (a) If it starts from rest, how long will it take to acquire a speed one-tenth that of light, which travels at 3.0 x10 m/s? (b) How far will the ship travel in reaching this speed?

Explanation / Answer

1)Given,

u = 0 m/s ; v = 640 m/s ; d = 1.2 m

let t be the required time

we know from eqn of motion

v^2 = u^2 + 2aS

a = (v^2 - u^2)/2S = 640^2 - 0^2/2 x 1.2 = 170666.67 m/s^2

v = u + at => t = v/a

t = 640/170666.67 = 0.00375 s

Hence, t = 0.00375 s = 3.75 milli-sec

2)It is obvious to assume that the intial speed is zero since the bullet was in rest before firing, the moment it gets fired within a fraction of second it leaves the barrel and as it has high value of acceleration.

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