Newton Third Law

2012-03-06 4:27 pm

1. If you were out on an icy pond and it was so frictionless that you weren't even able to walk, you could get yourself back to the edge of the pond by throwing stuff in th oppsite direction that you wanted to go. Let's say a 50 kg woman throws a 10 kg bag of pototes east, accelerating them at 3m/s^2.

a. What is the woman's acceleration?

b.The time for the throw is 0.5s. How fast is she moving after the bag of patatoes leaves her hands?

c. If the end of the ice is 30m west, how long does it take her to get the ice?
(Careful here. She accelerating during the first part of her motion, but then moving at constant speed after that.?

2. During the Korean Conflict, the weapon used by most soldier was .30 caliber M1 Carbine. A typical carbine has a mass of 2.9 kg and shoots a bullet
with a mass of 7.1 grams. The length of the barrel is 60 cm and the bullet exits at 650m/s.

a. What is the acceleration of the bullet in the barrel?

b. How much force is exerted by the carbine on the shoulder of the soldier?

c. How ling does the "kick" last ?

回答 (1)

天同

2012-03-06 10:21 pm

✔ 最佳答案

1. (a) Force given to the bag of potatoes = 10x3 N = 30 N
Acceleration of woman = 30/50 m/s2 = 0.6 m/s2

(b) Final speed of woman = 0.6x0.5 m/s = 0.3 m/s

(c) Distance travelled during acceleration = (1/2) x 0.3 x 0.5 m = 0.075 m
Time to get to the edge of the pond = [(30-0.075)/0.3 + 0.5] s = 100.25 s

2.(a) Use equation: v^2 = u^2 + 2a.s
with v = 650 m/s, u = 0 m/s, s = 0.6 m, a =?
hence, 650^2 = 2a.(0.6)
a = 3.52x10^5 m/s2

(b) Force on bullet = (7.1/1000) x 3.52x10^5 N = 2499 N
Hence, force on shoulder of soldier = 2499 N

(c) Change of momentum of bullet during firing
= 0.0071 x 650 Kg.m/s = 4.615 kg.m/s
Duration of "kick" = 4.615/2499 s = 1.8x10^-3 s = 1.8 ms

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