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標題:

Force and projectile motion

發問:

每題我都自己計左答案，如果唔岩可唔可以列step比我？1) calculate the average force needed to accelerate a 6.32g bullet from rest to 170m/s over a distance of 0.82m along the barrel of a rifle?我先搵左個a再搵F, 答案111N, 岩唔岩？2) A force of 75.2N is required to start a 15.2kg box moving across a horizontal concrete (that is has friction)... 顯示更多 每題我都自己計左答案，如果唔岩可唔可以列step比我？ 1) calculate the average force needed to accelerate a 6.32g bullet from rest to 170m/s over a distance of 0.82m along the barrel of a rifle? 我先搵左個a再搵F, 答案111N, 岩唔岩？ 2) A force of 75.2N is required to start a 15.2kg box moving across a horizontal concrete (that is has friction) floor. Calculate the coefficient of static friction between the box and the floor. Also, if the 80N force continues, the box accelerates at 0.9m/s^2, what is the coefficient of kinetic friction? 我搵到的答案是Us=0.505, Uk=0.492, 岩唔岩？ 3) A ball is thrown off the top of a 176m tall building with an initial speed of 23.2m/s at an angle of 40 with the horizontal. Calculate the angle the ball makes with horizontal (the flat ground) at the moment of impact. Vy=-60.6m/s, Vy0=14.9m/s, Vx0=17.8m/s, h=187.3, t=6.18s Angle=73.4, 岩唔岩？

最佳解答:

1. Use: work-done = increase of kinetic energy of bullet Your answer is correct. 2. Your calculation of Us is correct. For Uk calculation, 80 - Ff = 15.2 x 0.9 where Ff is the frictional force Hence, Ff = 66.32 N Uk = 66.32/15.2g where g is the acceleration due to gravity (= 9.81 m/s^2) i.e. Uk = 0.44 3. Your answer on the angle of impact is correct.

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