Physics Motion (20pts)

2011-02-21 12:12 am

★Taking g=10 ms-2 and ignore air resistance.

1)A gun is fired straight up from the ground.The bullet takes 30s to return to the ground.What is the speed of the bullet when it is fired?

2) How long does it take for a diver to drop (without spinning) from a 10-m platform? What is his speed when he enters the water?

3) A puppy jumps up and returns to the ground at a speed of 4ms-1.Estimate the highest position reached by the puppy.

請列明 t=? s=? u=? v=? a=?
用那一條公式和為什麼用這條公式。

回答 (1)

天同

2011-02-21 1:08 am

✔ 最佳答案

1. Given : t = 30 s, s = 0 m, a = -g(=-10 m/s2), u = ?
Use equation of motion: s = ut + (1/2)at^2hence, 0 = 30u + (1/2).(-10).(30)^2solve for u gives u = 150 m/s

2. Given: u = 0 m/s, a = -10 m/s2, s = -10 m, t =?
Use equation: s = ut + (1/2)at^2
-10 = (1/2).(-10).t^2
solve for t gives t = 1.414 s

Given: u = 0 m/s, a = -10 m/s2, t = 1.414 s, v =?
Use equation: v = u + atv = (-10).(1.414) m/s = -14.14 m/s (the -ve sign indicates the velocity is downward)
3. Consider the fall from the highest point,
given: v = -4 m/s, u = 0 m/s, a = -10 m/s2, s = ?
use equation: v^2 = u^2 + 2as
(-4)^2 = 2.(-10).s
solve for s gives s = 0.8 m
After listing out all the given quantities including the unknown, just select which equation involves all these quantities.

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