Question
Please help! The answers are below. I just need know how to work these problems!
[i] When a ball is dropped out of a window it hits the ground at a speed of 12 m/s. If the ball were thrown horizontally out of the same window at a speed of 5 m/s, then what would be its speed when it hits the ground?
(a) 0 m/s (b) 5 m/s (c) 7 m/s (d) 12 m/s (e) 13 m/s (f) 17 m/s (g) none of the above
Answer is (d) 12m/s
[ii] A 120 lb man stands in an elevator that moves upward and has an downward acceleration of g/4. What is the normal force of the floor on the man?
(a) 30 lb (b) 90 lb (c) 120 lb (d) 150 lb (e) 480 lb (f) none of the above
answer is (b) 90 lb
Explanation / Answer
When a ball is dropped out of window it hits the ground
v = 12 m/s that is the ball have vertical component of velocity
vy = 12 m/s Now when the same ball thrown horizontally with speed
vx = 5 m/s. so the ball initial vertical component of velocity is zero and then it is just like a freely falling body. Therefore the sped when it hits the ground is 12 m/s (b) Weight of the man w = 120 lb = 54.43 kg when the elevator moves upwards the normal reaction is W = m(g+a) and when the elevator is moving downwards, the normal reaction is W = m( g-a) W = (54.43 kg) [ 9.8 m/s2 - 9.8 m/s2/4] = (54.43) (9.8) (3/4) = 400 N or 90 lb (b) Weight of the man w = 120 lb = 54.43 kg when the elevator moves upwards the normal reaction is W = m(g+a) and when the elevator is moving downwards, the normal reaction is W = m( g-a) W = (54.43 kg) [ 9.8 m/s2 - 9.8 m/s2/4] = (54.43) (9.8) (3/4) = 400 N or 90 lb