posted by Jon on .
14)The cable of the elevator you are riding in breaks and the elevator begins to freefall at terminal velocity to the ground. Why would jumping up right before impact still not help you survive?
A)Because your overall inertia is down
B)Because you would be accelerating up
C)Because you are weightless
D)It would help you survive
I got C
15)If a horizontal force applied on a resting object is greater than the force of friction, the object will.
A)move at a constant velocity
B)accelerate in the direction of the applied force
D)stay at rest
I got B
14)da ans is a
anyone else agree?
In 14 the best answer is D, although in it won't work well.
By jumping up you reduce the velocity w.r.t. to the ground a bit, so when the bottom of the lift hits the ground and stops, you are moving toward the ground at a lower speed.
In 15 all options are wrong. You are supposed to answer B) but strictly speaking that is not right because the applied force does not need to point in the direction opposite to the friction force.
I agree with Iblis on 14. You can slightly reduce your velocity relative to the Earth at impact by jumping up just before impact. The effect might be helpful when falling one story, but not much more than that, or when the elevator is falling at terminal velocity (as here), which would be huge. They probably want you to answer A
In 15, Iblis is refering to the possiblity of friction with a directional bias, such as one might abtain when sliding on a grooved surface where the groves are inclined to the applied force. They want you to answer B, but is not always true.