8) When a rocket ship accelerating in outer space runs out of fuel, it A) accele

Question

8) When a rocket ship accelerating in outer space runs out of fuel, it A) accelerates for a short time, slows down, and eventually stops. B) accelerates for a short time, then slows down to a constant velocity. C) no longer accelerates 9) Consider drops of water that leak at a steady rate from a dripping faucet. As the drops fall they 9 A) get closer together B) get farther apart C) remain at a relatively fixed distance from one another 10) An 80-km/h airplane flying against a 10 km/h head wind has a ground speed of C) 90 km/h ?)70kmh B) 80 km/h D) none of these 11) A ball is thrown upwards Neglecting air resistance, what initial upward speed does the ball need 1 to remain in the air for a total time of 10 seconds? A) about 100 m/s B) about 110 m/s C) about 60 m/s D) about 50 m/'s E) about 80 m/s 12) In each second of fall, the distance a freely falling object will fall is A) the same, but not 5 m or 10 m. B) about 10 m C) about 5 m D) increasing. E) none of these 13) If an object moves with constant acceleration, its velocity must A) change by varying amounts depending on its speed. B) be constant also. C) always decrease. D) change by the same amount each second. 14) At one instant an object in free fall is moving downward at 50 meters per second. One second later its speed should be about A) 50 m/s. B) 100 m/s. C) 25 m/s D) 60 m/s. E) 55 m/s. 15) An airplane that flies at 1 00 km/h in a 100-km/h crosswind has a ground speed of A) 141 kmh B) 0 km/h C) 200 km/h D) 100 km/h 16) If a car increases its velocity from zero to 60 km/h in 10 seconds, its acceleration is A) 600 km/h/s. B) 6 km/his C) 60 km/h/s D) 3 km/h/s E) 10 km/his.

Explanation / Answer

8)

Ans :- (c) No longer accelerates..

Explanation :- It runs out of fuel and keeps that speed at the moment it runs out. It does not accelerate for a short time because there is no force on the rocket. And it does not stop because there is no opposing force.

9)

Ans ;- (B) they farther apart

Explanation :- The drop descending earlier goes faster than the ones after it because it has been traveling a longer time under gravities acceleration of about 9.81 m/s^2 which is being added to it's velocity every second.

10)

Ans:- (A) :- 70 km/h

11)

Ans :- (D) about 50 km/h

Calculate instead the initial velocity needed to make the ball reach the peak (where velocity is zero) in half of 10 s.

Vi = ?, Vf = 0 m/s , a = g = -9.81 m/s^2

t = 10 sec

Vf = Vi + a*t

Vi = Vf - a*t

Vi = 0 m/s - (-9.81 m/s2) * 5 sec

Vi = about 50 m/s because 9.81 is about 10

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