During the time period that the engine is on, the rocket is accelerating upward (force from rocket is greater than force from gravity). Using a=F/m, we get the acceleration to be (60-2*10)/2=20 m/s^2. It's accelerating for 2 seconds, so we can use kinematics to figure out how far it's traveled so far. x(t)=Vi*t+1/2at^2. Vi=0, a=20, and t=2. Subbing in those values, we obtain x=40. Once the engine shuts off, the rocket is in free fall, so its acceleration will be -10 m/s^2. The velocity of the rocket once the engine shuts off can be found using vf=vi+at. vi=0, so vf=40. Using this velocity, we can find the additional height the rocket attains using vf^2=vi^2+2ad. vi=40 vf=0 (peak of ascent) a=-10. Solving, we get d=80. The total height the rocket attains is therefore 120 or choice D
Charles W.
A.0.1
B.0.2
C.0.4
D.0.8
E.1
2.A straight wire of 120 cm carries directional current of 0.25A.If the space is affected by magnetic field of 0.5T that perpendicular to the current,thus the magnitude of Lorentz force of the wire is...
A.0.15N
B.0.30N
C.0.60N
D.30N
E.60N
3.Magnetic flux produced by magnetic filed B that through perpendicularly a surface of width A is P.If the magnetic field is reduced into B/2,but the width surface enlarged to 2A,then flux produced is...
A.P/4
B.P/2
C.P
D.2P
E.4P
4.A rock falls form the edge of a cliff height h from the abyss.At the same time another rock is tossed up vertically with initial velocity v.Both rocks will be in the same height after a time interval t.The value of t is...
A.h/v
B.h/2v
C.2v/h
D.3v/h
E.2hv
5.A car moves with velocity v and it decreases to 1/3 after a distance x because the car's braked.The distance taken from the start until the car stop is...x
A.8/9
B.9/8
C.8/10
D.9/10
E.3
6.Two electric bulbs of 40W,60V are serially installed,then connected to the source of 60 volt that are able to apply electric current of 0.5A.The magnitude of effective power of the bulbs is...Watt
A.20
B.22.5
C.25
D.30
E.40
06/10/15