114 - Kinematics Questions Answers
A ball is thrown up with velocity 20m/s relative to ground in open lift moving up with speed 10m/s at time of throwing and has an acceleration of 2 m/s upward, ball meets lift after
relative to lift
u of the ball = 20-10 = 10
a = 10-(-2) = 12 downward
and s = 0 because ball returns to initial point
now use s = ut + 1/2 a t2
Velocity of a particle is given by v= 3-t m/s. The distance travelled by particle in 1st four second is
∫04 |v|dt = ∫03 (3-t)dt + ∫34(t-3)dt
now solve
A bus is begins to move with an acceleration of 1 m/s2. A boy who is 48 m behind the bus starts running at 10 m/s towards the bus. After what time bus will cross the boy?
according to the given condition
1/2 t2+ 48 = 10t
solutions of this eq. = 12, 8
so boy will cross the bus after 8 sec and then bus will cross the boy after 12 sec from start.
The acceleration of a particle, varies with time according to the relation a=-mw2sinwt.The displacement of this particle at a time t will be
1) m sin(wt)
2) mwcos(wt)
3) mwsin(wt)
4) -1/2(mw2sinwt)t2
a = dv/dt = -mw2sinwt
so ∫dv = ∫-mw2sinwtdt
so v = mwcoswt + C
at t=0, v=0 so 0 = mw+C or C = -mw
so v = -mw(1-coswt)
similarly put v = ds/dt and integrate again
A motor boat going down stream ,overtakes a floating object in water .One hour later ,the motor boat turned back .If speed of motor boat in still water is constant and flow velocity is also assumed constant,motor boat will again pass the floating object after time 't ' given by
1)Greater than 2 hrs
2)Equal to 2 hrs
3)less than 2 hrs
4)less than 1 hr
see video in you tube under the name iitbrain
video i e irodov 1.1
time will be 1 hour
A paper weight is dropped from the roof of a 35 storey building each storey being 3m high. It passes the ceiling of the 20th storey at x m/s [g=10 m/s2]. Find x.
total height = (35-20+1)*3 = 48 m
now solve
A lift with open top is moving up with acceleration f. A body is thrown up with speed u relative to lift and it comes back to lift in time t. Then speed u is ...
{g = acceleration due to gravity}
relative acc. = g+f
and relative velocity = u
now solve
A projectile is thrown with speed 40 ms-1 at angle θ from horizontal. It is found that projectile as at same height at 1s and 3s. What is the angle of projection?
It is found that projectile as at same height at 1s and 3s, so half time of flight = 2 sec
now solve
A particle is thrown with a velocity of u m/s. It passes A and B at time t1 = 1s and t2 = 3s. The value of u is (g=10 m/s2).
question is not complete
When a force F acts on a particle of mass m, the acceleration of particle becomes a. Now if two forces of magnitude 3F and 4F acts on the particle (both are at an angle of 90° from mass m) simultaneously , then the acceleration of the particle is ..............
resultant force = 5F
so acceleration = 5a