1.The ratio of S.I units to C.G.S units of retardation is
Ans.S.I unit is ms-²
CGS unit is cms-²
SI unit / CGS unit = ms-² / cms-²
= 100cms-² / cms-² (1m= 100cm)
=100/1 (both units gets cancelled)
= 10²/10⁰
= 10²/ 1
= 10²
2.When the displacement of an object travels is directly proportional to the length of time, it is said to travel with
Ans.Uniform velocity
3.A body has an acceleration of - 6ms-².what is its retardation?
Ans.6ms-²
4.If the velocity of a body does not change, its acceleration is
Ans. Zero.
5.A body whose speed in a particular direction is constant
Ans.Has a constant velocity
6.A car increases its speed from 20 km/h to 50 km/h in 10 s.its acceleration is
Ans.u=20kmh-¹
v=50 kmh-¹
t= 10s
a=?
a= (v- u )/t
a=(50kmh-¹ - 20kmh-¹)/10s
= 30 kmh-¹ / 10s
= 30×1kmh-¹ / 10s
= 30× 5/18 ms-¹ / 10 s
= 25/3 ms-¹ / 10s
= 8.3333 ms-¹ / 10s
= 0.83333 ms-²
None of these
7.A ball is thrown up with a certain velocity. It attain a height of 40m and comes back to the thrower. Then the
Ans.Total displacement is zero.
Initial and final position of the ball is same.so, displacement is zero.
8.A body strikes the floor vertically with a velocity u and rebounds at the same speed. The change in velocity would be
Ans.2u
Initial velocity=u
Final velocity=-u (direction of the body has been changed but speed remains same)
Change in velocity = initial velocity-final velocity
= u - (- u)
= u + u
= 2u
Change in velocity= 2u
9.In the equation of motion , x=at + 1/2 bt² the units of a & brespectively are
Ans.x=at + 1/2bt²
S=ut + 1/2 at²
On comparing above two equations
a represents velocity , unit is ms-¹
b represents acceleration, unit is ms-²
ms-¹,ms-²
10.A body covers the first half of the distance with a velocity v and the second half in double the time taken for the first half. The average velocity is
Ans. total displacement= x
First half , displacement=x/2
Initial velocity=v
t₁=(x/2)/v
=x/2v
Second half, displacement=x/2
Velocity for second half= v/2
t₂= (x/2)/(v/2)
= x/v
Average speed=total displacement/2
=(x/2 + x/2 )/ (t₁+t₂)
= (x/2 +x/2)/ (x/2v + x/v)
=x / (x+ 2x )/2v
= x/ (3x/2v)
= x ×( 2v/3x)
= 2v/3
2v/3
11.The distance travelled by a freely falling body is proportional to
Ans.The square of the time of fall
12.A trolley while going down an inclined plane has an acceleration of 2m/s².what will be its velocity 3 s after the start
Ans.a=2ms-²
u= 0 ms-¹ (starts)
t=3s
v= u+at
= 0 + 2ms-² × 3s
= 6ms-¹
6m/s
13.A body initially at rest ,moves with an acceleration of 4m/s². Find the distance travelled in the 5th second
Ans.u=0
a=4ms-²
t₁= 4s
Distance travelled in first four seconds,
s₁=ut + 1/2 at²
= 0 + 1/2 × 4 ms-² × 4s ×4s
= 32m
Distance travelled in first 5 seconds,
s₂= ut + 1/2 at²
= 0 + 1/2 × 4ms-² × 5s×5s
= 50m
Distance travelled in 5th second
= s₂ - s₁
= 50m - 32m
= 18m
14.A body has an average speed of 30 m/s. If its final speed is 40 m/s, did the body start from rest or motion? If it started from motion what was the initial speed?
Ans. Final velocity=40 ms-¹
Average velocity=30 ms-¹
Initial velocity=?
Average velocity=(initial velocity+final velocity)/2
2×Average velocity=initial velocity+final velocity
2×30ms-¹ =initial velocity+ 40ms-¹
60ms-¹ - 40ms-¹ =initial velocity
20ms-¹ =initial velocity
15.A railway train 200m long ,passes over a bridge 800m long.Find the time it takes to cross the bridge when it is moving with a uniform velocity of 36 km/h
Ans. Speed=36kmh-¹
==36×1km/h
= 36× 5/18 ms-¹
= 2×5ms-¹
= 10ms-¹
Length of train=200m
Length of bridge=800m
Total to be covered for crossing the bridge=800m + 200m
= =1000m
Speed=distance/time
Time= distance/speed
= 1000m/10 ms-¹
=100s
= 60s + 40s
= 1 minute 40 s (1minute= 60s)
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