Class 9 Science

# Motion

### Part 2

Question 8: During an experiment, a signal from a spaceship reached the ground station in five minutes. What was the distance of the spaceship from the ground station? The signal travels at the speed of light, that is, 3 × 108 ms-1.

Answer: Here, we have, speed =3xx10^8\ ms^(-1)

Time = 5 minute

=5xx60s=300 second

We know that, Distance = Speed x Time

⇒ Distance =300xx3xx10^8\ ms^(-1)

= 900 xx 10^8\ m

=9xx10^10m

Question 9: When will you say a body is in

(i) uniform acceleration?

Answer: When rate of change of motion is same in equal intervals of time.

(ii) non-uniform acceleration?

Answer: When rate of change of motion is not same in equal intervals of time.

Question 10: A bus decreases its speed from 80 km/h to 60 km/h in 5 s. Find the acceleration of the bus.

Answer: Here we have, u=80 km/h =80xx5/(18)=(200)/9 m/s

v=60 km/h =60xx5/(18)=(50)/3 m/s and t=5s

∴ Acceleration (a) =?

We know that v=u+at

Or, a=(v-u)/t

=((50)/3-(200)/9)/5=-(50)/(9xx5)

=-(10)/9 m s-2

Question 11: A train starting from a railway station and moving with uniform acceleration attains a speed of 40 km/h in 10 minutes. Find its acceleration.

Initial velocity, u = 0,
Final velocity, v = 40km/h =40xx5/(18)=(100)/9 m/s
Time (t) = 10 minute = 60 × 10 = 600s
Acceleration (a) =?

We know that v=u+at

Or, a=(v-u)/t

=(100)/(9xx600)=1/(54) m s-2

Question 12: What is the nature of the distance-time graphs for uniform and non-uniform motion of an object?