Class 9 Science

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 × 10^{8} 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.

**Answer:** Here we have,

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?

**Answer:**

(a) The slope of the distance-time graph for an object in uniform motion is straight line.

(b) The slope of the distance-time graph for an object in non-uniform motion is not a straight line.

Question 13: What can you say about the motion of an object whose distance-time graph is a straight line parallel to the time axis?

**Answer:** When the slope of distance-time graph is a straight line parallel to time axis, the object is moving with uniform motion.

Question 14: What can you say about the motion of an object if its speed-time graph is a straight line parallel to the time axis?

**Answer:** When the slope of a speed time graph is a straight line parallel to the time axis, the object is moving with uniform speed.

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