Question

Physics Assignment No.1
1. Harry moves from A to B at a speed of 40km/h and a same distance from B
to C at a speed of 60km/h and he returns along the same path from C to A
at a speed of 70km/h. Find the average speed in the entire journey.
2. A boy moves on a rectangular track ABCD of length 4m and breadth 3m. He
completes 5 and half rounds of the track. Calculate the distance and the
displacement.
3. A car starts from moving with a velocity of 108km/h decelerates to 72km/h
in 4s by applying brakes and again from that point it accelerates further to
90km/h in 2s. Calculate the acceleration in both the cases.

Answer 1

Explanation:

Raghav had to walk a certain distance. He walked half the distance at a speed of 4 km/hour and the remaining half at a speed of 5 km/hour. If the total time taken was 9 hour, the distance was

9 km

20 km

40 km

50 km

Answer (c).

Let half of the distance travelled be d.

Time taken at the speed of 4 km/hr = (d/4)

Time taken at the speed of 5 km/hr = (d/5)

Therefore, total time = (d/4) + (d/5) = 9 hrs.

⇒ (5d + 4d)/20 = 9

⇒ 9d = 180

⇒ d = 20.

Therefore total distance = d x 2 = 40 kms

2. A person rides a bicycle round a circular path of radius 50 m. The radius of the wheel of the bicycle is 50 cm. The cycle comes to the starting point for the first time in 1 hour. What is the number of revolutions of the wheel in 15 minutes?

20

25

30

35

Answer (b)

Circumference of the circular path = 2Π50 = 100Π m

This means 100Π m is the distance covered in 60 minutes

Therefore distance covered in 15 minutes = 1/4 x 100Π m = 25Π m

Now distance covered in one revolution of the wheel = 2Π(1/2) m = Π m

Therefore number of revolutions in 15 minutes = 25Π/Π = 25