IV. Answer the following questions.
11.Distinguish between speed and velocity. (2)
12.Why do we fall in the forward direction when a moving bus brakes to a stop?(2)
13.A train accelerates uniformly from 36km/h to 72km/h in 20s. Find the distance
travelled. (3)
14.A bullet of mass 30g is fired horizontally with a velocity of 120 m/s from a pistol
of mass 2kg, find the recoil velocity of pistol?
Answers
Answer:
•Speed, being a scalar quantity, is the rate at which an object covers distance. The average speed is the distance (a scalar quantity) per time ratio. ... On the other hand, velocity is a vector quantity; it is direction-aware. Velocity is the rate at which the position changes.
•Inertia in a moving bus when brake is applied
As the driver applies brakes, the bus comes to rest. But, the passenger tries to maintain the inertia of motion. ... Hence, the passenger tends to fall back when the bus accelerates forward.
ᴀɴꜱᴡᴇʀ
- Speed, being a scalar quantity, is the rate at which an object covers distance. The average speed is the distance (a scalar quantity) per time ratio. ... Velocity is the rate at which the position changes. The average velocity is the displacement or position change (a vector quantity) per time ratio.
- When a moving bus brakes to a stop, our feet come to rest with the bus. But upper part of our body continues to move forward on account of inertia of motion. That is why we tend to fall in the forward direction
- GIVEN
initial velocity - 36km /h.
final velocity - 72km /h
Time taken -20 s
TO FIND
Distance
Conversion :
Convert unit of velocity from km/hr to
m/s.
1. 36 km/hr = 36 × 5/18 = 10 m/s
2. 72 km/hr = 72 × 5/18 = 20 m/s
Calculation:
Acceleration be denoted as "a"
a = ( v - u ) /t
a = (20 - 10 ) /20
a = 0.5 m/s^2
s= ut +0.5at ^2
s = 10×20+0.5×0.5×20^2
s = 300 m
4.Let recoil velocity of gun be V
Mass of bullet(m) = 30 g
Velocity of bullet(v) = 120m/s
Momentum of bullet = m*v = 30*120 gm/s = 3600 m/s
Mass of gun(M) = 2 kg = 2000 g
According to law of conservation of momentum,
m*v = -M*V
=>V = (m*v)/-M
=>V = -3600/2000
=>V = -1.8 m/s