Aga Khan Higher Secondary School Gahkuch, chapter 3, test solution

 

Aga Khan Higher Secondary School Gahkuch

Class 9^th                                                                                                                                                   Class test

Chapter 3 (Dynamics)                                 Instructor: Sadiq Saleem                                   Dated: 17-Oct-2022

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Q1. Why a gun recoils when fired?                                                                                                 (1 marks)

Answer: The gun recoils after fire to conserve the momentum of the system. Because the initial momentum of the gun is zero before firing the bullet. After firing, the bullet of mass ‘m’ moves forward with some speed ‘v’ then the gun of mass ‘M’ also moves backward with some speed v’ so that the product of mv and Mv’ remains same in opposite directions and the total momentum before and after firing the gun remains same.

                                                              Mv’ = mv                      

 

Q2. A speed time graph for a car traveling on a straight road is shown below                      (3 marks)

a)      What is the distance traveled during the first 15 seconds?

Answer: we have

                               Speed = distance / time

         So

                              Distance = speed x time

                              Distance:   20m/s x 15 s

                              Distance=   300 m

b)      What is the deceleration during the last 10 seconds?

Answer:

                       Acceleration = velocity / time

                           a = v / t

                          a = 0 – 35  /  10

                          a= -3.5 m/s2

 

c)       What is the average speed of whole journey?

Answer:

   average speed = total distance / total time taken

                        V(avg)       =   300 + 400 + 400 / 55

                         V(avg)     = 20m/s

Q3. A paratrooper jumps from a helicopter, he does not open his parachute straight away. 

The table shows his speed during the 10 seconds after his jump.                                            (3 marks)

a)      After how many seconds does he open his parachute?

Answer:

                As the speed is decreased at 6^th second so he opened his parachute after 6 seconds.

 

b)      Name the force that pulls the paratrooper down.

Answer: 

              The weight of the paratrooper pulls him towards the Earth.

 

c)       Name the force that acts in upward direction.

Answer:

             The drag force of the air acts in upward direction according to Newton’s 3^rd law of motion.

Q4. How much centripetal force is needed to make a body of mass 0.5 kg to move in a circle of radius 50 cm with a speed 3 m/s?                                                                                                                     (3 marks)

Answer:

           We know that

                             Fc = mv2 / r                                      :. 50cm = 0.5 m

           So

                           Fc = 0.5 x (3m/s)2  /  0.5m

                            Fc = 9 Newtons

Q5. Is friction always useful? Support your answer.                                                                      (2 marks)

Answer:

No, friction is not always useful. It takes more energy to slide an object over a surface, also it causes wear and tear of the machines thus decreasing the life expectancy of different machines and vehicles. But Friction is also useful for many other purposes, this the reason why friction is also called a necessary evil.

Q6. How Newton’s 2^nd law and momentum are mathematically related to each other?       (3 marks)

Answer:

           We know that

Where  

                               

so

                              F = m        

 Also

                            Momentum = P = mv

   So

                       

Hence

                          F = ∆P / ∆t

Or

                   Force = Rate of change of momentum

 

Best of Luck

Students of Class 9^th, comment your result below.                  

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