9th physics notes: Chapter 3 (Dynamics)

Chapter 3: Dynamics

• Q1.            Why does dust fly off when a hanging carpet is beaten with a stick?

Statement

When a hanging carpet is beaten with a stick, the dust particles come out.

Main idea

                  Inertia of rest.

Reason

This is because on beating the carpet comes in motion but the dust particles try to remain in rest due to inertia of rest and hence come out of carpet.

• Q2.            If your hands are wet and no towel is handy, you can remove some of the excess of water by shaking them. Why does this work?

Statement

     If our hands are wet and no towel is handy, we can remove some of the excess of water by shaking them.

Main idea

                  Inertia of rest.

Reason

This is because shaking brings the hands in motion but the water drops try to remain in rest due to inertia of rest and hence removed from our hands.  

• Q3.            Why a balloon filled with air move forward, when its air is released?

Statement

A balloon filled with air move forward, when its air is released

Main idea

                   Law of conservation of momentum.

Reason

When the air is released from a balloon then from law of conservation of momentum, the momentum of the escape air in backward direction is equal to the momentum of the balloon in forward direction. Therefore the balloon moves forward.

• Q4.            Why does a hose pipe tend to move, backward when the fireman directs a powerful stream of water towards fire?

Statement

A hose pipe tend to move, backward when the fireman directs a powerful stream of water towards fire

Main idea

                  Action and reaction.

Reason

When the water from the hose pipe rushing in the forward direction, its action is in the forward direction. From Newton’s third law of motion, the reaction of pipe is in backward direction due to which it tends to move backward direction.

• Q5.            Your car stuck in wet mud. Some students on their way to class see your predicament by sitting on the trunk of your car to increase its traction. Why does this help?

Statement

            Our car stuck in wet mud and some students sitting on the trunk of our car to increase its friction.

Main idea

                  Force of friction.

Conclusion

            When the car stuck in wet mud the friction between tyres and mud decreases due to which car cannot move. So the students sit on the trunk of car to increase the normal force (F_N). Then the friction also increases according to equation

f_k = µ_k F_N            or                  f_k α F_N

• Q6.            How does friction help you walk? Is it kinetic friction or static friction?

Statement

Friction help us walk.

Main idea

                  Force of friction.

Reason

            The friction force acting on a body when it is at rest is called static friction while the friction force acting on a body when it is in a state of motion is called kinetic friction. So when we walk on ground our feet are at rest for a moment which provide the static friction between our feet and ground. Hence we walk due to static friction and not kinetic friction.

• Q7.            The parking brake on a car causes the rear wheels to lock up. What would be the likely consequence of applying the parking brake in a car that is in rapid motion?

Statement

            The parking brake on a car causes the rear wheels to lock up.

Main idea

                  Inertia of motion.

Conclusion

            The parking brake on a car causes the rear wheels to lock up while the front wheels are in motion and want to be in motion due to inertia. Hence the car will skid in such situation.

• Q8.            Why is the surface of a conveyor belt made rough?

Statement

The surface of a conveyor belt is made rough.

Main idea

                   Force of friction.

Reason

Because to increase its force of friction. Greater force of friction reduces the motion of the objects on conveyor belt tied together.

• Q9.            Why does a boatman tie his boat to a pillar before allowing the passenger to step on the river bank?

Statement

A boatman tie his boat to a pillar before allowing the passenger to step on the river bank.

Main idea

                  Action and reaction.

Reason

Because when the passenger from the boat move towards the river bank, they pushes the boat in backward direction with his feet which is action. From Newton’s third law of motion the reaction of the boat is in opposite direction. However, when the boat is tied with pillar it cannot move and hence the passenger move out from boat easily.

• Q10.            In uniform circular motion, is the velocity constant? Is the acceleration constant? Explain.

Statement

            In uniform circular motion velocity is constant or the acceleration is constant.

Main idea

                 In uniform circular motion velocity is not constant while the acceleration is constant. 

Conclusion

            In uniform circular motion the velocity does not remain constant because the direction of velocity changes and is tangent to circle at each point. While the direction of acceleration is always directed towards center of circle and remain unchanged. Hence the centripetal acceleration remain constant during circular motion.

• Q11.            You tie a brick to the end of a rope and whirl the brick around you in a horizontal circle. Describe the path of the brick after you suddenly let go of the rope.

Statement

      The path of the brick after you suddenly let go of the rope.

Main idea

                   Centripetal force and centrifugal force.

Conclusion

        When we tie a brick to the end of a rope and whirl the brick around us in a horizontal circle, then we provide centripetal force to brick but the brick apply centrifugal force on our hand. After we suddenly let go the rope, the brick move away along tangent to the circle due to centrifugal force.

• Q12.            Why is the posted speed for a turn lower than the speed limit on most highways?

Statement

                  The posted speed for a turn is lower than the speed limit on most highways.

Main idea

                   Centripetal force

Reason

              Because during motion on most highways due to speed the friction is very low and during slow motion the friction increases. That is why for a turn speed is lower than the speed limit on most highways.