KPK (Mardan Board) Physics Class 9 Chapter 3 Dynamics Conceptual Questions
Q.1) Why does dust fly off, when a hanging carpet is beaten with a stick?
When a hanging carpet is beaten with a stick, it moves forward along with the dust particles but when the carpet moves back due to inertia, the dust continues its motion in the forward direction, therefore, dust flies off from the carpet.
Q.2) If your hands are wet and no towel is handy, you can remove some of the excess water by shaking them. why does this work?
When we shake the hands, the shaking brings the hands in motion but the water drops try to remain in rest due to the inertia of rest and are removed from the hands, in this way, we can remove some of the excess water.
Q.3) Why a balloon filled with air move forward, when its air is released?
Explanation using Newton’s 3rd Law of Motion:
When the air is released from a balloon, the balloon pushes the air in backward direction while the reaction force of the air pushes the balloon in the forward direction. As a result, balloon moves in the forward direction.
Explanation using Law of Conservation of Momentum:
Due to the law of conservation of momentum,
when the air is released, it rushes out in the backward direction; in order to the conserve the momentum, the balloon moves in the forward direction keeping the total momentum of the system constant.
Q.4) Why does a hosepipe tend to move backwards when the fireman directs a powerful stream of water towards fire?
When the water from the hosepipe rush in the forward direction, a reaction force in the backward direction is applied to the pipe by the forward moving water due to Newton’s 3rd law motion and hence the hosepipe tends to move in the backward direction.
Q.5) Your car is stuck in wet mud. some students on their way to class see your predicament and help out by sitting on the trunk of your car to increase its traction. why does this help?
When the car stuck in wet mud the friction between tires and mud decreases due to which car cannot move. So the students sit on the trunk of the car to increase the weight, as a result, normal force (FN) also increases. Then the friction force increases according to the equation
fk = µk FN or fk α FN
Q.6) How does friction help you walk? Is it kinetic friction or static friction?
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 the ground our feet are at rest for a moment which provides the static friction between our feet and ground. Hence we walk due to static friction and not kinetic friction.
Q.7) 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?
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.
Q.8) Why is the surface of a conveyor belt made rough?
The surface of a conveyor belt is made rough in order to increase its force of friction. As a result, the things remains safe from falling.
Q.9) Why does a boatman tie his boat to a pillar before allowing the passengers to step on the river bank?
When the passengers from the boat move out towards the riverbank, they push the boat in backward direction with their feet.
According to Newton’s third law of motion, the reaction of the boat is in opposite direction, hence the boat can move in the forward direction. In order to restrict the motion of the boat for the safety of passengers, boatman tie the boat with the pillar before allowing the passengers to step on the river bank.
Q.10) In a uniform circular motion. Is the velocity constant? Is the acceleration constant? Explain.
In a uniform circular motion, the body continuously changes its direction, therefore, its velocity is also changing at each point. The acceleration is always directed inwards, towards the center of the circle.
Q.11) 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.
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 and the brick applies centrifugal force on our hand. After we suddenly let go the rope, the brick will travel in a straight line, which show that the velocity vector is directed along tangent.
Q.12) Why is the posted speed for a turn lower than the speed limit on most highways?
The posted speed for a turn lower than the speed limit on most highways because on most of the highways if the banking of road is enough to provide centripetal force to the car to take the turn in circular track then the speed limit mentioned will be normal speed but in some roads, we have sharp curves that even banking would not provide the necessary centripetal force to the car therefore for such turns the posted speed limit mentioned will be low.
Physics Class 9 Chapter 3 Dynamics
Q.1) What is force? What are its units? Distinguish between contact and non-contact forces?
“A force is a kind of a push or a pull on an object”.
“Force is a physical quantity which moves or tends to move a body, stops or tends to stop a moving body”.
Unit of Force:
In SI the unit force is Newton ‘N’. One newton is defined as
“the force that produces an acceleration of one meter per second squared (a = 1m/s2) in a body of mass one kilogram (1 kg).
1 N = 1 kg × 1 m / s2 or N = kg m / s2
Contact and Non-Contact forces:
We can think of different ways in which we can move textbook. We can push or pull it, or we can tie a string around it and pull on the string. We often call these contact forces because the force is exerted when one object comes in contact with another object. As we are holding physics textbook right now, our hands are exerting a contact force on it. There are other ways in which we can change the motion of the textbook. It would accelerate as if it falls to the ground. This time the gravitational force of Earth (which is not a contact force) is acting on the book which is causing this acceleration. We call such forces as non- contact forces, field forces or action-at-a-distance forces.
Q.2) State Newton’s three laws of motion. Give one example for each.
First law of motion
“If the net (external) force acting on an object is zero, the object will maintain its state of rest or of uniform motion (constant velocity)”.
The study of first law of motion can be divided into two parts.
Bodies At Rest:
The first part of the law sates that a body at rest will remain at rest if no net force acts on it. This part of first law is easy to understand and goes with our common observation. For example a chair laying in a room will remain stationary and will not start moving or flying around by itself unless some one moves it by applying a net force.
Bodies In Motion:
The second part of the law states that a body in motion will continue to move in a straight in line with uniform speed if no net force acts on it. However our daily observation is against this. For example if we roll a ball it comes to rest after some time. But careful study of the moving ball shows that there are forces (like friction and air resistance) which oppose the motion of the ball.
This means that object would continue to move in a straight line for ever, with uniform speed if the forces opposing the motion of the object are removed.
Newton’s second law of motion
“A net force(unbalanced force) applied on the body produces an acceleration ‘a’ in the body. This acceleration is directly proportional to the magnitude of the net force and inversely proportional the as of the object”.