Matric Notes Physics 9th Ch 2 Kinematics Long Questions

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Q.1) What is motion? Describe that motion is relative. How two observers in relative motion can have conflicting views about the same object? 

Answer: Motion: “A body is in a state of motion with respect to an observer if it changes its position with respect to that observer“. Motion is relative

For the same event, two observers can have different observations. For example, a body in a train is in motion with respect to an observer on the ground. Whereas the same object is at rest with respect to another observer in the train moving with the object. Thus the motion and rest are not absolute but relative, This means that we have to specify the observer while telling about the rest or motion of the body.

 As position needs a reference, therefore rest and motion also need the specification of an observer.

For example when a teacher changes her position in the classroom while students are sitting on their chairs. According to students observation, the teacher is in motion. Interestingly, teacher while moving also observes the students move as well.

Similarly, when Sara Leaves in train and her cousin John sees her off. As the train starts moving Sara see John moving to the right with the same speed as John see Sara moving to the left.

Q.2) Define scalar and vector quantities. Explain with examples the graphical representations of vector quantities.

Answer: Scalars quantities: In physics, scalar are those quantities, which are completely described by their magnitudes and with a proper unit are called scalars. For example, speed, volume, time, work, energy, power, and density etc.

Explanation: There is one characteristic is associated with scalar quantities, that is their magnitude. When comparing two scalar quantities of the same type, you have to compare only their magnitude.

Vectors quantities: In physics, vectors are those quantities, which are completely described by their magnitudes and with a proper unit and direction are called vectors. For example, velocity and acceleration.

Explanation: There are two characteristics of vector quantities, a magnitude, and a direction. When comparing two vector quantities of the same type, you have to compare both the magnitude and as well as their direction. When doing any mathematical operation on a vector quantity (like adding, subtracting, multiplying) you have to consider both the magnitude and the direction. This makes dealing with vector quantities a little more complicated than scalars.

Q.3) What is a position. Explain the difference between distance traveled, displacement, and displacement magnitude. 

Answer: Position: “A Position is the location of an object relative to some observer”.

Earth is usually taken as a reference, and we often describe the position of an object as it relates to stationary objects on earth.

Distance traveled: “The length of a path traveled between two positions is called distance”.

Distance has no direction and therefore it is a scalar quantity and its SI unit is a meter.

Displacement: “The shortest directed distance between two positions is called displacement”. Straight distance from one point to another is called displacement.

Displacement has direction, therefore, it is a vector quantity. Its SI unit is meter same as distance.

Q.4) Use velocity-time graph to prove the following equations of motion 

a) vf = vi + at           b) S = vi t + 1/2 at2        c) 2aS = vf2 – vi2

Answer: First equation of motion

First equation of motion gives the relation of final velocity ‘vf‘ in terms of initial velocity ‘vi‘ and acceleration ‘a‘ in time t.

From the graph it is clear that

                           DB = DC + CB                    ………..    (i)

In figure:

Line segment DB represents, final velocity vf                  DB = vf

Line segment DC represents, initial velocity vi                DC = vi

Line segment CB in terms of the slope givesCB = at

Putting these value in equation (i) from the graph we get

                                                         vf = vi + at                              ………..    (A)                         

Second equation of motion

Second equation of motion relates displacement ‘s‘ with the initial velocity ‘vi‘ and acceleration ‘a‘ in time ‘t’. As the area under the velocity-time curve represents the displacement ‘s‘ as shown in the figure below.

Q.5) What is free-fall, what is its value near the surface of the earth. Explain with examples that rock and sheet of paper will fall at the same rate without air resistance.

Answer: Everyone has observed the effect of gravity as it causes objects to fall downward. In the absence of air resistance, it is found that all bodies at the same location above the earth fall vertically with the same acceleration. Furthermore, if the distance of the fall is small compared to the radius of the earth, the acceleration can be considered constant throughout its fall. The motion, in which air resistance is neglected and the acceleration is nearly constant, is known as free-fall.

The acceleration of a freely falling body is called the acceleration due to gravity, and its magnitude (without any algebraic sign) is denoted by the symbol g. The acceleration due to gravity is directed downward, toward the center of the earth. Near the earth’s surface, g is approximately

                                    g = 9.80 m/s2             or      32.2 ft/s2

A well-known phenomenon of a rock falling faster than a sheet of paper in which under the effect of air resistance the rock will fall faster than the paper, the air resistance is responsible for the slow fall of paper. If we have a tube in which the air is removed then the rock and air have exactly the same acceleration due to gravity and in the absence of air, the rock and the paper both fall freely.