Mechanics, Motion & Rest, Distance & Displacement, Speed & Velocity

This is a general science article on mechanics. Here we have discussed concepts like statics, kinematics, dynamics, rest and motion, distance and displacement, speed, velocity and acceleration.

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What Is Mechanics

Mechanics is the branch of Physics which deals with the study of motion of material objects. In our daily life, we see many things moving around us. for example, we see birds flying across the sky, buses plying on roads, insects crawling on ground, animals running across fields. All these moving things are studies under Mechanics.

Mechanics can be broadly classified into following branches:-

1. Statics

It is that branch of mechanics which deals with the study of those objects which are at rest. An object can be at rest, even if the number of forces acting on it are in equillibrum. So we can say, that Statics is that branch of mechanics which deals with the study of motion of objects under the effect of such forces which are in equillibrum.

2. Kinematics

Kinematics has been derived from a Greek word, “Kinema“, which means motion. It is that branch of mechanics, which deals with the motion of the objects without taking into account the factors that are responsible for motion.

3. Dynamics

Dynamics has been derived from a Greek word, “Dynamis“, which means power. It is that branch of mechanics which deals with the study of motion of objects taking into account the factors responsible for causing motion in a body. Since the factor responsible for causing motion is Force, hence dynamics is based on the concept of force.

Objects In Motion

Rest: An object is said to be at rest if it does not change its position with time, with respect to its surroundings. A book kept on a table is at rest as it does not change its position with time with respect to its surroundings.

Motion: An object is said to be in motion if it changes its position with time, with respect to its surroundings. A bus moving on a road is in motion as it changes its position with time with respect to its surroundings.

Types of Motion In A Body

There are mainly three types of motion that a body can have. They are:

1 Rectilinear or Translatory motion.

Rectillinear Motion is that type of motion in which a particle or a point mass body is moving along a straight line.

Translatory Motion is that type of motion in which a body which is not a point mass body is moving such that all its constituent particles move simultanously along parallel lines and shift through equal distances in equal intervals of time.

2 Circular or Rotatory Motion.

Circular Motion is that motion in which a particle or a point mass body is moving along a circle.

Rotatory Motion is that type of motion in which a body, which is not a point mass body is moving such that all its constituent particles move simultanously along concentric circles, whose centres lie on a line called the axis of rotation and shift through equal angles in equal intervals of time.

3 Oscillatory or Vibratory Motion.

Oscillatory Motion is that type of motion in which a body moves to and fro or back and forth repeatedly about a fixed point in a definite interval of time.

Distance And Displacement

Distance

The length of the actual path travelled by an object during motion in a given interval of time is called distance travelled by that object.

If a body travels 3 km along north and then turns right and travels a distance of 4 km, then the distance travelled by him is equal to 5 km + 3 km = 8 km

Distance is a scalar quantity and its value can never be zero during the motion of a body. Its S.I unit is metre (m).

Displacement

The displacement of an object in a given time is defined as the shortest distance between its final position and initial position.

If a body travels along a straight line for 50 metres, then the distance travelled by it is equal to 50 m. The displacement covered by the body is also 50m. So, remeber that when a body travels along a straight line, its displacement is equal to the distance covered by it.

If a body, after travelling a distance of 50 metres comes back to its initial position, then its initial and final positions coincide. In this case the shortest distance between the initial and the final position is equal to zero. Therefore, displacement in this case is equal to zero. Remember, if a body returns to its initial position, the displacement covered by it is equal to zero.

Displacement is a vector quantity and its direction is always from initial position to the final position. If a body travels from point A to point B, then the direction of displacement is from A to B.

Displacement can be positive, zero or negative and its S.I Unit is metre (m).

Speed And Velocity

Speed

Speed of an object is defined as the time rate change of position of the object in any direction. It is measured by the distance travelled by the object in a unit time in any direction.

Mathematically, Speed = Distance covered/time taken

Speed is a scalar quantity an its S.I units are m/s

Uniform speed 

An object is said to be moving with a uniform speed if its covers equal distances in equal intervals of time. for example, if a body covers a distance of 2 metres in each second, it is said to have a uniform speed.

Variable Speed

An object is said to have a variable or non uniform speed, if it covers equal distances in unequal intervals of time, or unequal distnaces in equal intervals of time.

If a body covers a distance of 1m in first second, 3m in another second, 6m in another second, it is said to have a variable speed as it covers unequal distances in equal intervals of time.

Similalrly if a body covers a distance of 2m in one second, another 2m in 4 seconds, another 2m in 8 seconds, it is said to have a variable speed, as it covers equal distances in unequal intervals of time.

Instantaneous Speed

The speed of a body at any instant of time is known as its instantaneous speed. The speedometer of a car gives us the value of its instantaneous speed.

Velocity

Velocity of an object is defined as the time rate of change of displacement of the object. It is also defined as the speed of an object in a given direction.

Velocity = Speed + Direction

It is measured by the displacement covered by an object in a unit time.

Velocity = Displacement covered/time taken

Velocity is a Vector quantity an its S.I units are m/s

Uniform Velocity

An object is said to be moving with a uniform velocity if its covers equal displacements in equal intervals of time. for example, if a body covers a displacement of 2 metres in each second, it is said to have a uniform velocity.

Variable Velocity

An object is said to have a variable or non uniform velocity, if it covers equal displacements in unequal intervals of time, or unequal displacements in equal intervals of time.

If a body covers a displacement of 1m in first second, 3m in another second, 6m in another second, it is said to have a variable velocity as it covers unequal displacements in equal intervals of time.

Similalrly if a body a covers a displacement of 2m in one second, another 2m in 4 seconds, another 2m in 8 seconds, it is said to have a variable velocity, as it covers equal displacements in unequal intervals of time.

Instantaneous Velocity

The velocity of an object at a given instant of time or at a given point during motion is called its instantaneous velocity.

Acceleration

In non uniform motion, the velocity of a body is different at different instants. That is the velocity of the body keeps on changing continuously. Such a motion, where velocity of a body changes continuously is known as an accelerated motion.

Acceleration is defined as the time rate of change of velocity of an object. It may also be defined as the change in velocity in a unit time. Mathematically,

Acceleration = change in velocity/time taken

Acceleration is a vector quantity and its S.I unit is m/s².

The acceleration of a body is said to be positive if the velocity of the body is increasing and negative if the velocity of the body is decreasing. The negative acceleration is known as Retardation. If you are continuously pushing the accelerator of your car, its velocity continuously increases, so it has positive acceleration. Now if you start applying brakes, its velocity continuously decreases, so it has negative acceleration or retardation.

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Now that you have revised the basics of mechanics. It is time for you to test your knowledge of physics in general and Mechanics in particular by taking part in this science quiz.

MCQs on Mechanics

Noe is the turn to solve some important MCQs on mechanics. Lets go.

You clean a dusty mat by hitting it with a stick. It is possible only because of ;

1. inertia of rest of dust particles
2. inertia of motion of dust particles
3. inertia of direction of dust particles
4. none

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A paper and a feather are dropped from a tower of height 100m in vacuum. which one will reach the ground first?

1. paper
2. feather
3. both will reach at the same time
4. can not say anything

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An apple falls from a tree of height 10 m. what is the acceleration of the body during its motion?

1. 9.8 metre per second square
2. 19.6 metre per second square
3. 4.9 metre per second square
4. none

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A man starts moving from point A toward north and covers a distance of 200m. He then turns back and returns to point A. what is the displacement covered by him?

1. 200m
2. 400m
3. 0 m
4. 100m

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Which of the following is true?

1. units of velocity and speed are same.
2. unit of acceleration is m/s²
3. if a body is moving along a straight line, its velocity and speed are same.

1. all of them
2. only 1 and 2
3. only 2 and 3
4. only 1 and 3

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What is the unit of speed?

1. metre per second
2. metre per second square
3. kilogram metre per second square
4. none

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A body moves with a non uniform velocity along a straight line. it means that it will;

1. cover equal displacements in equal intervals of time
2. cover equal distances in unequal intervals of time
3. cover unequal distances in equal intervals of time

1. all of them
2. only 2 and 3
3. only 1 and 2
4. none

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A body is moving with uniform velocity along a straight line. It covers a displacement of 2 meter in first 3 seconds. What will be the displacement covered by it in next 3 seconds?

1. 2 metres
2. 3 metres
3. 1 metre
4. 4 metres

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Velocity is a _____ quantity.

1. scalar
2. vector
3. tensor
4. none

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The pendulum of a clock moves continuously in a particular motion. That motion is called?

1. rectillinear motion
2. circular motion
3. oscillatory motion
4. curvillinear motion

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Speed is a _______ quantity.

1. scalar
2. vector
3. tensor
4. contravariant tensor

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Total distance travelled divided by total time taken is called?

1. speed
2. velocity
3. acceleration
4. jerk

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What is the time rate change of position of an object in any direction called?

1. speed
2. velocity
3. acceleration
4. instantanous velocity

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The motion in which a point mass body is moving along a straight line is called?

1. rectillinear motion
2. curvillinear propagation
3. oscillatory motion
4. rotatory motion

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Is earth at rest or in a state of motion?

1. it depends upon the location of the observer
2. it is at rest
3. it is in motion
4. it is in rotational motion

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What is the study of motion of material objects while taking into account the factors that cause the motion, called?

1. kinematics
2. statics
3. dynamics
4. none

Show Answer

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So, this was all about mechanics. We are sure that you have understood the concepts like statics, kinematics, dynamics, rest and motion, distance and displacement, speed, velocity and acceleration