Motion - Class 6 Notes & Olympiad Questions

Motion

a) Motion is when an object moves or changes its position over time.
b) Imagine a soccer ball rolling across the field or a car speeding down the road. Both of these examples show motion because the objects are changing their positions as time goes on.
c) On the other hand, when something is at rest, it means it's not moving at all. For instance, think of a book sitting on a table or a stationary bicycle. These objects are not changing their positions over time, so we say they are at rest.
d) So, motion is all about things moving and changing their positions, while being at rest means staying still and not moving around.

Types of Motion

Motion can be divided into different types based on how objects move. Some of these types are:

Translatory or Linear Motion

In this type of motion, all the parts of an object move the same distance in the same amount of time.

There are two specific kinds of translatory motion: rectilinear and curvilinear.

1. Rectilinear motion

a) Rectilinear motion is when an object moves in a straight line.
b) For example, a car driving on a long, straight road or a person walking in a straight line.

c) In rectilinear motion, the object keeps moving forward without changing direction.

2. Curvilinear motion

a) Curvilinear motion, on the other hand, is when an object moves along a curved path.
b) For example, a roller coaster ride where the track twists and turns, or a bicycle going around a circular path.

c) In curvilinear motion, the object still moves, but its path follows a curve or a bend.

Differences between Rectilinear and Curvilinear Motion

Circular Motion

a) Circular motion occurs when an object moves around a fixed point or follows a circular path.
b) For example, swinging a yo-yo in a circle or spinning around on a playground carousel.
c) Circular motion can be called uniform when an object moves around a circle at a constant speed.
d) Non-uniform circular motion, on the other hand, is when an object moves around a circle but changes its speed.

There are two types of circular motion: revolutionary motion and rotational motion.

1. Revolutionary motion

a) In revolutionary motion, the entire object moves in a circular path.
b) This means that the object as a whole is moving around the same fixed point like the Earth moving around the Sun or a car driving on a circular track.

c) The object changes its position as it moves, continuously circling around the fixed point.

2. Rotational motion

a) This type of motion involves the object spinning or rotating around an axis that passes through it.
b) For example, the blades of a rotating fan or a potter's wheel. The object stays in the same place with respect to time, but different parts of it move at varying distances.

Periodic Motion

a) Periodic motion refers to the type of motion where an object repeats its movement over and over again in a regular pattern.
b) It means that after a certain amount of time, the object goes back to its original position and starts the same motion again.
c) There are many examples of periodic motion.
d) One example is the motion of a simple pendulum, like a weight hanging from a string. When you push the weight to one side and let it swing, it will keep swinging back and forth in a regular pattern. The pendulum goes through the same motion, swinging back and forth, over and over again.

e) Another example is the motion of a swing. When you swing back and forth on a playground swing, you go through the same motion repeatedly. Each time you swing forward and then backwards, it's a cycle of motion that keeps repeating.

Oscillatory motion

a) Oscillatory motion refers to the type of motion where an object moves back and forth around a fixed point, also known as its mean position. It means that the object keeps repeating its motion in a to-and-fro pattern.
b) There are several examples of oscillatory motion. One example is the oscillation of a simple pendulum.
c) The motion of a needle in a sewing machine is another example of oscillatory motion. The needle moves up and down, continuously repeating the same motion as it stitches fabric.
d) It's important to note that every oscillatory motion is periodic because it repeats after a fixed interval of time. However, not every periodic motion is oscillatory.
e) Periodic motion refers to any motion that repeats in a regular pattern, but it doesn't necessarily have to be back and forth like in oscillatory motion.
f) For example, the circular motion of a planet around the Sun is periodic, but it's not oscillatory since it doesn't move back and forth.

Vibratory motion

a) Vibratory motion is a specific type of oscillatory motion where an object doesn't move as a whole but instead undergoes changes in shape or vibrates.
b) In this type of motion, the object rapidly moves back and forth, creating vibrations.
c) There are various examples of vibratory motion. The swinging movement of a swing is a common example. When you swing on a playground swing, you move back and forth, creating a vibratory motion.

d) Another example is the movement of vocal cords when we speak or sing. The vocal cords vibrate rapidly, producing sound.
e) Additionally, the motion of a stretched wire in a guitar, when plucked, is a form of vibratory motion. The wire vibrates back and forth, creating musical tones.

Objects in Simultaneous Motion

a) It's worth noting that objects can exhibit multiple types of motion simultaneously. For example, the Earth demonstrates both rotational motion (as it spins on its axis) and revolutionary motion (as it revolves around the Sun).
b) Some more examples of objects exhibiting multiple types of motion simultaneously:

1. A spinning top: When a spinning top rotates around its axis (rotational motion) while also moving in a circular path on a table (circular motion).
2. A car moving: The wheels of a car rotate (rotational motion) while the car moves along a path (translatory motion)
3. A cyclist pedalling uphill: The cyclist's legs rotate the pedals (rotational motion) while the bicycle moves in a linear path (translatory motion).

Distance and Displacement

Distance

a) Distance refers to the total length of the path travelled by an object.
b) It is always positive.
c) For example, imagine you walk in a park. If you walk from point A to point B and then return back to point A, the total distance you covered would be the sum of the distances from A to B and from B back to A.

Displacement

a) Displacement, on the other hand, refers to the change in the position of an object from its starting point to its final position.
b) Displacement can be positive, negative, or zero.
c) For instance, if you walk from point A to point B in a straight line, the displacement would be the straight-line distance and the direction from A to B.

How Distance and Displacement are Different?

To understand it better, let's consider an example.

a) Suppose you start at your house (point A) and walk 2 kilometres to the park (point B). After spending some time at the park, you walk back home following the same path. In this case, the distance you covered would be the sum of the distance from home to the park and from the park back to home, which is 2 + 2 = 4 kilometres.
b) However, the displacement would be zero because you ended up at the same point where you started. Even though you walked a distance of 4 kilometres, your displacement is zero because there is no change in your position relative to your starting point.