Class 11 Physics Motion in a Straight Line Notes

Rest and Motion

An object is said to be:

• At rest if its position does not change with time
• In motion if its position changes with time

Motion and rest are relative and depend on the observer.

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🔹 Position and Displacement

• Position: Location of an object with respect to a reference point
• Displacement: Change in position of an object

Key Points:

• Displacement is a vector quantity
• It can be zero even when distance is not zero

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🔹 Distance

• Total path length covered by an object
• Scalar quantity
• Always positive

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🔹 Difference Between Distance and Displacement

Distance	Displacement
Scalar quantity	Vector quantity
Path dependent	Path independent
Always positive	Can be positive, negative, or zero

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🔹 Speed

Speed is the rate of change of distance.$$\text{Speed} = \frac{\text{Distance}}{\text{Time}}$$Speed=TimeDistance​

Types of Speed:

• Uniform speed
• Non-uniform speed
• Average speed

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🔹 Velocity

Velocity is the rate of change of displacement.$$\text{Velocity} = \frac{\text{Displacement}}{\text{Time}}$$Velocity=TimeDisplacement​

Key Points:

• Vector quantity
• Can be positive, negative, or zero

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🔹 Average Velocity

$$\text{Average Velocity} = \frac{\text{Total Displacement}}{\text{Total Time}}$$Average Velocity=Total TimeTotal Displacement​

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🔹 Acceleration

Acceleration is the rate of change of velocity.$$\text{Acceleration} = \frac{\text{Change in Velocity}}{\text{Time}}$$Acceleration=TimeChange in Velocity​

Types of Acceleration:

• Uniform acceleration
• Non-uniform acceleration
• Negative acceleration (retardation)

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🔹 Equations of Motion

For uniformly accelerated motion:

1. $v = u + at$v=u+at
2. $s = ut + \frac{1}{2}at^2$s=ut+21​at2
3. $v^2 = u^2 + 2as$v2=u2+2as

Where:

• $u$u = initial velocity
• $v$v = final velocity
• $a$a = acceleration
• $t$t = time
• $s$s = displacement

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🔹 Graphical Representation of Motion

1️⃣ Position–Time Graph

• Slope gives velocity
• Straight line → uniform velocity

2️⃣ Velocity–Time Graph

• Slope gives acceleration
• Area under graph gives displacement

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🔹 Uniform and Non-Uniform Motion

• Uniform motion: Equal distances in equal intervals of time
• Non-uniform motion: Unequal distances in equal intervals of time

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🔹 Relative Velocity

Velocity of an object with respect to another object.$$v_{AB} = v_A – v_B$$vAB​=vA​−vB​

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