# Motion in a Straight Line Notes, Class 11 Physics | SwaVid

Motion in a Straight Line, NCERT Class 11 Physics: notes with key ideas, formulas, common mistakes and 3 quick checks with worked answers.

Canonical: https://www.swavid.com/cbse/class-11/physics/chapter/motion-in-a-straight-line

Source: https://www.swavid.com/cbse/class-11/physics/chapter/motion-in-a-straight-line

# Motion in a Straight Line

## Formulas and definitions

## Key ideas

## Common mistakes

## Quick checks

## Keep going

## Questions parents ask

## Class 11 Physics on SwaVid

### Position, path length, and displacement

### Average and instantaneous velocity

### Average and instantaneous acceleration

### Kinematic equations for uniform acceleration

### Free fall under gravity

### Stopping distance and reaction time

### Next chapter

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SwaVid › CBSE › Class 11 › Physics › Motion in a Straight Line

Notes written by SwaVid from the NCERT textbook.

Class 11 Physics, Chapter 2. Key ideas, formulas and common mistakes.

CBSE prints 23 marks for Units I-III together, not for this chapter alone.

Short answer. This chapter describes rectilinear motion of a point object using position, displacement, velocity, and acceleration. It develops the kinematic equations for uniformly accelerated motion and analyzes vertical motion under gravity.

Source: NCERT Physics Part I (Reprint 2026-27) , Chapter 2, Motion in a Straight Line

A frame of reference specifies the position of an object along a coordinate axis. Path length is the total distance traversed, which is always positive. Displacement is the change in position vector, Δx = x₂ - x₁, and can be positive, negative, or zero.

Average velocity is displacement divided by the time interval: v avg = Δx / Δt. Instantaneous velocity is the derivative of position with respect to time, v = dx/dt, which represents the slope of the position-time curve at that instant.

Average acceleration is the change in velocity divided by the time interval: a avg = Δv / Δt. Instantaneous acceleration is the limiting rate of change of velocity, a = dv/dt = d²x/dt², which is the slope of the velocity-time graph.

When acceleration is constant, velocity and displacement can be related through simple algebraic equations: v = v₀ + a t, x = v₀ t + (1/2) a t², and v² = v₀² + 2 a x. The area under the v-t graph represents displacement.

An object released near Earth's surface experiences a constant downward acceleration due to gravity, g ≈ 9.8 m s⁻², neglecting air resistance. Galileo's law of odd numbers states that distances traversed during equal successive intervals of time by a body falling from rest stand to one another in the ratio 1 : 3 : 5 : 7...

Stopping distance is the distance travelled before coming to rest: d s = -v₀² / (2 a). Reaction time is the time taken to observe, think, and initiate action, adding an extra distance v₀ × t r before braking starts.

Try each question first. Then tap it to see the answer.

v₀ = 20 m s⁻¹, v = 0 m s⁻¹, x = 40 m

v² = v₀² + 2 a x → 0 = 20² + 2 × a × 40

80 a = -400 → a = -400 / 80 = -5 m s⁻²

-5 m s⁻²

v₀ = 20 m s⁻¹, v = 0 m s⁻¹, a = -10 m s⁻²

v² = v₀² + 2 a y → 0 = (20)² + 2 × (-10) × y

20 y = 400 → y = 400 / 20 = 20 m

20 m

v = dx/dt = 6 t² - 6 t

a = dv/dt = 12 t - 6

At t = 2 s: a = 12(2) - 6 = 24 - 6 = 18 m s⁻²

18 m s⁻²

Motion in a Plane .

Laws of Motion .

The 14 chapters of Class 11 Physics .

CBSE does not give marks to one chapter. It prints 23 marks for Units I-III together (Physical World and Measurement; Kinematics; Laws of Motion). Motion in a Straight Line is part of that group.

No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed.

Motion in a Plane.

SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works.

- Ch 2 NCERT book
- 23 marks, Units I-III together
- 3 quick checks
- Treating distance and magnitude of displacement as always equal . Magnitude of displacement equals distance only if the particle moves along a straight line without reversing its direction. If the particle turns back, total distance exceeds the magnitude of displacement.
- Assuming acceleration must be zero when velocity is zero . A particle at the highest point of vertical throw has instantaneous velocity zero, but its acceleration is non-zero and equal to g directed downwards.
- Using constant-acceleration kinematic equations when acceleration varies with time . Check whether acceleration is constant before applying v = v₀ + a t. If acceleration depends on time or position, integrate using calculus definitions v = dx/dt and a = dv/dt.
- Assigning inconsistent signs to vectors along the vertical line . Choose one coordinate direction as positive at the start, usually upward. Then assign signs strictly: if upward is positive, downward initial velocity or acceleration due to gravity must be entered with a negative sign.

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