Motion and Speed for SSC & Railway Exams – Complete Physics Notes
Motion is one of the basic topics of Physics and is important for SSC, Railway and other competitive examinations.
Questions from this chapter generally test basic concepts such as distance, displacement, speed, velocity, acceleration and equations of motion.
Understanding these concepts also helps students solve numerical questions more confidently.
What is Motion?
An object is said to be in motion when its position changes with time relative to a reference point.
For example, a moving car changes its position continuously with respect to the road.
Distance
Distance is the total path travelled by an object.
Distance is a scalar quantity.
It has magnitude but no direction.
SI unit of distance = metre (m)
Displacement
Displacement is the shortest straight-line distance between the initial and final positions of an object, along with direction.
Displacement is a vector quantity.
Important Point
Distance is always greater than or equal to the magnitude of displacement.
Example of Distance and Displacement
A person walks 3 km east and then 4 km west.
Total distance:
3 + 4 = 7 km
Final position is 1 km west of the starting point.
Displacement = 1 km west.
Speed
Speed tells us how fast an object is moving.
Speed = Distance / Time
SI unit = metre per second (m/s)
Average Speed
Average Speed = Total Distance / Total Time
Example
A vehicle travels 120 km in 2 hours.
Average speed:
120/2 = 60 km/h
Answer: 60 km/h
Velocity
Velocity is the rate of change of displacement.
Velocity = Displacement / Time
Velocity is a vector quantity because it has both magnitude and direction.
Speed vs Velocity
Speed is based on distance.
Velocity is based on displacement.
Speed is scalar.
Velocity is vector.
Acceleration
Acceleration is the rate of change of velocity.
Acceleration = Change in Velocity / Time
a = (v - u)/t
Where:
u = Initial velocity
v = Final velocity
t = Time
a = Acceleration
SI unit of acceleration = m/s²
Uniform Motion
An object is said to be in uniform motion if it covers equal distances in equal intervals of time.
Non-Uniform Motion
If an object covers unequal distances in equal intervals of time, it is said to be in non-uniform motion.
Equations of Motion
For uniformly accelerated motion:
First Equation
v = u + at
Second Equation
s = ut + 1/2 at²
Third Equation
v² = u² + 2as
Where:
u = Initial velocity
v = Final velocity
a = Acceleration
t = Time
s = Displacement
Example 1
A car starts from rest and accelerates at 2 m/s² for 5 seconds.
u = 0
a = 2 m/s²
t = 5 s
Using:
v = u + at
v = 0 + 2 × 5
= 10 m/s
Answer: 10 m/s
Example 2
A body starts from rest with acceleration 4 m/s². Find the distance travelled in 3 seconds.
Using:
s = ut + 1/2 at²
u = 0
s = 0 + 1/2 × 4 × 3²
= 2 × 9
= 18 m
Answer: 18 m
Conversion Between km/h and m/s
To convert km/h into m/s:
Multiply by 5/18.
Example
72 km/h:
72 × 5/18 = 20 m/s
To convert m/s into km/h:
Multiply by 18/5.
Example
10 m/s:
10 × 18/5 = 36 km/h
Rest and Motion are Relative
An object may be at rest with respect to one observer but moving with respect to another.
For example, a passenger sitting inside a moving train is at rest relative to another passenger but is moving relative to a person standing on the platform.
Circular Motion
When an object moves along a circular path, it is said to be in circular motion.
Even if its speed remains constant, its velocity changes because its direction continuously changes.
Therefore, an object moving in a circle with constant speed still has acceleration.
Important One-Liners
Distance is a scalar quantity.
Displacement is a vector quantity.
Speed is distance travelled per unit time.
Velocity is displacement per unit time.
Acceleration is the rate of change of velocity.
SI unit of speed is m/s.
SI unit of acceleration is m/s².
Distance can never be less than the magnitude of displacement.
Average speed is total distance divided by total time.
An object moving in a circle at constant speed has changing velocity.
Uniform motion means equal distances in equal intervals of time.
The equation v = u + at applies to uniformly accelerated motion.
18 km/h = 5 m/s.
36 km/h = 10 m/s.
72 km/h = 20 m/s.
Practice MCQs
1. Which of the following is a scalar quantity?
A) Velocity
B) Displacement
C) Acceleration
D) Distance
Answer: D) Distance
2. Which quantity has both magnitude and direction?
A) Speed
B) Distance
C) Velocity
D) Time
Answer: C) Velocity
3. What is the SI unit of acceleration?
A) m/s
B) m/s²
C) km/h
D) metre
Answer: B) m/s²
4. Convert 54 km/h into m/s.
A) 10 m/s
B) 12 m/s
C) 15 m/s
D) 18 m/s
Answer: C) 15 m/s
5. A car covers 150 km in 3 hours. Find its average speed.
A) 40 km/h
B) 50 km/h
C) 60 km/h
D) 75 km/h
Answer: B) 50 km/h
6. If a body starts from rest, its initial velocity is:
A) 1 m/s
B) 10 m/s
C) 0
D) Infinite
Answer: C) 0
7. Which equation is correct for uniform acceleration?
A) v = u + at
B) v = u - st
C) s = v/t
D) a = vt
Answer: A) v = u + at
8. A body starts from rest and accelerates at 3 m/s² for 4 seconds. Find its final velocity.
A) 7 m/s
B) 10 m/s
C) 12 m/s
D) 15 m/s
Answer: C) 12 m/s
9. In uniform circular motion, which quantity changes continuously?
A) Mass
B) Speed
C) Direction of velocity
D) Time
Answer: C) Direction of velocity
10. Distance travelled can be:
A) Always less than displacement
B) Equal to or greater than displacement magnitude
C) Always zero
D) Negative
Answer: B) Equal to or greater than displacement magnitude
Conclusion
Motion is a foundation topic of Physics. Students should clearly understand the differences between distance and displacement, speed and velocity, and speed and acceleration.
After learning these concepts, practise numerical questions based on unit conversion and equations of motion.
This will provide a strong base for further topics such as Force, Newton's Laws of Motion, Work, Energy and Gravitation.