Describing Motion - Physics in Motion

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2 km North

↑

O (Origin)
0 km

C
25 km

B
35 km

A
60 km

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Distance

Total path length covered

0 km

Displacement

Shortest distance from initial to final position

0 km

Uniform vs Non-Uniform Motion

Watch how objects move with constant and varying speeds

● Uniform Motion (equal distances in equal time)

● Non-Uniform Motion (varying speeds and distances)

NCERT Activities

Activity 7.3: Basketball Court

Start

End

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Distance: 0 m

Displacement: 0 m

Take a metre scale and a long rope
Walk from one corner of a basketball court to its opposite corner along its sides
Measure the distance covered by you and magnitude of the displacement
What difference would you notice between the two in this case?

Explanation: This activity demonstrates the difference between distance and displacement. When you walk along the sides of a basketball court from one corner to the diagonally opposite corner, you cover a greater distance than the straight-line displacement between those points.

If the court is a square with sides of length 'a', then:

- Distance covered = 2a (sum of two sides)

- Displacement = a√2 (diagonal of the square)

This shows that distance is always greater than or equal to displacement.

Activity 7.4: Odometer Reading

Bhubaneshwar

New Delhi

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Odometer: 0 km

Displacement: 0 km

Automobiles are fitted with a device that shows the distance travelled (odometer)
A car is driven from Bhubaneshwar to New Delhi
The difference between final and initial odometer reading is 1850 km
Find the magnitude of displacement using Road Map of India

Explanation: This activity illustrates how an odometer measures the total distance traveled, which is different from displacement. When driving from Bhubaneshwar to New Delhi:

- The odometer records 1850 km (total path length along roads)

- The displacement is approximately 1500 km (straight-line distance)

The odometer reading is always greater than or equal to the displacement because roads rarely follow a straight line between two cities.

Activity 7.5: Motion Analysis

Distance (m)

Time

Object A (Uniform)

Object B (Non-uniform)

Examine the data for two different objects A and B
Object A covers equal distances in equal time intervals
Object B covers unequal distances in equal time intervals
Determine whether motion is uniform or non-uniform

Explanation: This activity helps distinguish between uniform and non-uniform motion by analyzing distance-time data:

- Object A: Covers 10m every 15 minutes consistently. On a distance-time graph, this creates a straight line with constant slope, indicating uniform motion.

- Object B: Covers varying distances (7m, 4m, 12m, etc.) in equal time intervals. On a distance-time graph, this creates a curved line with varying slope, indicating non-uniform motion.

The slope of a distance-time graph represents the speed of the object. A constant slope means constant speed (uniform motion), while a changing slope indicates changing speed (non-uniform motion).

Table 7.1: Motion Data Analysis

Time	Distance travelled by Object A (m)	Distance travelled by Object B (m)
9:30 am	10	12
9:45 am	20	19
10:00 am	30	23
10:15 am	40	35
10:30 am	50	37
10:45 am	60	41
11:00 am	70	44

Object A Analysis

Object A covers 10m every 15 minutes consistently. This is uniform motion because equal distances are covered in equal time intervals.

Object B Analysis

Object B covers varying distances (7m, 4m, 12m, 2m, 4m, 3m) in equal time intervals. This is non-uniform motion.

Practice Questions

Q1.

An object has moved through a distance. Can it have zero displacement? If yes, support your answer with an example.

Answer: Yes, an object can have zero displacement even after moving through a distance.

Example: If you walk around a circular track and return to your starting point, you have covered a distance equal to the circumference of the track, but your displacement is zero because your initial and final positions are the same.

Q2.

A farmer moves along the boundary of a square field of side 10 m in 40 s. What will be the magnitude of displacement of the farmer at the end of 2 minutes 20 seconds from his initial position?

Solution:
• Perimeter of square field = 4 × 10 = 40 m
• Time to complete one round = 40 s
• Total time = 2 min 20 s = 140 s
• Number of complete rounds = 140 ÷ 40 = 3.5 rounds
• After 3.5 rounds, farmer is at the opposite corner
• Displacement = diagonal of square = 10√2 = 14.14 m

Q3.

Which of the following is true for displacement?
(a) It cannot be zero
(b) Its magnitude is greater than the distance travelled by the object

Answer: Neither (a) nor (b) is correct.

Explanation:
• (a) is false: Displacement can be zero when initial and final positions are the same
• (b) is false: Displacement magnitude is always less than or equal to distance travelled
• The magnitude of displacement equals distance only when motion is in a straight line without changing direction

Key Concepts of Motion

To describe the position of an object, we need to specify a reference point called the origin. Motion is described relative to this reference point, and different reference points can be chosen based on convenience.

Distance

• Total path length covered
• Always positive
• Scalar quantity
• Can never be zero for a moving object

Displacement

• Shortest distance from initial to final position
• Can be positive, negative, or zero
• Vector quantity
• Magnitude ≤ Distance

Uniform Motion

An object is in uniform motion when it covers equal distances in equal intervals of time, regardless of how small the time intervals are.

Non-Uniform Motion

An object is in non-uniform motion when it covers unequal distances in equal intervals of time. Most real-world motions are non-uniform.

Important Points to Remember

• The magnitude of displacement can be equal to distance only when motion is in a straight line without changing direction
• For circular motion, displacement can be zero while distance is non-zero
• Motion is always described relative to a reference point
• Time intervals should be small when analyzing uniform motion