Case Study:

Every morning, students carry their school bags from home to school. At Green Valley School in Lucknow, a science teacher asked students to observe how much effort is required when they lift their school bags from the floor to a desk.

One student, Riya, lifted her school bag from the floor to a desk that was 0.8 m high. The mass of her bag was 6 kg. According to physics, work is said to be done when a force causes displacement in the direction of the force. 

When Riya lifted the bag upward, she applied an upward force to overcome gravity. The bag moved vertically upward, meaning the force and displacement were in the same direction. In such a case, work done is calculated using the formula:

W = F * s

where F is the applied force and s is the displacement. 

Since the force required to lift the bag equals its weight, the force can be written as:

F = mg

Thus, work done in lifting an object vertically becomes:

W = mgh

If we take (g = 9.8 , m/s^2), Riya performs work while lifting the bag. However, if she simply holds the bag while standing still, no work is done in the physics sense because there is no displacement.
This example helps students understand how everyday actions-like lifting a school bag-demonstrate the scientific concept of work and energy. It also shows that work depends on force and displacement, not on how long the force is applied.

Questions

Section A - MCQs

1. When a student lifts a school bag from the floor to a table, the work done is mainly against:

A. Friction
B. Air resistance
C. Gravity
D. Magnetic force

2. If the mass of a school bag increases, the work done in lifting it to the same height will:

A. Decrease
B. Remain the same
C. Increase
D. Become zero

3. A student holds a heavy bag while standing still for 10 seconds. The work done by the student on the bag is:

A. Maximum
B. Zero
C. Equal to weight
D. Infinite

4. Work done in lifting an object vertically can be calculated using:

A. W = mgh
B. W = mv
C. W = ma
D. W = v/t

Section B - Short Answer Questions

1. Why is work said to be done when a student lifts a school bag from the floor to a desk?

2. Explain why holding a school bag without moving it does not involve work in physics.

3. How does increasing the height of the desk affect the work done in lifting the bag?

Section C - Long Answer Question

1. A student lifts a school bag of mass 5 kg from the floor to a table of height 1 m.

a) Calculate the work done by the student in lifting the bag.
b) Explain how work done would change if the bag mass were doubled.
c) Suppose the student carries the bag horizontally for 10 m at the same height. Is any work done in physics? Explain.

Answer Key

MCQ Answers

1. C - Work is done against gravity.
2. C - Work increases because force (weight) increases.
3. B - Work done is zero because displacement is zero.
4. A - Work done in lifting an object vertically is (W = mgh).

Short Answer Solutions

1. Work is done because the student applies an upward force to lift the bag and the bag moves upward. Both force and displacement are present in the same direction.

2. When the student holds the bag without moving it, displacement is zero. Since work depends on force and displacement, the work done is zero.

3. Work done increases if the height increases because displacement becomes larger in the formula  W = F * s.

Long Answer Solution

Given:
Mass m = 5 kg
Height h = 1 m
g = 9.8 m/s²

Step 1: Calculate force

F = mg = 5 * 9.8 = 49 N

Step 2: Calculate work

W = F * h
W = 49 * 1 = 49 J

Work done = 49 Joules

b) If mass doubles, force doubles; therefore work done also doubles.

c) When carrying the bag horizontally, the applied force is vertical while displacement is horizontal. Since the directions are perpendicular, work done is zero.