Q.A person of mass 60 kg wants to lose 5 kg by going up and down a 10 m high stairs. Assume he burns twice as much fat while going up than coming down. If 1 kg of fat is burnt on expending 7000 kilo calories, how many times must he go up and down to reduce his weight by 5 kg?

The Work-Energy Principle: From Intuition to Precision

Imagine pushing a heavy box across a rough floor. The harder you push and the farther it slides, the faster it moves when you let go. That connection — between the effort you put in (force × distance) and the change in the box's motion — is exactly what the Work-Energy Principle captures.

The Intuition First

Think of work as "energy transferred by a force." When you do work on an object, you're essentially pumping energy into it. That energy has to go somewhere — and in the simplest case, it shows up as a change in the object's speed. The object's kinetic energy (energy of motion) increases by exactly the amount of work you did.

The Precise Statement

Where:

• $W_{\text{net}}$ = net work done on the object (total work from all forces combined)
• $K$ = kinetic energy = $\frac{1}{2}mv^2$
• $m$ = mass, $v$ = speed

The net work done on an object equals the change in its kinetic energy.

Why "Net Work" Matters

If you push a box forward while friction pulls it backward, only the net force matters. Suppose you push with 50 N and friction opposes with 30 N over 2 m:

• Work done by you: $50 \times 2 = 100$ J
• Work done by friction: $-30 \times 2 = -60$ J (negative because force opposes motion)
• Net work: $100 - 60 = 40$ J

That 40 J is exactly the increase in the box's kinetic energy. The individual works don't matter — only the sum.

A Common Trap

When Does It Hold?

The principle works for:

• Any constant or varying force
• Straight-line or curved paths
• Objects that don't rotate (for now)

It fails if:

• The object deforms permanently (like crumpling a car) …