In which case bodies become hot due to mechanical energy losses
(i) Rub our hands for sometimes
(ii) Two vehicles collied with each other
(iii) Aeroplane crases
(iv) Sliding of legs on roof surface
(v) Transfer of energy from hot energy from hot body to cold body

Options

A. `5`
B. `4`
C. `3`
D. `2`

Answer

To determine in which case bodies become hot due to mechanical energy losses, we will analyze each option step by step.

### Step 1: Analyze the first case - Rubbing our hands
When we rub our hands together, we create friction. This friction converts kinetic energy (from the motion of rubbing) into thermal energy, causing our hands to warm up. However, this process is primarily physical and not classified as mechanical energy loss in the context of this question.

**Hint:** Consider whether the energy transformation involves a physical process or a mechanical one.

### Step 2: Analyze the second case - Two vehicles colliding
In a collision between two vehicles, kinetic energy is involved. Depending on the type of collision (elastic or inelastic), some energy may be transformed into sound, heat, or deformation. However, this does not specifically refer to mechanical energy loss, as the primary focus is on the kinetic energy of the vehicles.

**Hint:** Think about the type of energy involved in collisions and whether it qualifies as mechanical energy loss.

### Step 3: Analyze the third case - Aeroplane crashes
An airplane crash is also a collision event. Similar to the previous case, the kinetic energy is involved, and energy may be lost in the form of sound and heat. However, it is still not categorized as mechanical energy loss.

**Hint:** Reflect on whether the energy loss in crashes is due to mechanical processes or other forms of energy transformation.

### Step 4: Analyze the fourth case - Sliding of legs on the roof surface
When sliding legs on a roof surface, friction occurs between the legs and the surface. This friction is a mechanical force that converts kinetic energy into thermal energy, resulting in heat generation. Therefore, this case is a clear example of mechanical energy loss.

**Hint:** Identify if friction is present and how it contributes to energy transformation in this scenario.

### Step 5: Analyze the fifth case - Transfer of energy from hot body to cold body
This scenario describes thermal conduction, where heat energy is transferred from a hot object to a cold object. This process does not involve mechanical energy loss; instead, it is a thermal process.

**Hint:** Consider the nature of energy transfer and whether it involves mechanical energy loss or thermal conduction.

### Conclusion
After analyzing all the cases, the only scenario where bodies become hot due to mechanical energy losses is the fourth case: sliding of legs on the roof surface.

**Final Answer:** The correct option is (iv) Sliding of legs on roof surface.