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## Question Bank in Class XI Physics – Mechanical Properties of Solids

**Important Points to remember:**

- Deforming forces are external force applied on a body to change its size or shape.
- Restoring force is the internal force developed in a body trying to bring it back to the original configuration when it is acted upon by an external force trying to change its size or shape.
**Elasticity**is the property of a body by virtue of which it tries to regain its original configuration when the deforming forces are removed.- Perfectly elastic body is a body which regains its original configuration completely on removal of deforming forces.
- Plasticity is the property of a body by virtue of which it
**does not**regain its original configuration on removal of deforming forces. - Perfectly plastic body is a body which does not show any tendency to regain its original configuration on removal of deforming forces
- Stress: The restoring force developed per unit area of a deformed body is called stress.
- Tensile stress: If a deforming force is trying to produce an increase in length, then the restoring force developed per unit area of cross section is called tensile stress
- Compressive Stress: When a deforming force acting normal to a surface tries to produce a change in volume, then the restoring force produced per unit area is called compressive stress.
- Shearing stress: When a tangential force tries to change the shape of a body whose bottom is fixed, then the restoring force developed per unit area is called Shearing stress.
- Strain: The ratio of the change in dimension to the original dimension of a deformed body is called strain
- Linear or longitudinal strain

## Pressure, Thrust, Area and Pascal’s Law

“In Pascal’s law we find that pressure does not increase with area. But when we study about pressure, we learn that pressure is inversely proportional to area. How is this possible? Please explain.”

**Fahad Imtiaz **asked via Speak Pipe

Answer:

Pressure is defined as the thrust (the total force acting normal to a surface) per unit area.

Pascal’s law deals with fluid pressure and the statement goes –

“The pressure

exertedanywhere in anenclosedincompressibleandnon-viscousfluid is transmitted equally and undiminished in all directions through out the fluid, provided the effect of gravity is neglected”

Read the statement carefully.

Here we are not changing the definition or meaning of pressure.

the fact to note that, in an **enclosed fluid, **the pressure is **transmitted **equally **throughout **the fluid. Therefore, if we apply some pressure somewhere in the fluid, the same pressure will be felt at any other place on the enclosed fluid. This gives us an opportunity to multiply the force. Since the **pressure is equal everywhere**, if we increase the area the force (thrust) is increased.

P = F/A or **F = PA**

So, **pressure remaining constant, **greater the area, greater is the **force.**

Hope you understand the matter now.

## Heat! Heat! Heat!

A glass ball at -10 degrees C is placed in 500 ml of water at 20 degrees C. The final temp is 15 degrees C.

A. How much heat is lost by the water

B. what is the heat capacity of the water

C. How much heat is gained by the glass ball

D. What is the heat capacity of the glass ball

E. what is the ratio of the heat capacities of the water and the glass ball

F. What is the ratio of the temp changes of the water and glass ball

G. How are the answers to e and f related

## SURFACE TENSION

**How does surface tension act? What is basic cause?**

**Answer**: Surface tension is the property of a liquid by virtue of which its free surface behaves like a stretched elastic membrane.

When a blade is carefully kept on a liquid surface, it floats; but it sinks if it is dropped carelessly.

## Change in Barometric Height with position

Biswaroop Dasgupta asks:

“How would the barometric height change when a mercury barometer is taken to a mine? Give reasons to explain.”

Answer: Barometer is a device used to determine the atmospheric pressure. The atmospheric pressure is calibrated in terms of height of mercury column in the barometer.

Atmospheric pressure is the weight of air acting normally per unit area on the surface ofof earth or the point under consideration.

Inside a mine, the weight of air above the point will be more than that on the surface of earth. Therefore, the **barometric height increases when taken to a mine. **