Which one of the following physical quantity has the same unit as that of pressure
Stress
Understanding Physical Quantities and Units
In physics, every physical quantity has a specific unit that helps us measure it. Different quantities can sometimes share the same unit if they represent similar physical concepts or ratios. The question asks us to find a physical quantity that has the same unit as pressure.
What is Pressure?
Pressure is defined as the force applied perpendicularly to the surface of an object per unit area over which that force is distributed. Mathematically, it is given by:
\( \text{Pressure} = \frac{\text{Force}}{\text{Area}} \)
The standard unit of force is Newton (N), and the standard unit of area is square meter (\(m^2\)). Therefore, the unit of pressure is Newton per square meter (\(N/m^2\)). This unit is also known as Pascal (Pa).
\( 1 \text{ Pa} = 1 \text{ N/m}^2 \)
Analyzing the Given Options and Their Units
Let's examine the units of the physical quantities given in the options:
Angular Momentum
Angular momentum is a measure of the rotational inertia of a body. For a point mass, it is given by \( \mathbf{L} = \mathbf{r} \times \mathbf{p} \), where \( \mathbf{r} \) is the position vector and \( \mathbf{p} \) is the linear momentum (\( \mathbf{p} = m\mathbf{v} \)).
The unit of position (\( r \)) is meter (m). The unit of mass (\( m \)) is kilogram (kg). The unit of velocity (\( v \)) is meter per second (m/s).
So, the unit of linear momentum (\( p \)) is \( kg \cdot m/s \).
The unit of angular momentum (\( L \)) is \( m \cdot (kg \cdot m/s) = kg \cdot m^2/s \).
This unit is different from the unit of pressure (\( N/m^2 \)).
Stress
Stress is a physical quantity that describes the internal forces that neighboring particles of a continuous material exert on each other. It is defined as the force acting on a unit area within a material.
Like pressure, stress is defined as:
\( \text{Stress} = \frac{\text{Force}}{\text{Area}} \)
Since the definition is the same as pressure (Force per Area), the unit of stress is also Newton per square meter (\( N/m^2 \)), or Pascal (Pa).
This unit is the same as the unit of pressure.
Strain
Strain is a measure of the deformation of a material caused by stress. It is defined as the ratio of the change in dimension to the original dimension.
For example, linear strain is \( \text{Strain} = \frac{\text{Change in length}}{\text{Original length}} \).
Since strain is a ratio of two lengths (or other dimensions), its units cancel out. Strain is a dimensionless quantity, or it can be expressed as a pure number or a percentage.
This unit (dimensionless) is different from the unit of pressure (\( N/m^2 \)).
Work
Work is defined as the energy transferred to or from an object via the application of force along a displacement. It is given by \( W = \mathbf{F} \cdot \mathbf{d} \), where \( \mathbf{F} \) is the force and \( \mathbf{d} \) is the displacement.
The unit of force (\( F \)) is Newton (N). The unit of displacement (\( d \)) is meter (m).
So, the unit of work (\( W \)) is Newton meter (N·m). This unit is also known as Joule (J).
This unit is different from the unit of pressure (\( N/m^2 \)).
Comparing Units of Physical Quantities
Let's summarize the units in a table:
| Physical Quantity | Definition | Unit | Equivalent Unit |
|---|---|---|---|
| Pressure | Force / Area | \(N/m^2\) | Pascal (Pa) |
| Angular Momentum | \(r \times p\) | \(kg \cdot m^2/s\) | \(J \cdot s\) |
| Stress | Force / Area | \(N/m^2\) | Pascal (Pa) |
| Strain | Change in dimension / Original dimension | Dimensionless | None (Ratio) |
| Work | Force · Displacement | \(N \cdot m\) | Joule (J) |
From the table, it is clear that Stress has the same unit as Pressure (\(N/m^2\) or Pascal).
Conclusion on Pressure and Stress Units
Based on the analysis of the units of pressure, angular momentum, stress, strain, and work, we find that stress is the physical quantity that shares the same unit as pressure. Both are defined as force per unit area.
Revision Table: Key Units in Mechanics
| Physical Quantity | Formula/Definition | SI Unit | Derived Unit |
|---|---|---|---|
| Force | Mass × Acceleration | \(kg \cdot m/s^2\) | Newton (N) |
| Area | Length × Width | \(m^2\) | - |
| Pressure | Force / Area | \(N/m^2\) | Pascal (Pa) |
| Stress | Force / Area | \(N/m^2\) | Pascal (Pa) |
| Work/Energy | Force × Displacement | \(N \cdot m\) | Joule (J) |
| Power | Work / Time | \(J/s\) | Watt (W) |
| Angular Momentum | \(r \times p\) | \(kg \cdot m^2/s\) | \(J \cdot s\) |
| Strain | Ratio of lengths | Dimensionless | - |
Additional Information on Stress and Pressure
While stress and pressure have the same units and are both defined as force per unit area, there's a conceptual difference. Pressure is typically associated with fluids (liquids and gases) and acts perpendicular to a surface, usually uniformly in all directions at a given point. Stress, on the other hand, is used to describe forces within solid materials and can be tensile (pulling), compressive (pushing), or shear (tangential). Stress can exist in different directions within the material, often described by a stress tensor.
Different types of stress include:
- Normal Stress: Force acting perpendicular to the surface (tensile or compressive). This is most analogous to pressure.
- Shear Stress: Force acting parallel or tangential to the surface.
Despite these conceptual differences, their dimensions and standard units remain the same: force per unit area.
Important Questions from Units, Dimensions and Measurements
Which one of the following is the usual unit of measurement for Air Pressure used in India?
Which one of the following is the correct relation between Ȧ and nm?
A student measure certain lengths using a meter scale having least count equal to 1 mm. Which of the following measurements are more precise?
The SI unit of acceleration is
Light year is a unit for measurement of
