Physics Questions and Answers

Answer: C

In a static liquid, the pressure is the same at every point at the same horizontal level.

Pressure depends only on depth, \(P = P_0 + \rho gh\), so all points at one depth are at the same pressure whatever the shape of the vessel. If they were not, liquid would flow until they were - which is why connected vessels settle at a common level.

Answer: B

This is the Doppler effect.

The observed frequency shifts because successive wavefronts have less distance to travel when source and observer approach: \[f' = f\left(\frac{v \pm v_o}{v \mp v_s}\right)\] It explains the falling pitch of a passing horn and the redshift of receding galaxies.

Answer: C

This is streamline, or laminar, flow.

Every particle passing a given point follows the same path with the same velocity, so the streamlines never cross. Above a critical speed the flow becomes turbulent and the motion at each point varies irregularly with time.

Answer: A

This is viscous force.

Adjacent layers of fluid moving at different speeds exert a tangential drag on each other: \[F = \eta A \frac{dv}{dx}\] Viscosity is in effect internal friction, and it is what makes honey pour more slowly than water.

Answer: A

Hooke's law states that stress is directly proportional to strain, within the elastic limit.

\[\text{stress} = E \times \text{strain}\] where the constant \(E\) is the modulus of elasticity. Beyond the elastic limit the proportionality fails and the material does not return to its original shape.

Answer: A

True. This is Archimedes' principle.

The apparent loss in weight equals the weight of the fluid displaced: \[F_B = \rho_{fluid}V_{displaced}\,g\] It follows from the pressure being greater at the bottom of the immersed body than at the top.

Answer: A

This is Stefan's law, or the Stefan-Boltzmann law.

\[E = \sigma T^4\] with \(\sigma = 5.67\times10^{-8}\ \mathrm{W\,m^{-2}\,K^{-4}}\). The fourth power means a small rise in temperature increases radiation sharply - doubling the absolute temperature multiplies the emission by sixteen.

Answer: A

This is adhesive force.

Adhesion acts between molecules of different substances - water and glass, ink and paper. Cohesion acts between molecules of the same substance. Whether a liquid wets a surface or beads up depends on which of the two is stronger.

Answer: A

Both are types of mechanical wave.

Mechanical waves need a material medium to travel through, and they differ in the direction of particle motion: along the wave for longitudinal, at right angles for transverse. Electromagnetic waves are non-mechanical and need no medium.

Answer: B

This is the critical velocity.

Above it the flow turns turbulent. It follows from the Reynolds number: \[v_c = \frac{R_e\,\eta}{\rho D}\] Flow is laminar below about \(R_e = 2000\) and turbulent above about 3000.