All Free Physics MCQs with Answers

31. Bernoulli's equation is essentially a statement of

• A. conservation of mass
• B. conservation of momentum
• C. conservation of energy for a flowing fluid
• D. Newton's third law

Explanation: The equation adds pressure energy, kinetic energy and potential energy per unit volume and states that the total is the same all along a streamline for steady, non viscous, incompressible flow. Conservation of mass is expressed separately by the continuity equation. The restriction to non viscous flow is why the equation only approximates real pipes.

32. According to Bernoulli's principle, where the speed of a fluid is high, its pressure is

• A. high
• B. low
• C. unchanged
• D. zero

Explanation: Since the total energy per unit volume is fixed, any gain in kinetic energy must be paid for by a fall in pressure energy. This is why a shower curtain is pulled inwards by the fast moving water stream and why two ships moving side by side are drawn together. The effect is easy to state and easy to misremember, so it is heavily tested.

33. The lift on an aeroplane wing arises because

• A. air moves faster over the curved upper surface, lowering the pressure there
• B. air moves faster under the wing
• C. the wing is lighter than air
• D. the engines push the wing upwards

Explanation: The aerofoil shape makes the air travel further and faster across the top, so by Bernoulli's principle the pressure above is lower than below and the difference multiplied by the wing area gives the lift. The wing also deflects air downwards, and by Newton's third law that contributes lift as well. Both descriptions are valid ways of accounting for the same force.

34. A Venturi meter measures the rate of flow of a fluid by using

• A. the temperature rise in a constriction
• B. the pressure difference between a wide section and a narrow section
• C. the weight of the fluid
• D. the colour change of the fluid

Explanation: The fluid speeds up in the throat, so its pressure falls, and measuring that pressure drop with a manometer gives the speed through the continuity and Bernoulli equations. Because it has no moving parts it is reliable and causes little permanent pressure loss. The same principle drives a carburettor and a filter pump.

35. The SI unit of coefficient of viscosity is

• A. N s per m squared
• B. N per m squared
• C. N m
• D. m squared per s

Explanation: Viscosity is defined through the relation between shear stress and velocity gradient, giving units of newton second per square metre, equivalently the pascal second. The pascal, newton per square metre, is pressure rather than viscosity. In the older CGS system the unit was the poise, and one pascal second equals ten poise.

36. As the temperature rises, the viscosity of a liquid

• A. increases
• B. decreases
• C. stays the same
• D. becomes zero

Explanation: Heating gives the molecules enough energy to slip past one another more easily, weakening the intermolecular attractions that resist flow, which is why warm honey pours readily. Gases behave in the opposite way: their viscosity rises with temperature because it comes from molecular collisions rather than cohesion. Confusing the two cases is the usual mistake.

37. Blood pressure measured in a major artery is highest

• A. where the artery is narrowest
• B. where the blood flows fastest
• C. where the vessel is widest and the blood moves slowest
• D. at the capillaries

Explanation: Bernoulli's principle applies inside the body as well: where the cross section is large the speed is low and more of the energy remains as pressure. Where a vessel narrows, the blood accelerates and the lateral pressure on the wall falls. This is one reason an aneurysm, a widened weak spot, experiences a higher pressure and tends to grow.

38. Upthrust on a body immersed in a fluid is equal to

• A. the weight of the body
• B. the weight of the fluid displaced by the body
• C. the volume of the body
• D. the density of the fluid

Explanation: This is Archimedes's principle, and the upthrust arises because pressure increases with depth, so the upward force on the bottom of the body exceeds the downward force on the top. A body floats when the upthrust at partial immersion already equals its weight. The principle also explains why a body appears lighter when weighed in water.

39. Two spheres of the same size but different densities fall through the same viscous liquid. The denser one has

• A. the same terminal velocity
• B. a lower terminal velocity
• C. a higher terminal velocity
• D. no terminal velocity

Explanation: Terminal velocity depends on the difference between the density of the sphere and that of the fluid, so a denser sphere needs a larger drag force to balance its greater weight and must therefore fall faster to generate it. Radius and viscosity being equal, density is the only variable left. If the sphere were less dense than the liquid it would rise instead.

40. A skydiver opens a parachute. The terminal velocity

• A. falls, because the larger area increases the drag at a given speed
• B. rises, because the parachute adds weight
• C. is unchanged
• D. becomes zero immediately

Explanation: Opening the canopy multiplies the effective area and therefore the drag, so the forces balance again at a much lower speed and the diver decelerates towards the new terminal velocity. The weight is essentially unchanged, so it is the drag side of the balance that has altered. The speed never reaches zero, which is why a landing roll is still needed.