All Free Physics MCQs with Answers

41. In streamline flow through a horizontal pipe of uniform cross section, Bernoulli's equation predicts that the pressure

• A. rises steadily along the pipe
• B. stays the same along the pipe for an ideal fluid
• C. falls to zero at the outlet
• D. depends on the colour of the pipe

Explanation: With constant area the speed is constant by continuity, and with no height change the potential term is constant too, so the pressure term must also be constant for an ideal fluid. Real fluids do lose pressure along such a pipe because viscosity dissipates energy, which is exactly the effect Bernoulli's equation leaves out. Comparing the ideal prediction with the real behaviour is how the role of viscosity is demonstrated.

42. The flow of a fluid becomes turbulent when

• A. the speed falls below a critical value
• B. the speed exceeds a critical value and the Reynolds number becomes large
• C. the pipe is perfectly smooth
• D. the fluid is very viscous

Explanation: Turbulence sets in when inertial effects dominate viscous ones, which is what a large Reynolds number expresses, and it appears as eddies and rapid mixing. A high viscosity favours laminar flow rather than turbulence, since it damps the disturbances out. Turbulent flow carries far more resistance, which matters in pipelines, aircraft design and blood vessels alike.

43. Water flows at 2 m per second through a pipe of cross sectional area 0.02 m squared. The volume flow rate is

• A. 0.04 cubic metres per second
• B. 0.01 cubic metres per second
• C. 4 cubic metres per second
• D. 0.4 cubic metres per second

Explanation: Flow rate is area multiplied by speed, so 0.02 times 2 gives 0.04 cubic metres per second, that is 40 litres every second. Dividing rather than multiplying gives the 0.01 distractor. The same product appears on both sides of the continuity equation, which is why it is constant along a pipe.

44. A spinning cricket ball swings in flight because

• A. gravity acts sideways on a spinning body
• B. the spin makes the ball lighter
• C. air moves faster on one side of the ball, creating a pressure difference across it
• D. the ball changes shape in the air

Explanation: The spinning surface drags air round with it, so the airflow is faster on one side and slower on the other, and by Bernoulli's principle the resulting pressure difference pushes the ball sideways. This sideways force on a spinning body is the Magnus effect, and it is why a swing bowler polishes one side of the ball. The same physics curves a football and lifts a golf ball.

45. One radian is defined as the angle subtended at the centre of a circle by an arc whose length is

• A. equal to the radius
• B. equal to the diameter
• C. equal to the circumference
• D. one metre

Explanation: Since the arc length equals the radius times the angle in radians, an arc of one radius length subtends exactly one radian, about 57.3 degrees. A full circle is therefore 2 pi radians, because the circumference is 2 pi times the radius. Defining the angle this way is what makes the arc length formula and the link between linear and angular quantities so simple.

46. An angle of 180 degrees expressed in radians is

• A. pi over 2
• B. pi
• C. 2 pi
• D. 3 pi over 2

Explanation: A complete revolution of 360 degrees is 2 pi radians, so half a revolution is pi radians, roughly 3.14. A quarter turn of 90 degrees is pi over 2. Converting between the two is simply multiplying by pi over 180 in one direction and by 180 over pi in the other.

47. The relationship between linear speed v and angular speed omega for a body moving in a circle of radius r is

• A. v equals omega divided by r
• B. v equals r divided by omega
• C. v equals r omega
• D. v equals r squared omega

Explanation: In one second the body sweeps an angle omega, covering an arc of length r omega, so the linear speed is r omega with omega in radians per second. This is why two points on a rotating disc share the same angular speed while the outer point moves faster in a straight line sense. The same relationship links angular and linear acceleration as a equals r alpha.

48. A wheel rotating at 300 revolutions per minute has an angular speed of about

• A. 5 radians per second
• B. 31.4 radians per second
• C. 300 radians per second
• D. 1885 radians per second

Explanation: 300 revolutions per minute is 5 revolutions per second, and each revolution is 2 pi radians, so omega is 5 times 2 pi, which is 31.4 radians per second. Forgetting to convert revolutions into radians leaves the answer as 5, the commonest error here. The value 1885 comes from using minutes rather than seconds.

49. A body moving in a circle at constant speed

• A. has zero acceleration
• B. is accelerating, because the direction of its velocity is continuously changing
• C. has a constant velocity
• D. experiences no net force

Explanation: Velocity is a vector, so changing its direction is a change in velocity even when the magnitude is fixed, and that change per unit time is the centripetal acceleration directed towards the centre. A net force must therefore act, supplied by tension, gravity, friction or the normal force depending on the situation. This is the single most tested idea in circular motion.

50. The centripetal acceleration of a body moving in a circle of radius r with speed v is

• A. v squared divided by r, directed towards the centre
• B. v squared times r, directed away from the centre
• C. v divided by r squared, directed along the tangent
• D. zero, since the speed is constant

Explanation: The acceleration has magnitude v squared over r, equivalently r omega squared, and always points at the centre, which is why it is called centripetal or centre seeking. It changes only the direction of motion and never the speed, since it is perpendicular to the velocity at every instant. That perpendicularity is also why it does no work.