General Science MCQs – Page 10 (Q91-100)

In uniform circular motion, an object's speed remains constant, but its velocity direction continuously changes because velocity is a vector quantity, and the object is always heading in a new direction as it traces its circular path. This constantly changing direction means the object experiences centripetal acceleration directed toward the center of the circle, even though its speed never changes. Newton's second law shows this acceleration requires a continuous centripetal force, such as gravity for orbiting satellites or tension for a ball on a string, keeping the object curving inward rather than flying off in a straight line.

In uniform circular motion, speed remains constant throughout the motion, even though the object's velocity direction, and therefore its acceleration, is continually changing as it moves around the circular path. This is why the term uniform refers specifically to constant speed rather than constant velocity, since velocity as a vector also depends on direction. The centripetal force responsible for this circular path acts perpendicular to the object's velocity at every instant, which is precisely why it changes direction without doing any work on the object or altering its speed, following the physics of circular motion.

The natural tendency of a body to resist any change in its state of motion, whether starting to move, stopping, or changing direction, is called inertia, a concept formalized in Newton's first law of motion. Inertia is directly proportional to an object's mass, meaning more massive objects require greater force to accelerate, decelerate, or redirect than lighter ones. This is why a loaded truck is much harder to start moving or stop than an empty bicycle. The word inertia comes from the Latin for idleness, and Galileo's early experiments on motion laid important groundwork before Newton formalized the concept.

Between a loaded truck and a bicycle moving at the same speed, the truck has far greater inertia, since inertia is directly proportional to an object's mass rather than its speed or velocity. This is why stopping a moving truck requires much more braking force and distance than stopping a bicycle traveling at the same speed, and why collisions involving heavier vehicles tend to be more destructive. Inertia should not be confused with momentum, which combines both mass and velocity, though the two concepts are related since a more massive object also carries more momentum at a given speed.

Work done in lifting an object against gravity is stored in the object as gravitational potential energy, calculated as mass times gravitational acceleration times height (PE = mgh). This stored energy can later convert back into kinetic energy as the object falls, illustrating the principle of conservation of mechanical energy in the absence of friction or air resistance. This is the operating principle behind hydroelectric dams, pendulum clocks, and pile drivers, all of which raise a mass to store energy that is later released to do useful work. The higher an object is lifted, the more potential energy it stores.

Passengers in a bus jerk forward when the bus suddenly stops because of Newton's first law of motion, the law of inertia, which states that a body in motion tends to remain in motion unless acted upon by an external force. While the bus itself decelerates due to braking friction, the passengers' bodies continue moving forward at the original speed until seatbelts, seats, or their own muscles apply a stopping force. This same principle explains why seatbelts are essential safety devices, and why sudden acceleration instead pushes passengers backward into their seats.

The term describing total distance traveled divided by total time taken is average speed, a scalar quantity that summarizes overall motion without capturing variations in speed along the way. This differs from instantaneous speed, which describes how fast an object is moving at a single specific moment, such as a speedometer reading at any instant. For example, a car's average speed over a long trip may be much lower than its peak instantaneous speed if it stopped at traffic lights or slowed in traffic. Average velocity, by contrast, considers displacement and direction rather than total distance traveled.

A ball rolling on a frictionless horizontal surface would continue moving forever at constant velocity according to Newton's first law of motion, the law of inertia, since no external force acts to slow it down or change its direction. In the real world, friction between the ball and the surface, along with air resistance, gradually removes kinetic energy as heat, eventually bringing the ball to rest. This idealized frictionless scenario is a common thought experiment used to illustrate the pure concept of inertia, isolated from the complicating real-world effects that normally mask it.

The rate of change of momentum of a body is directly proportional to the applied force, a statement that is essentially a more general form of Newton's second law of motion, since force equals mass times acceleration, and acceleration is itself the rate of change of velocity. This momentum-based formulation, F = dp/dt, is especially useful in situations where mass itself changes over time, such as a rocket burning fuel and losing mass as it accelerates. Newton originally expressed his second law in terms of momentum rather than the simpler F = ma form commonly taught today.

The joule, the SI unit of energy, is equivalent to one newton of force multiplied by one meter of displacement, reflecting the definition of work as force times distance moved in the direction of that force. This means lifting an object with a weight of one newton to a height of one meter does exactly one joule of work against gravity. The joule, named after James Prescott Joule, is used to measure all forms of energy, including kinetic, potential, thermal, and electrical energy, unifying these concepts under a single consistent unit throughout physics.