General Science MCQs – Page 5 (Q41-50)

The turning effect of a force about a point or axis is called torque, calculated as the force applied multiplied by the perpendicular distance from the pivot point, known as the lever arm or moment arm. Torque explains why it is easier to loosen a tight bolt with a longer wrench, since a greater lever arm produces more turning effect for the same applied force. It plays a crucial role in engineering applications from car engines, where torque determines pulling power, to simple everyday actions like turning a doorknob or opening a jar lid.

Torque is measured in newton-meters (N·m) in the SI system, reflecting its calculation as force multiplied by the perpendicular distance from the pivot, or lever arm. Although a newton-meter has the same dimensional units as a joule, the unit used for energy and work, torque and energy are conceptually different, so the newton-meter is kept separate from the joule to avoid confusion. Car manufacturers commonly advertise engine torque in newton-meters or pound-feet, since it indicates how much rotational force an engine can generate, which is especially important for towing and acceleration.

The center of gravity of a body is the point where its entire weight appears to act, effectively balancing all the gravitational forces distributed throughout the object's mass. For a uniform, symmetrical object like a sphere or cube, the center of gravity coincides with its geometric center, but for irregular shapes it can lie elsewhere, even outside the physical object, as with a boomerang or a donut. Engineers and athletes both rely on understanding center of gravity, whether designing stable vehicles and buildings or improving balance in sports like gymnastics and diving.

A body is said to be in equilibrium when the net force acting on it is zero, meaning all forces balance out so the object either remains at rest (static equilibrium) or continues moving at constant velocity (dynamic equilibrium). True equilibrium also typically requires zero net torque, so the object does not begin rotating. Everyday examples include a book resting on a table, where gravity pulling down is balanced by the table's normal force pushing up, and a car moving at a constant speed on a straight highway. Equilibrium is central to structural engineering, ensuring bridges and buildings remain stable.

Scissors are a common example of a first-class lever because the pivot, or fulcrum, sits between the applied effort (your hand) and the load (the material being cut), similar to a see-saw. Each blade of a pair of scissors acts as a separate first-class lever, and the cutting edges act like a pair of wedges that concentrate force to slice through material. First-class levers can either multiply force or increase range of motion depending on where the fulcrum sits relative to the effort and load. Other first-class lever examples include crowbars, pliers, and see-saws.

A wheelbarrow is a common example of a second-class lever, where the load sits between the fulcrum (the wheel) and the effort (the handles being lifted), allowing a person to move heavy loads with reduced effort. This arrangement always provides a mechanical advantage greater than one, since the load is closer to the fulcrum than the effort is, though it requires moving the handles a greater distance than the load moves. Other second-class lever examples include nutcrackers, bottle openers, and doors, all of which trade distance for reduced force to lift or move something.

The human forearm lifting a weight, such as during a bicep curl, is an example of a third-class lever, where the effort (the bicep muscle pulling near the elbow) sits between the fulcrum (the elbow joint) and the load (the weight in the hand). Third-class levers always require more effort than the load being moved, but they allow for greater speed and range of motion at the load end, which is why the human body uses this arrangement extensively for fast, dexterous movements. Fishing rods and tweezers work on the same third-class lever principle.

The efficiency of a machine is the ratio of its output work to its input work, usually expressed as a percentage, and it indicates how much of the energy put into a machine is actually converted into useful work rather than lost. No real machine achieves 100 percent efficiency because some energy is always lost to friction, heat, sound, or deformation within the moving parts. Engineers strive to maximize efficiency through lubrication, better materials, and improved design, since higher efficiency machines waste less energy and often perform better, whether in car engines, electric motors, or simple hand tools.

No real machine can be 100 percent efficient mainly due to energy losses from friction between moving parts, which converts some of the input mechanical energy into heat and sound rather than useful output work. Even well-lubricated machines lose some energy this way, and other factors like air resistance and material deformation can contribute further losses. This is consistent with the second law of thermodynamics, which states that some energy in any real process is inevitably dissipated as unusable heat. Engineers measure efficiency to compare machine designs and identify where improvements can reduce wasted energy.

Work done on an object is calculated as the force applied multiplied by the displacement of the object in the direction of that force (W = Fd cos theta), meaning no work is done if there is no movement, regardless of how much force is applied. For example, pushing against a stationary wall, no matter how hard, technically does zero work in the physics sense because the wall does not move. Work is measured in joules, the same unit as energy, since doing work on an object transfers energy to it, such as increasing its kinetic or potential energy.