TURNING EFFECT OF FORCE
Q-What is the main difference between the “moment of force” and a “couple”?
A-Moment of Force: It is the turning effect of a single applied force about a pivot point. It can produce both translational and rotational motion if the pivot isn’t fixed.
Couple: It consists of two equal and opposite parallel forces whose lines of action do not coincide. A couple produces pure rotational motion without any translational acceleration
Q-Why is it easier to open a heavy door by pushing it at the edge furthest from the hinges rather than near the hinges?
A-The turning effect, or torque, depends on both the applied force (F) and the perpendicular distance (r) from the axis of rotation (torque = F x r). By pushing at the edge furthest from the hinges, you maximize the distance r, which minimizes the effort (force) required to produce the same torque needed to rotate the door.
Q-A steering wheel is turned using two hands rather than one. What mechanical advantage does this offer?
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A-Using two hands applies two equal and opposite forces on opposite sides of the wheel, forming a couple. The forces cancel each other out translationally, resulting in no net linear force on the steering column. This minimizes friction and wear on the axle while efficiently generating pure rotational torque.
Q-Can a body be in translational equilibrium but not in rotational equilibrium? Give an example.
A-Yes. When a couple acts on a body, the net force is zero (F – F = 0), so the body is in translational equilibrium. However, because the forces act along different lines, they produce a non-zero net torque, causing the body to rotate (not in rotational equilibrium). A classic example is a compass needle aligning with a magnetic field.
Q-If the net force acting on a system is zero, is the net torque about any point necessarily zero?
A-No. If a system is subject to a couple, the net force is exactly zero, but the net torque about any point is non-zero and equal to the product of one of the forces and the perpendicular distance between them
Q-Is centrifugal force a real force? Explain why a passenger feels thrown outward when a car takes a sharp turn.
A-No, centrifugal force is a pseudo (or fictitious) force. It only appears to exist when observed from a rotating (non-inertial) frame of reference.
When a car turns sharply, the passenger feels thrown outward due to inertia. The passenger’s body wants to continue moving in a straight line, while the car forces a change in direction inward. To the passenger inside the accelerating car, it feels like an outward force is acting on them.
Q-What provides the necessary centripetal force in the following scenarios?
A-A planet orbiting the Sun: Gravitational force between the Sun and the planet.
An electron orbiting a nucleus: Electrostatic force of attraction between the positive nucleus and negative electron.
A car taking a turn on a flat road: Friction between the car’s tires and the road surface.
Q-Why are racing tracks or highway curves “banked” (sloped inwards)?
A- On a flat road, only friction provides the centripetal force. At high speeds, friction might not be enough, causing the vehicle to skid. By banking the curve, the normal reaction force (N) of the road tilts inward. The horizontal component of this normal force (N sin x ) provides the required centripetal force, reducing reliance on tire friction and making high-speed turns safer. ( x is angle)
Q-A washing machine spinner uses centrifugal action to dry clothes. How does this mechanism work?
A-The drum rotates at high speeds. The wet clothes are forced into a circular path by the walls of the drum. However, the water droplets on the clothes cannot experience enough centripetal force (adhesion to the fabric isn’t strong enough) to maintain that tight circular path. As a result, due to their inertia, the droplets fly off tangentially through the perforations in the drum.
Q-What is the difference between the Center of Gravity (CG) and the Center of Mass (CM)?
A-Center of Mass: The unique point where the entire mass of the body is concentrated, independent of a gravitational field.
Center of Gravity: The point where the entire weight of the body acts (the resultant of all parallel gravitational forces acting on each particle).
Q-Can the center of gravity of a body lie outside its physical material? Give two examples.
A-Yes, the center of gravity does not need to be located on the physical material of an object. Examples include:
A hollow ring or doughnut (CG is at the geometric center, in empty space).
A boomerang or a L-shaped metallic wire (CG lies in the air space between the bends).
Q-Why does a tightrope walker carry a long, heavy pole?
A-The pole serves two major purposes:
It increases the walker’s rotational inertia. If the walker begins to tip, rotating the heavy pole creates a counteracting torque, allowing them more time to adjust their feet back over the rope.
It lowers the overall center of gravity of the walker-pole system, making the equilibrium more stable.
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Q-Explain the three states of equilibrium with respect to the displacement of the Center of Gravity.
A-1. Stable Equilibrium: If displaced slightly, the CG rises. A restoring torque brings the body back to its original position (e.g., a ball at the bottom of a bowl).
2. Unstable Equilibrium: If displaced slightly, the CG lowers immediately. The body moves further away from its original position (e.g., a cone balanced perfectly on its apex).
3. Neutral Equilibrium: If displaced, the height of the CG remains constant. The body stays in its new position (e.g., a ball rolling on a flat table).
Q-Why does a person lean forward when carrying a heavy backpack?
A-When a person puts on a heavy backpack, the combined center of gravity of the person and the pack shifts backward, falling behind their base of support (their feet). To prevent falling backward, the person leans forward to shift the combined CG back over their feet, restoring stable balance.
Q-Why do double-decker buses have heavy steel frames at the bottom and are kept light or open at the top?
A-This design ensures that the center of gravity of the bus remains as low as possible. A lower CG means the bus can tilt to a relatively sharp angle during turns or on uneven roads without the vertical line dropping from its CG falling outside its wheel base, preventing it from toppling over.
Q-When a car accelerates rapidly forward, why does the front end tend to lift slightly while the rear dips?
A-The forward force driving the car acts at the contact point between the tires and the road. However, the mass of the car resists this change at its center of mass, which sits higher than the road. This mismatch creates a rotational torque that rotates the front of the car upward and compresses the rear suspension.