Final answer:
The primary force acting on a pendulum's bob during its swing is the force of gravity, which causes a restoring force and enables the bob to oscillate in simple harmonic motion. No work is done on the pendulum in the absence of external forces or friction, as the force and displacement are perpendicular at the extreme positions of the swing.
Step-by-step explanation:
When a pendulum swings back and forth, the primary force acting on the hanging object is the force of gravity. However, in terms of work, if we assume that the pendulum is swinging without any external interference or air resistance, then no work is being done on the object.
This is because for work to be done on an object in physics, the force must cause displacement in the direction of the force. Since the force of gravity acts downwards while the displacement of the pendulum bob is at a right angle to the force of gravity at its extreme positions, no work is done by gravity.
The force of gravity acts on the pendulum bob's center of mass, causing a restoring force that is proportional to the displacement for small angles of swing, a characteristic of simple harmonic motion. Additionally, the pendulum is affected by the period, which is a function of the pendulum's length and the acceleration due to gravity, but not by the mass of the bob.
The restoring force provides a torque that causes the pendulum to swing; this force is the component of the bob's weight that acts tangent to the arc of the pendulum's swing. According to Newton's first law, this oscillation implies that forces are acting on the object; otherwise, it would move in a straight line at a constant speed.
The force acting in the opposite direction of the angle's increase generates a torque T = rFsinθ, with r being the length of the pendulum string and θ the angle of displacement from the equilibrium.