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What is true of the velocity of a pendulum bob at its lowest point?

A pendulum is a weight suspended from a pivot so that it can swing freely back and forth under the influence of gravity. Pendulums are a classic example of periodic motion, where the motion repeats in a regular cycle. The velocity of a pendulum refers to how fast it is moving at a particular point during its cycle. Understanding the velocity of a pendulum at different points helps explain its periodic motion.

The Motion of a Pendulum

A simple pendulum consists of a mass (the pendulum bob) suspended by a string or rod from a fixed pivot point. It moves with a repetitive to-and-fro motion under the influence of gravity. On its downward swing, it accelerates due to gravity. At the lowest point, its velocity momentarily becomes zero as the bob changes direction. It then decelerates on its upward swing as gravity brings it to a momentary halt at its highest point before gravity accelerates it downward again, repeating the cycle.

The time taken for a complete back-and-forth cycle depends on the length of the pendulum. Longer pendulums have a longer time period than shorter ones. But regardless of the pendulum length, the velocity at different points follows the same pattern.

Factors Affecting Pendulum Motion

Three key factors affect the motion of a pendulum:

  • The length of the pendulum (the distance from the pivot to the center of mass of the bob)
  • The mass of the bob
  • The gravitational field strength (on Earth this is approximately 9.8 m/s2)

For small displacements, changing the mass and gravitational field strength has no effect on the time period of a pendulum. Only the length affects the time period. Longer pendulums take longer to complete one full cycle.

Velocity of a Pendulum Bob

The velocity of a pendulum bob refers to its instantaneous speed at any given point in its swing. It is constantly changing over the course of one full cycle.

Starting from rest at its lowest point, the velocity increases steadily as the bob swings upwards. At the maximum height, the velocity momentarily becomes zero again before the bob accelerates downward under gravity. The velocity increases until the bob reaches its lowest point again, where velocity is momentarily zero.

The velocity follows a sinusoidal pattern, reaching equal maximum magnitudes in the upward and downward directions but with opposite signs. The maximum velocity occurs at the midpoint of its swing.

Key Points About Pendulum Bob Velocity

  • Velocity is zero at the highest and lowest points.
  • Velocity reaches a maximum at the midpoint of the swing.
  • Velocity direction reverses between upward and downward motion.
  • Velocity magnitude follows a regular sinusoidal pattern.

Velocity is Zero at the Lowest Point

At its lowest point, the pendulum bob has zero velocity. This is because it has reached its maximum displacement from the equilibrium hanging position and momentarily comes to a halt before beginning motion in the opposite direction.

The velocity is zero because:

  • All its potential energy has been converted to kinetic energy.
  • The kinetic energy is instantly converted back to potential energy as it changes direction.
  • With no kinetic energy, its velocity is momentarily zero.

This instant where its velocity is zero is called the “turning point” of the pendulum. It represents the transition from downward motion to upward motion.

Why Velocity Must Be Zero

Looking at the forces acting on the pendulum bob at its lowest point explains why the velocity is zero:

  • Only force acting is the restoring force due to the suspending rod or string.
  • This acts perpendicular to the arc motion, not along it.
  • So at that instant, there is no force parallel to the direction of motion.
  • Since force = mass x acceleration, and mass is constant, acceleration must be zero.
  • And since velocity = acceleration x time, if acceleration is zero, velocity must also be zero.

In summary, at its lowest point, the pendulum bob has no force acting along its arc motion to cause any continued acceleration. So its velocity has to become zero.

Explaining Zero Velocity With Energy Conservation

The conversion between potential and kinetic energy also explains why velocity must become zero:

  • At maximum displacement, all potential energy is converted to kinetic energy
  • All this kinetic energy is instantly converted back to potential energy at the turning point
  • With no kinetic energy, the velocity becomes zero

The kinetic energy K is proportional to the square of velocity:

K = (1/2)mv2

So if K becomes zero, v must also become zero.

Velocity-Time and Displacement-Time Graphs

Graphs of velocity and displacement against time also illustrate why velocity must become zero:

  • On a v-t graph, velocity reaches zero at the peaks and troughs
  • On an x-t graph, peaks and troughs represent the maximum displacements
  • Velocity is zero when displacement is maximum, and vice versa

So when analyzing pendulum motion, graphs provide further confirmation that the velocity is momentarily zero whenever displacement is at a maximum.

Importance of Zero Velocity at Lowest Point

The fact that velocity is zero at the lowest point is a key characteristic of pendulum motion. Some key implications are:

  • It represents a transition between kinetic and potential energy
  • It shows the velocity reversing direction as the bob changes direction
  • It determines the period of small oscillations for a given pendulum length
  • It confirms there is no net force acting along the arc at this point

This characteristic of zero velocity is what makes the pendulum periodic. If velocity remained non-zero at the lowest point, the bob would not change direction and would not keep oscillating. The zero velocity is essential to allow continual motion.

Zero Velocity and Pendulum Period

For small angular displacements, the time period T of a simple pendulum depends only on its length L and the gravitational field strength g:

T = 2π√(L/g)

This relationship comes from an analysis of the motion, specifically involving the velocity becoming zero at the lowest point. So the zero velocity is key to deriving the time period.

Examples and Applications

The zero velocity at the lowest point applies in any example of pendulum motion. Some examples include:

  • A clock pendulum
  • A mass suspended from a spring
  • A wrecking ball
  • A child swinging on a playground swing

In all these cases, the object will have zero velocity when it reaches its maximum displacement to either side, before swinging back the other way.

Pendulum Clock Example

In a pendulum clock, the pendulum swings back and forth, with the velocity becoming zero briefly each time it reaches its lowest point. The time period of the pendulum determines the clock’s tick rate. Knowing the velocity characteristics allows an appropriate pendulum length to be chosen to keep correct time.

Mass-Spring System Example

A mass bouncing up and down on a spring exhibits similar motion to a pendulum. At the maximum stretches, the velocity is momentarily zero as the mass changes direction. The zero velocities at the turnaround points allow the repetitive oscillating motion.


At its lowest point, the velocity of a pendulum bob is zero. This occurs because:

  • All potential energy has been converted to kinetic energy
  • This kinetic energy is instantly converted back to potential energy
  • With no kinetic energy, velocity becomes zero
  • There is no force parallel to the arc motion at this point

This zero velocity represents the transition from downward to upward motion. It is a key characteristic that makes the motion periodic. The zero velocity allows derivation of the pendulum time period.

Understanding that the velocity is zero at the lowest point provides insight into the nature of pendulum motion. It is an important concept in physics with many applications.