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### Course: Class 11 Physics (India)>Unit 18

Lesson 3: Simple pendulums

# Simple pendulum review

Overview of key terms, equations, and skills for simple pendulums, including how to analyze the forces on the mass.

## Key terms

TermMeaning
Simple pendulumA mass suspended from a light string that can oscillate when displaced from its rest position.

## Equations

EquationSymbolsMeaning in words
${T}_{p}=2\pi \sqrt{\frac{l}{g}}$${T}_{p}$ is period, $l$ is pendulum length, and $g$ is the acceleration due to gravityThe pendulum’s period is proportional to the square root of the pendulum’s length and inversely proportional to the square root of $g$

## Analyzing the forces on a simple pendulum

An object is a simple harmonic oscillator when the restoring force is directly proportional to displacement.
For the pendulum in Figure 1, we can use Newton's second law to write an equation for the forces on the pendulum. The only force responsible for the oscillating motion of the pendulum is the $x$-component of the weight, so the restoring force on a pendulum is:
$F=-mg\mathrm{sin}\theta$
For angles under about $15\mathrm{°}$, we can approximate $\mathrm{sin}\theta$ as $\theta$ and the restoring force simplifies to:
$F\approx -mg\theta$
Thus, simple pendulums are simple harmonic oscillators for small displacement angles.

## Common mistakes and misconceptions

Sometimes people think that a pendulum’s period depends on the displacement or the mass. Increasing the amplitude means that there is a larger distance to travel, but the restoring force also increases, which proportionally increases the acceleration. This means the mass can travel a greater distance at a greater speed. These attributes cancel each other, so amplitude has no effect on period. The pendulum’s inertia resists the change in direction, but it’s also the source of the restoring force. As a result, the mass cancels out too.

For deeper explanations, see our video introducing pendulums.
To check your understanding and work toward mastering these concepts, check out the exercise on period and frequency of simple pendulums.

## Want to join the conversation?

• Hi! I don't really know why you're using Pi 2 in the equation. What does pi have to do with that? I really want to know. Although im 13 i still love physics and math. Greetings from Armin.
• The reason Pi is used is because if you think about the shape a pendulum makes as it swings back and forth, it makes a shape similar to a circular arc. When dealing with circles, you mainly deal with the value of Pi, as what my physics teacher explained to my class.
• In the equation for the period of the pendulum, is the value g always constant or can it change
• g can change if you're on a different planet. Otherwise, it's going to be a constant 9.8 m/s^2.
• Hello. I don't understand why
this equation "F≈−mgθF" makes simple pendulums to be simple harmonic oscillators
• The approximation is

F ≈ −m g θ,

not what you wrote. This makes it a simple harmonic oscillator because there is a restoring force (here: F) that is (approximately) proportional to the (magnitude of the) displacement (here: θ). (As explained in the video.)
• In figure 1 isn't there a y-component to gravity as well? Which's responsible to balance tension out, which actually is added over by centrifugal force as well?
• First of all, centrifugal force is not a thing. It is just an illusion that is due to the objects inertia. It is not an actual force. Second of all, the diagram is a free-body diagram. So, the solid lines show the forces but not the component of the forces. The dotted line shows the component of gravity that causes the motion. The thing is that, the force of tension is not balanced out. If it were, the ball would not travel in a circular path, it would just go in a straight line. The tension force is larger than the component of gravity that goes in the same direction, thus there is a net centripetal force causing circular motion. Hope this helps!
• Is the motion of the simple pendulum uniform or non-uniform
• How do you find w,a and v