mechanical · 22 minutes · Official OETT Aligned
Pendulums
Why a hanging weight speeds up and slows down
What to know
- bob: the hanging weight
- fastest and slowest points
- length of the string
- period: time for one full swing
The position of the weight tells you whether it is speeding up or slowing down.
The core rule
A pendulum is fastest at the bottom and stops briefly at each end. A longer pendulum takes more time to swing.
The hanging weight, called the bob, speeds up as it falls toward the bottom. It slows as it climbs the other side. One full swing goes from one end to the other and back. This time is the period. For small swings without friction, changing the weight does not change the period.
Worked examples
Example 1: easy
A hanging weight released from one end speeds up toward the bottom.
- The hanging weight starts at one end, briefly stopped.
- It speeds up as it drops and is fastest at the bottom.
As the weight moves downward, energy from its height becomes energy of motion.
Example 2: medium
A longer pendulum takes more time per swing than a shorter one.
- For small swings without friction, compare the string lengths.
- The longer pendulum takes more time for a full swing. Doubling the weight does not change that time.
The motion takes longer with a longer string. Changing the weight does not change the full-swing time in this example.
Example 3: hard
At a marked point after the bottom, identify both the upward direction and decreasing speed.
- After the bottom, the weight moves upward toward the other end.
- It slows as it climbs, then stops briefly and turns back.
Moving upward does not mean speeding up.
Common mistakes
The weight moves fastest at the end of the swing.
It stops briefly at each end before changing direction. Its greatest speed is at the bottom.
Put it into practice
Use guided explanations, check your answers, and try a new variation.