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Feedback Comments on Motion

by Ron Kurtus

A total of 228 comments and questions have been sent in. They are listed according to date.

List of most recent 10 letters




Pendulum Equations Graphing a pendulum Nigeria
Pendulum Equations Frequency when reducing length by 1/2 Pakistan
Kinematics Joke on average speed USA
Pendulum Equations Where is the pendulum speed the maximum? Pakistan
Pendulum Equations How to calculate the KE of a bob UK
Pendulum Equations What happens to pendulum period USA
Newtons Cradle Newton's Cradle slows down too fast USA
Pendulum Equations Derivation of pendulum equation Kenya
Pendulum Equations Pendulum equation Bangladesh
Relative Motion Car in reverse is slowing down South Africa

Next 10


Graphing a pendulum

Topic: Pendulum Equations


March 7, 2016

why does the graph fail to pass through the origin in an experiment of simple pendulum

favour - Nigeria



What are you trying to graph?

What is the origin you are talking about? Is it where you start the pendulum motion?

I need a better description to understand your problem.

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Frequency when reducing length by 1/2

Topic: Pendulum Equations


February 25, 2016

What will be the frequency of simple pendulen if its lenght is l/2

muhammad - Pakistan



The frequency equation is: f =[?(g/L)]/2?. If the length is reduced by 1/2, the frequency will increase by the square root of 2 or 1.414

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Joke on average speed

Topic: Kinematics


October 25, 2015

Good Morning,
I am trying to reconstruct an old favorite joke and need some help identifying the person behind the following principle. It takes the distance between 2 fixed points and the time it takes to determine the (average) rate of speed.

Hopefully I have not complicated the question. :) And here is the joke.

Mystery scientist was pulled over on the freeway for speeding. The Trooper approached his car and asked him, "Do have any idea how fast you were going?"

He replied, "No but I know exactly where I was at."

Thanks and haqve a great day, Larry

Larry - USA



From Wikipedia: The Italian physicist Galileo Galilei is credited with being the first to measure speed by considering the distance covered and the time it takes. Galileo defined speed as the distance covered per unit of time.

I guess you could use Galileo in your joke.

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Where is the pendulum speed the maximum?

Topic: Pendulum Equations


May 8, 2015

The speed of a simple pendulum will be maximum at?
A. Pole
b. Equator
c. Centre of earth
d. Surface of earth.

asad - Pakistan



The speed of a simple pendulum is dependent on the force of gravity.

At the centre of the Earth, gravity is in all directions and thus is zero.

Since the Earth bulges by the equator, the speed would be greater than at either pole. In both cases, the pendulum is near the surface of the Earth. If it was AT the surface, it couldn't swing.

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How to calculate the KE of a bob

Topic: Pendulum Equations


February 9, 2015

How to calculate the KE of a bob without knowing its velocity?

Paul - UK



You can calculate the velocity at the bottom of a pendulum swing from the equation: v = ?{2gL[1-cos(a)]}. The the KE = mv^2/2.

But also, the KE at the bottom equals the potential energy (PE) athe the top of the swing PE = mgh = KE, where h is the vertical distance from where the bob is released to the bottom of the swing.

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What happens to pendulum period

Topic: Pendulum Equations


January 4, 2015

As the length of the pendulum increase, what happens to the period or time? When gravity increases, what happens to period?

Jeftaline - USA



According to the simple pendulum equation T = 2pi*SQRT(L/g), the period T increases as the length L increases.

Also, when the gravity g increases, the period decreases.

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Newton's Cradle slows down too fast

Topic: Newtons Cradle


December 18, 2014

Can the Balls be easily fixed so the balls bounce more than a few seconds. You would think 5 minutes or more it should bounce. Mine is new and only bounces a few seconds.

Phil - USA



The Newton's Cradle should continue for more than a few seconds, but they do slow down a lot faster than you would like.

If the balls are hard metal and well aligned, the slow-down should be less. But still, there seems to be a fair amount of lost energy. I noticed that my balls do not stay in place after a collision. It looks like the energy and momentum is not perfectly transferred.

I don't know if expensive, scientific versions work any better.

A analysis can be seen at Newton's Cradle Analysis. It probably goes into more detail that you need to know.

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Derivation of pendulum equation

Topic: Pendulum Equations


October 3, 2014

proof that T=2pisquaroot ofL/G




You can see the Derivation: Period of a Simple Pendulum. However, it is not an easy derivation.

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Pendulum equation

Topic: Pendulum Equations


September 6, 2014

what is the effect on the period of the pendulum when the experiment is repeated with
1.aheavier pendulum bob of the same size.
2.a larger number of oscillations
3.a smaller angle of swing
4.a longer pendulum

monika - Bangladesh



The pendulum equation is: T = 2pi*SQRT(L/g), where T is the period in seconds, pi = 3.14, SQRT is the square root, L is the length of the wire, and g is the acceleration due to gravity = 9.8

In a simple pendulum, the weight of the bob is not a factor.

The period is how long it takes to swing back and forth. That can determine the number of oscillations.

With little or no friction, a smaller angle would not change anything except the amplitude of the swing.

The greater the length of the wire, the greater the period.

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Car in reverse is slowing down

Topic: Relative Motion


August 12, 2014

Please help me with this problem!!

A car initially moving in reverse.The driver applies the brakes,slowing the car.What is the direction of the acceleration,relative to the car?

Nokuphiwa - South Africa



Both the car and the driver are moving in the reverse direction. If the car slows down or stops, the driver will tend to continue in the reverse direction, causing him to be pushed against the back of the seat.

The car is decelerating or has a negative acceleration with relative to the ground.

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