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Feedback Comments on Motion

by Ron Kurtus

A total of 229 comments and questions have been sent in. They are listed according to date.

List of next 10 letters




Relative Motion Car in reverse is slowing down South Africa
Relative Motion Lion chasing a deer Nepal
Law of Inertia Weight of ball swinging around on a rope UK
Motion Laws Man jumps off platform India
Motion Laws Dropping a tennis ball from a height India
Pendulum Would like a book in physics Pakistan
Pendulum Equations Why is pendulum angle small? Nigeria
Pendulum Equations Solving homework problem Nigeria
Relative Motion Good metric for velocity South Africa
Collisions Is energy conserved in an inelastic or elastic collision? South Africa

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First 10 letters

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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Lion chasing a deer

Topic: Relative Motion


June 12, 2014

a lion and a deer is at a distance of 30 m. lion is chasing a deer & it gains 3m in 5sec then after 10 sec what is the distance between them?? plz help me as soon as possible.

- Nepal



If the lion gained on the deer 3 m in 5 sec, then in 10 sec it will have gained 6 m.

If the total time is 10 seconds, then the distance between them would be 30 - 6 = 24 m. However, if the total time measured in 15 seconds, the separation would be 21 m.

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Weight of ball swinging around on a rope

Topic: Law of Inertia


May 12, 2014

hi.I have a question if a solid ball weighing say 1 pound attached to a rope and spon around at arms length would the weight of the ball increase the faster you spon it.

Dennis - UK



The faster you spun the ball around, the greater the force pulling on the rope, thus making the ball seem heavier.

The equation is F = mv^2/r. See Centrifugal Force is Not Real for more information.

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Man jumps off platform

Topic: Motion Laws


May 8, 2014

a man is standing at a spring platform.Reading of spring balance is 60kg wt. if man jumps outside the platform,then the reading of the spring balance:
(a)first increases then decreases to zero
(d)remains same

priyanka - India



When the man jumps, he creates a force against the platform to accelerate his body upwards. That force increases the reading on the spring platform. Once he is off the platform, the value goes to zero.

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Dropping a tennis ball from a height

Topic: Motion Laws


May 7, 2014

a tennis ball is dropped on the floor from a height of 20m. it rebounds to a height of 5m. if the ball was in contact with the floor for 0.01 secs.what was its average acceleration during contact?

priyanka - India



You want to find the velocity of the ball when it falls 20 m. See Velocity Equations for Falling Objects for the equation v = SQRT(2gy) where y = 20 m.

Average acceleration is the change in velocity for the given time. Since the ball is changing directions, a = 2v/t.

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Would like a book in physics

Topic: Pendulum


May 7, 2014

Salam ! sir I wanted general book of physics in easy approach, which cover all over the topics related to F.SC level, you sent me either the book and author name, or send via your kind. it is very standard school, and how pakistani students will enroll in this institution ? I see your topics via through website, which is very helpful.

Imran - Pakistan



Our physics lessons help to supplement your studies. They are at a basic level to help with your understanding.

There is no registration or grading in our school. Besides the sciences, there is information on how to succeed in your jobs and in life.

Best wishes in your studies. You seem to be looking into how to improve yourself, and that is the sign of a potential champion.

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Why is pendulum angle small?

Topic: Pendulum Equations


April 16, 2014

i Have two separate questions 1) it is often recommended that the angle of swing of a pendulum be made as small as possible, WHY?
2) will the value of g be affected if the size of bob be varied, WHY?

bright - Nigeria



A pendulum will swing properly at larger angles. However, the equations for period and velocity are much simpler if the angle is small. This is to approximate having to deal with the sine of the angle and other factors.

Since gravity affects objects of different mass equally, the pendulum will swing the same with a heavy bob as a light weight bob. However, if the bob is physically large, air resistance may come into effect.

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Solving homework problem

Topic: Pendulum Equations


March 25, 2014

1. Three coplanar vectors are expresse, with respect 2a certain rectangular coordinate system as a= -4i-j, b=-3i+2j, c=-j, in which d components of ds vectors are given in arbitary units. find d vector r which is d sum of ds vectors. 2.when a 2kg mass is hung from a vertical supported spring,it stretches d spring by 10cm. what mass must be hung on d spring if its to oscillate with a period of 1s? 3. the equation of motion for a particular mass at d end of a spring is x=0.40cos(0.70t-30)m.for ds oscillation find d amplitude and d period.

- Nigeria



I'm sorry, but we don't solve homework problems.

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Good metric for velocity

Topic: Relative Motion


March 6, 2014

why can the distances traveled be a good metric of the velocity?

Carina - South Africa



Since distance can be round-about and not in a straight line, it is better to consider displacement or separation between two locations. Then the time is takes to go from one point to another in a straight line determines your velocity.

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Is energy conserved in an inelastic or elastic collision?

Topic: Collisions


February 4, 2014

Energy conserved is an inelastic or elastic collision?

Thovhakale - South Africa



A perfectly elastic collision is when no kinetic energy is lost to heat or other forms. Kinetic energy is conserved.

With an inelastic collision, the object may be deformed and some energy is lost to heat. In such a situation, the total kinetic energy is not conserved.

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