# 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 next 10 letters

## Topic |
## Title |
## Country |

Newtons Cradle |
Should collision be inelastic | Canada |

Law of Inertia |
Acceleration for a given force | USA |

Relative Motion |
Confused about units | Canada |

Pendulum Equations |
What is a velocity time graph? | Ghana |

Relative Motion |
Police shoots at speeding car | India |

Pendulum Equations |
Purpose of simple pendulum | South Africa |

Relative Motion |
Show efficiency of a machine | Uganda |

Pendulum |
Is gravity like acceleration in a force field? | Nigeria |

Law of Inertia |
Inertia and gravity | USA |

Newtons Cradle |
Newton's Cradle not working | USA |

## Should collision be inelastic

Topic: **Newtons Cradle**

### Question

December 9, 2013

Under the heading "Slowly Slows Down" you wrote:

"Steel balls are usually used, because they deform very little upon collision and are highly elastic..."

I think you meant "highly inelastic"???

J.D. - **Canada**

24306

### Answer

There is often confusion about the terms "elastic" and "inelastic" in collisions.

When the balls are elastic, it means that they readily return to their original shape with little loss in energy. With an inelastic collision, there is a loss of kinetic energy, such that the balls slow down.

## Acceleration for a given force

Topic: **Law of Inertia**

### Question

November 15, 2013

Will two objects of different mass accelerate at different velocities if the same net force is acting upon them both, in space?

Will - **USA**

24194

### Answer

Since the relationship between force and acceleration is F = ma, the equation can be rewritten as a = F/m. Thus, for a given force, the acceleration is less with a greater mass.

## Confused about units

Topic: **Relative Motion**

### Question

October 20, 2013

In the topic of relative motion, there is the part where someone accelerates from 240 m/s to 560 m/s. So the acceleration ends up to be 32 m/s squared. Where does squared come in? You wrote it 32m/s/s, I don't understand why?

Sam - **Canada**

24115

### Answer

560 m/s - 240 m/s = 320 m/s. Divide by the time of 10 s, you get (320 m/s)/10 s = 32 m/s/s or 32 m/s^2.

When you divide the units m/s by s, you can think of it as multiplying m/s * 1/s, which would be m/s^2.

Perhaps it would be better if I simply eliminated the m/s/s, since it could be confusing.

## What is a velocity time graph?

Topic: **Pendulum Equations**

### Question

September 5, 2013

What is velocity time graph?

Prince - **Ghana**

23993

### Answer

You can draw a graph or chart where the vertical axis is velocity and the horizontal axis is time. Then a line on the graph will show how velocity changes with passing time.

In the case of a pendulum, the velocity starts at zero, speeds up and then slow down to zero at a time equal to the period of the pendulum.

## Police shoots at speeding car

Topic: **Relative Motion**

### Question

July 10, 2013

A police man moving on a highway with a spoeed of 30km/h fires a bullet at a thief's car speeding away in the same direction with a speed of 192 km/h.if the muzzle speed of the bullet is 150 m/s with what speed does the bullet hit the thief's car?

suryanshu - **India**

23849

### Answer

For the sake of this problem, assume that air resistance on the bullet is negligible.

The muzzle of the policeman's gun is traveling along with the car at 30 km/h.

Since 1 m = 1/1000 km and 1 s = 1/3600 hr, the muzzle velocity of 150 m/s converts to 150*3600/1000 km/h = 540 km/h.

The velocity of the bullet is 540 + 30 = 570 kn/h.

The bullet hits the thief's car at 570 - 192 = 378 km/h.

## Purpose of simple pendulum

Topic: **Pendulum Equations**

### Question

April 13, 2013

What is the aim of the simple pendulum, what is the real purpose of the simple pendulum? Thank you

Sifiso - **South Africa**

23615

### Answer

A simple pendulum can be used to demonstrate periodic motion. It is also useful as a form of timer or clock. See Pendulum Exhibits Periodic Motion for more information.

## Show efficiency of a machine

Topic: **Relative Motion**

### Question

August 12, 2012

show that the efficiency of a machine is given by;

Efficiency=M.A/VR*100%

tindeka - **Uganda**

22835

### Answer

The efficiency of a machine is the work done divided by the energy used. Work is force times distance. Energy used includes losses due to friction and heat.

See: Overview of Machines.

## Is gravity like acceleration in a force field?

Topic: **Pendulum**

### Question

June 29, 2012

Acceleration due to gravity is it the same thing as acceleration in a force field.

Joy - **Nigeria**

22750

### Answer

Gravity can be considered a force field. However, the acceleration due to gravity is approximately constant at distances up to 7 km from the Earth. Other force fields vary with the square of the distance from the source.

## Inertia and gravity

Topic: **Law of Inertia**

### Question

April 26, 2012

you write of inertia on earth ,but what of space,i have a invention that creates inertia without gravity[in outer space]travel

gregory - **USA**

22614

### Answer

Inertia is a property of matter in space. Once an object starts moving in space, it continues in a straight line until some force changes its velocity.

Inertia has nothing to do with gravity. Although the pull of gravity can change the inertia of an object.

## Newton's Cradle not working

Topic: **Newtons Cradle**

### Question

April 18, 2012

Dear Sir,

I do not seem to find a flash simulation on your webpage of Newton's cradle. I may be missing a link. There are instructions on how to interact but I am missing where the flash simulation is. Could you please guide me where to find it?

Thank you

M.C. Berisso

Shasta Community College

M. Cristina - **USA**

22594

### Answer

Thanks for letting me know about it. For some reason it disappeared. I corrected the problem, and it works fine now.

## Summary

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