My Math Forum  

Go Back   My Math Forum > Science Forums > Physics

Physics Physics Forum


Thanks Tree1Thanks
  • 1 Post By studiot
Reply
 
LinkBack Thread Tools Display Modes
July 24th, 2016, 05:26 PM   #1
Member
 
Joined: Jan 2016
From: United States

Posts: 61
Thanks: 7

Questions about kinetic energy

These are a few end of chapter questions to see if you understood the theory. Unfortunately there's no answers for these in the book could someone double check my answers? It shouldn't be too much trouble, I appreciate the help as always ^.^

1) An elevator is hoisted by its cables at constant speed. Is the total work done on the elevator positive, negative, or zero?

The work done on the elevator is positive since the direction of motion is the same as the direction of force from the pull of the cables. (assuming the elevator is moving up)


2) If it takes total work W to give an object a speed v and kinetic energy K, starting from rest, what will be the object’s speed (in terms of v) and kinetic energy (in terms of K) if we do twice as much work on it, again starting from rest?

Since the object starts at rest then the initial kinetic energy is zero. In terms of v, since
K = 1/2(m * v^2) then 2K = m * v^2 and sqrt(v) = 2K / m


3) If there is a net nonzero force on a moving object, can the total work done on the object be zero? Explain, using an example.

Yes. if the force is perpendicular to the object the work done will be zero. If an object is being pushed along an even floor, then it will have the constant downwards perpendicular force of gravity acting on it, but the work done on the object by this force is zero since it's perpendicular to the motion. Although if the object were being pulled upwards then gravity would be anitiparallel and apply negative work.


4) A falling brick has a mass of 1.5 kg and is moving straight downward with a speed of 5.0 m>s. A 1.5-kg physics book is sliding across the floor with a speed of 5.0 m>s. A 1.5-kg melon is traveling with a horizontal velocity component 3.0 m>s to the right and a vertical component 4.0 m>s upward. Do all of these objects have the same velocity? Do all of them have the same kinetic energy? For both questions, give your reasoning.

For velocity they all have 5 m/s (including the melon 3-4-5 triangle), but they are not the same for kinetic energy. The fall bricks motion is parallel to the force of gravity which does positive work on it. The book sliding across the floor has movement perpendicular to the force of gravity so gravity does zero work, and lastly the melon which is travelling at an upward angle is antiparallel to the force of gravity, therefore gravity does negative work on it.


5)When a certain force is applied to an ideal spring, the spring stretches a distance x from its unstretched length and does work W. If instead twice the force is applied, what distance (in terms of x) does the spring stretch from its unstretched length, and how much work (in terms of W) is required to stretch it this distance?

This answer is almost all guess work (by that I mean I'm not very sure if it's right or not), but if you stretch the spring a certain distance, then by newtons laws there will be an antiparallel force with equal magnitude. To further stretch the spring by some base unit of distance you will have to overcome the negative work done by this force, and every time you stretch the spring by another of these base units of distance the negative work you have to overcome will grow. If there's some formula for this I really have no idea.
GumDrop is offline  
 
July 25th, 2016, 05:20 AM   #2
Senior Member
 
Joined: Jun 2015
From: England

Posts: 509
Thanks: 130

These are tricky and definitely designed to test your understanding.

Look carefully at my notes and ask any questions you have about them, they are rather brief since there are 5 questions.

I have done thme in an attachment since there is some maths involved.

Trust the maths and be careful to get to the end of the calculation, you dod not do this.

Also trust the maxim

No work is done if a system of forces in equilibrium moves its point of application.
Attached Images
File Type: jpg mech1.jpg (51.0 KB, 8 views)
Thanks from GumDrop

Last edited by studiot; July 25th, 2016 at 05:22 AM. Reason: add missing attachment.
studiot is online now  
July 26th, 2016, 02:42 AM   #3
Member
 
Joined: Jan 2016
From: United States

Posts: 61
Thanks: 7

Thanks! and yea number 2 was a typo
GumDrop is offline  
Reply

  My Math Forum > Science Forums > Physics

Tags
energy, kinetic, questions



Search tags for this page
Click on a term to search for related topics.
Thread Tools
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
3D Energy mark eaton24 Physics 21 December 7th, 2015 02:57 AM
Is v in kinetic energy formula speed or velocity? Mr Davis 97 Physics 2 April 30th, 2015 07:44 AM
Kinetic and potential energy of an object Mr Davis 97 Physics 3 April 20th, 2015 05:39 PM
How was the formula for kinetic energy found, and who found it? Ganesh Ujwal Physics 3 January 5th, 2015 02:58 AM
Energy Problem jakeward123 Calculus 0 January 1st, 1970 12:00 AM





Copyright © 2017 My Math Forum. All rights reserved.