Register to reply 
F=ma Confusion about answer given for Fby King
Tags: confusion 
Share this thread: 
#1
Nov1412, 05:38 PM

P: 43

Hi,
I'm reading a book and one of the questions asks: how much force is required to give an object weighing 3000lb an acceleration of 12ft/sec^{2}? I calculated this as F=3000x12=36,000pdl. The answer given is "1125 force" (36,000/32). Why is this? Why isn't it just 36,000pdl? 


#2
Nov1412, 06:37 PM

Emeritus
Sci Advisor
HW Helper
Thanks
PF Gold
P: 6,525

If an object weighs 3000 lbs, that 3000 lbs is not the same as its mass.



#3
Nov1412, 06:39 PM

P: 100

I think you need to find the mass. They give you the weight.
The acceleration on earth due to gravity is 32.174 ft/s^2. Divide the weight of the object by the acceleration on earth due to gravity and that will be the mass in pounds. I have not worked in imperial units that much but I think the pound has two definitions; mass pounds and force pounds. In the metric system the unit for mass is the gram and the unit for weight is the newton. Weight is simply force exerted by an object with a field of gravity on an object. After when you have the mass you multiply the mass by the acceleration given. I'm on my phone so I can't write it out but I calculated it and it works out. 


#4
Nov1412, 07:46 PM

P: 43

F=ma Confusion about answer given for F
Thanks TheAbsoluTurk! I was so confused by the fact that the book states that pounds is the mass and poundals is the weight, such that when he stated pounds of weight I was unsure which of the two he meant.
This makes sense now: I find out the mass of the automobile on earth and then find the force required to make it reach an acceleration of 12ft/sec^{2} in the horizontal direction. 


#5
Nov1412, 09:49 PM

Sci Advisor
HW Helper
PF Gold
P: 6,039




#6
Nov1412, 10:13 PM

P: 100

Thank you for the information. I had no knowledge of the slug.
Here in Canada we're sort of in the middle; no one gives weight or height in kilograms or centimeters but our traffic signs are in km/h. Giving weight in kilograms would be wrong but the newton I'm assuming is either unknown or completely unintuitive to most people. 


#7
Nov1412, 10:20 PM

Sci Advisor
HW Helper
PF Gold
P: 6,039




#8
Nov1412, 10:34 PM

P: 100

I've never had my weight or height measured in school.
Just to give you an idea of how far along I am in the 'metrification' process, I graduated from high school this year. 


#9
Nov1512, 10:09 AM

Mentor
P: 11,890




#10
Nov1512, 10:27 AM

P: 643




#11
Nov1512, 10:31 AM

Sci Advisor
HW Helper
PF Gold
P: 6,039




#12
Nov1512, 10:33 AM

P: 43

I've not heard the term "slugs" used before :D. In Europe grams and Newtons are used. 


#13
Nov1512, 12:00 PM

Mentor
P: 11,890




#14
Nov1512, 12:20 PM

P: 643




#15
Nov1612, 08:58 AM

P: 855

This doesn't come up in SI where, if you push 1 kg with a force of 1 N, it accelerates at 1 m/sec2. 


#16
Nov1612, 09:08 AM

P: 227




#17
Nov1612, 09:17 AM

P: 100




#18
Nov1612, 09:22 AM

P: 855




Register to reply 
Related Discussions  
Tough distance formula. (Answer provided), but how to get the answer?  Introductory Physics Homework  11  
Series. AP GP. Some confusion. Answer checking needed also.  Precalculus Mathematics Homework  3  
Solution stoichometry, attempted ANSWER IS WRONG, can anyone fix answer  Biology, Chemistry & Other Homework  2  
Statics Problem  Not sure why answer is answer  Engineering, Comp Sci, & Technology Homework  5  
Simple harmonic motion and frequency; answer disagrees from answer key  Introductory Physics Homework  1 