Physics Help Forum Help with physics problems?

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 Aug 17th 2017, 04:15 AM #2 Senior Member     Join Date: Jun 2016 Location: England Posts: 338 Your issues seem mostly to do with recognising the correct ways of mathematically manipulating the equations to get the answer you want on the left of the = sign. Q1 You have correctly identified F=ma to get Newtons from Mass You are allowed 750 million Newtons for each square meter of cable cross-section Work out the actual Cross-Section Area of an 8mm diameter cable (in Meters) How much smaller is this actual cross section than One square Meter? This is how much smaller the Force must be than 750 million. This gives you the Maximum Force, use F=ma (where a is the acceleration due to gravity) to find the maximum mass. Q2 Try looking at the problem in reverse. The box starts at 7.5 m/s and decelerates at 9.8m/s/s until stationary. Remember if you differentiate the displacement formula, you get the velocity formula. I will leave other questions for Ron (later Ron) __________________ ~\o/~ Last edited by Woody; Aug 17th 2017 at 10:56 AM.
 Aug 17th 2017, 07:04 AM #3 Senior Member     Join Date: Jun 2016 Location: England Posts: 338 Q4 seems sensible, Q5 part 1 looks OK, part 2 I would suggest that the dust might act (like mini ball bearings) to reduce friction Thus after cleaning the floor is LESS slippy the coefficient of friction has risen it requires more than 150N to move the boxes. Q3 don't know. My practical side would be worried about sparks Not sure if this is what the question is after though. __________________ ~\o/~
 Aug 17th 2017, 05:19 PM #4 Junior Member   Join Date: Aug 2017 Posts: 3 Thanks Woody! I have worked through your comments and have some progress to show. Really appreciate your help. BUT SOMETHING IS REALLY WRONG - WHAT HAVE I DONE WRONG...please help. Question: A safety harness is connected to a cable with a report breaking stress of 800MNm-2. What mass could be hung from a harness if the diameter of the cable was 6mm? We need to use formula Stress = force / area to get the force value. To calculate the area we look to the diameter of cable (6mm), so radius is 3mm. To find area (A) of the cable surface (cross-section) the formula is A = π r2 A = π x 32 = 28.27mm2 (to 2 dp) =28.27 x103m2 (in SI Units of m) We also need to convert the stress into SI units So 800MNm-2 becomes 800 x 106Nm-2 So back to σ = F / A. We know the stress and the area so we need to find force (F). So lets swap the formula around to F = σ A F = σ A = 800 x 106Nm-2 x 28.27 x 10-3m2 = 800 x 106Nm-2 x 0.02827m2 = 22,616,000 N Then to get mass we can use F = ma where F = force, m = m and a = acceleration to rearrange the formula to find mass M = F A = 22,616,000 9.8 = 2,307,755.1kg But surely this is way too big – where have I gone wrong….help please! Last edited by NEWtoPHYSICS; Aug 20th 2017 at 03:53 PM.
 Aug 21st 2017, 11:11 AM #5 Senior Member     Join Date: Jun 2016 Location: England Posts: 338 One mistake I have seen is the number of square millimetres in a square meter You have used 1000 in your calculations, it should be 1,000,000! This will reduce all of your results by a factor of 1000 giving a final answer of 2308kg or about 2 tonnes. I am sure it was just a simple slip of the brain, at least you recognised that there was a mistake. The best way to avoid this sort of thing is (boring) practice, you will eventually build up a pattern of common mistakes and how to find them. __________________ ~\o/~
 Aug 21st 2017, 09:44 PM #6 Junior Member   Join Date: Aug 2017 Posts: 3 Thank you so much Woody!!!! I really appreciate your help! Now it stands out but I couldn't see the wood for the tress before! have a great week.

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