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# Mechanics question.... help?! watch

1. I've tried everything but I really cannot get my head around mechanics and despite all my efforts I cannot answer this question, any help would really be appreciated:

The driving force produced by the engine of a car of mass one tonne causes a car to accelerate uniformly from rest to a speed of 8ms-1 in 6 seconds along a rough horizontal road. If the coefficient of friction between the car and the plane if 0.3, find the driving force of the car
2. Find acceleration, Find Friction using coefficient of friction and reactant force vertically ( fr=0.3R). Then Driving Force-friction=mass x acceleration. And rearrange to find Driving Force hope this is right, haven't done mechanics in a year or two, if not try drawing a diagram to help.

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3. So we know that over the course of 6 seconds the car goes from stationary to 8 ms-1 s.

u= 0 v =8 t= 6 a=??

v=u + at

8=0 + 6a
a= 8/6 ms-2 (you could have simply divided 8/6 straight but using equations is good practice)

So since its horizontal the weight component has only one direction, vertically downwards (if it was inclined on a slope then it would exert a force down the slope) this is because as you know every force has a vertical and horizontal component, but cos90 = 0, therefore the resolved horizontal component of weight is 0..as 10,000*cos90 = 0.

Force - resistance = mass * acceleration

We know that the resistance is only friction..which is determined by the equation of fric=coefficient of friction * normal contact force.

Here the normal contact force is the weight since its horizontal.

So

F - 0.3*1000= 1000*(8/6)

F = 1000*(8/6) + 0.3*1000

I hope the driving force is 1633.4 N or 1634 to this day I dk how to round it off

http://www.cimt.plymouth.ac.uk/proje...vel/alevel.htm

http://www.mathcentre.ac.uk/courses/...ics/mechanics/ (quick reference)

dw I used to struggle a lot with mechanics too, its just about practice..there are key words you must understand.. smooth means no friction is to be considered. Rough means friction should be considered. Horizontal means its not inclined. Always draw force diagrams!

Theres also another way to do this. That involves potential and kinetic energy, You use the principle that the change in kinetic energy = work done by driving force - work done by resistive force. Basically

0.5m(v^2-u^2) = F*d - 0.3*m*d

where distance is calculated s= 0.5(u +v)t
4. (Original post by Bethanyduck)
Find acceleration, Find Friction using coefficient of friction and reactant force vertically ( fr=0.3R). Then Driving Force-friction=mass x acceleration. And rearrange to find Driving Force hope this is right, haven't done mechanics in a year or two, if not try drawing a diagram to help.

Posted from TSR Mobile
Thanks, this really helped me out
So we know that over the course of 6 seconds the car goes from stationary to 8 ms-1 s.

u= 0 v =8 t= 6 a=??

v=u + at

8=0 + 6a
a= 8/6 ms-2 (you could have simply divided 8/6 straight but using equations is good practice)

So since its horizontal the weight component has only one direction, vertically downwards (if it was inclined on a slope then it would exert a force down the slope) this is because as you know every force has a vertical and horizontal component, but cos90 = 0, therefore the resolved horizontal component of weight is 0..as 1000*cos90 = 0.

Force - resistance = mass * acceleration

We know that the resistance is only friction..which is determined by the equation of fric=coefficient of friction * normal contact force.

Here the normal contact force is the weight since its horizontal.

So

F - 0.3*1000= 1000*(8/6)

F = 1000*(8/6) + 0.3*1000

I hope the driving force is 1633.4 N or 1634 to this day I dk how to round it off

http://www.cimt.plymouth.ac.uk/proje...vel/alevel.htm

http://www.mathcentre.ac.uk/courses/...ics/mechanics/ (quick reference)

dw I used to struggle a lot with mechanics too, its just about practice..there are key words you must understand.. smooth means no friction is to be considered. Rough means friction should be considered. Horizontal means its not inclined. Always draw force diagrams!

Theres also another way to do this. That involves potential and kinetic energy, You use the principle that the change in kinetic energy = work done by driving force - work done by resistive force. Basically

0.5m(v^2-u^2) = F*d - 0.3*m*d

where distance is calculated s= 0.5(u +v)t
Thank you for going through this step by step, it helped me to understand the question a whole lot more and thanks for the pdfs they're really useful, I definitely need more practise

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Updated: January 24, 2015
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