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# A car, of mass Tonnes, accelerates from rest of over 100 m in 12 seconds. it then immediately applies the brakes. given that the car stops nine seconds later, find the magnitude of the braking force?

Hi Faziah! Since this question is about force, it looks like we’ll be using the f=ma and SUVAT equations. First thing we need to do is figure out what velocity the car is going at before it breaks. Using v = s/t, we get v = 100/12 = 8.33ms^-1. With this, we know when the car breaks; t = 9 (it takes 9 seconds to stop moving) u = 8.33 (calculated above) v = 0 (the car breaks until it stops) a = ? So using the SUVAT equations, we can choose: v=u + at Rearranging this equation to get a we arrive at the equation: (v - u)/t = a Plugging in the numbers from above: (0- 8.33)/ 9.81 = -0.85 (2 s.f) Since we require force, we use the equation stated above: F = ma F = mass x 0.85 We use the positive value for acceleration here as the question asks for the magnitude of the force, which is always a positive number! The mass hasn’t been given in the question you wrote but of you know it, convert the mass of the car from tonnes to KG (because we have to use SI units!). The conversion equation is: 1 tonne = 1000kg Plug the mass into the equation and you’ll arrive at your answer, Hope this helps!  Conor O’Halloran
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