PE exam practice - What is the answer?

What is the answer? question 3

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Today’s What is the answer blog has a dynamics question.  Here we go…

You are driving a car at 90 km/hr on a dry road and suddenly brake to stop.  If the coefficient of friction between the tires and the road is 0.63 what is the minimum stopping distance?

A. 34 m

B. 40 m

C. 51 m

D. 73 m

What is the Answer?

So, solve the problem and leave a comment below with you answer.  As always, please share this with others so they can have practice.  Now would also be a great time to go like the Mechanical PE Academy Facebook page!

 

14 comments on “What is the answer? question 3

    1. Answer is
      V squared equals u squared plus 2 x deceleration x distance. V squared is equated to zero, deceleration to g x 0.63.

      The answer is 50.57 metres.

  1. C is closest answer. Equate initial kinetic energy(1/2mV^2) to work done (F.x) by braking where the braking force is .63 of the normal force of the vehicle. “m” cancels on both sides of the equation; the Normal force (mg) (assuming level surface), and friction force is Friction coefficient * Normal force(mg). You get a constant acceleration, a, (deceleration, since the braking force is opposite the direction of travel) and thus constant force working through the stopping distance, x.

    1. The correct answer is C (50.57 m). Besides the conservation of energy, you can use the friction force formula equals Newton’s 2nd law formula which eliminates the weight. Then, apply the constant acceleration formulas vx = vox + ax t to find time, and substitute t in df = do = vo t + 0.5 ax t to calculate the stop distance or (df – do).

  2. The answer is c, 50.56m. And one more thing. You are as good as dead if your car takes that distance to halt in emergency. ?

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