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!

Kinetic energy equals potential energy. Answer C: 51m

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.

The answer is C.

C

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.

How does the coefficient of friction factor in when you don’t have a weight for the car?

Mass cancels out of the force balance equation.

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).

Answer is c. 51 m (approx.)

51 Meters

I am continually searching online for posts that can assist me. Thank you!

C

What’s the other way to solve this problem without using energy conservation?

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. ?