Forces- draw diagram and find force

Question

A 1500 kg car is driving at 20 m/s. The driver wishes to stop over a distance of 30 m. (a) Draw a force diagram for the car. (b) What force must the brakes apply to the car to do this?

Answer 1

This forum is not set up for drawing diagrams, but we can help you with the calculations.  Use the kinematic relation

 

v_f² - v_i² = 2ad

 

m = mass of car = 1500 kg

v_f = final speed = 0  [comes to full stop]

v_i = initial speed = 20 m/s

d = stopping distance = 30 m

 

Find the acceleration a, and then get the force from F = ma.

 

a = -v_i²/(2d)

 

F = ma = -mv_i²/(2d)

 

You know m, v_i, and d.  Just plug in the numbers to get F.  The negative sign means the force opposes the direction of motion.  You can finish from here.