calculate braking force

Question

A truck that travels on a level road at 60 km/h can be stopped by its brakes in a distance of 51 m. The mass of the truck is 5 tonnes and the resistance against the motion is 3 615 N. Calculate the braking force in kN.

Answer 1

Convert speed  of truck from km/h to m/s.

v1 = 60*(5/18) =16.667 m/s

Calculate the braking force by using the work -energy princpal:

T1 + U1-2 =T2

(1/2)m(v1)² +(Wh-F-Fk) =0

(1/2)(907.2)(16.667)² +[(907.2*9.81) -3615- Fk] 51=0

[(907.2*9.81) -3615- Fk] 51 =- 126000

[(907.2*9.81) -3615- Fk] =-2470.588

Fk = (907.2*9.81) -3615+2470.588

     =7755.22 N