Question

1. A car of mass 800 kg, moving at 54 km is brought to rest over a distance of 15 m. Find the
retarding force developed by the brakes of the car

Answer 1

Answer:

v=0km/s

u=54km/s

m=800kg

2as=v2 - u2

a=81m/s2

f=ma

f=800 x 81 = 64800N IN OPPSITE DIRECTION

Answer 2

Correct Question :

A car of mass 800 kg, moving at 54 km/h is brought to rest over a distance of 15 m. Find the retarding force developed by the brakes of the car.

Given :

• Initial velocity, u = 54 km/h
• Final velocity, v = 0 m/s
• Distance, s = 15 m
• Mass of a car, m = 800 kg

To find :

• Retarding force developed by the brakes, F

According to the question,

Note :-

We will change 54 km/h into m/s. So, the initial velocity will be 15 m/s.

➞ v² = u² + 2as

Where,

• v = Final velocity
• u = Initial velocity
• s = Distance
• a = Acceleration

➞ Substituting the values,

➞ (0)² = (15)² + 2 × a × 15

➞ 0 = 225 + 30a

➞ 0 - 225 = 30a

➞ - 225 = 30a

➞ - 225 ÷ 30 = a

➞ - 7.5 = a

• So,the acceleration is - 7.5 m/s².

Note :- Negative signs means retardation.

Now,

➞ Force = Mass × Acceleration

Or,

➞ F = ma

➞ F = 800 × (- 7.5)

➞ F = - 6000 N

• So,the retarding force developed by the brakes is 6000 Newton.