Question

anil is driving a car at 72km/hour. He suddenly applies a brake and the car stops after 20 s The acceleration of the car due to the brakes in m/s square is

A. 1
B. 3
C. -1
D. -2
​

Answer 1

Given :

• Initial velocity (u) = 72 km/h = 20 m/s
• Final velocity (v) = 0 m/s (as it applies brakes)
• Time (t) = 20 seconds

To find :

• Acceleration (a)

According to the question,

By using Newtons first equation of motion,

→ v = u + at

Where,

• v = Final velocity
• u = Initial velocity
• a = Acceleration
• t = Time

→ 0 = 20 + a × 20

→ 0 - 20 = 20a

→ -20 = 20a

→ -20 ÷ 20 = a

→ -1 = a

So,the acceleration is -1 m/s².

.°. The correct option is C) -1 m/s².

Note : Negative signs means retardation.

____________________

More formulas :

★ Newton's first equation of motion :

• v = u + at

★ Newton's second equation of motion :

• s = ut + ½ at²

★ Newton's third equation of motion :

• v² = u² + 2as