Question

18) A man is texting on his phone as he drives his
car at 40 km/h, he suddenly notices a child is
crossing the road so he rams his brake bringing the
car to stop in 2s. If the car's mass is 4000 kg and he
weighs 60 kg, what was the retarding force on the
car?​

Answer 1

The solution is in the attachment.

Answer 2

Given :-

A man is texting on his phone as he drives his  car at 40 km/h.

He suddenly notices a child is  crossing the road so he rams his brake bringing the  car to stop in 2s.

Mass₁ = 4000 kg

Mass₂ = 60 kg

To Find :-

The retarding force on the car.

Solution :-

We know that,

• u = Initial velocity
• t = Time
• v = Final velocity
• f = Force
• m = Mass

Given that,

Mass₁ = 4000 kg

Mass₂ = 60 kg

Total mass (m) = 4000 + 60 = 4060 kg

Initial velocity (u) = 40 kmph = 11.11 m/s

Final velocity (v) = 0

Time (t) = 2 s

By the formula,

$\underline{\boxed{\sf Acceleration=\dfrac{(v-u)}{t} }}$

Substituting their values,

$\sf a= \dfrac{(0-11.11)}{2}$

$\sf a = -5.55 \ m/s^{2}$

Thus,

$\underline{\boxed{\sf Force=Mass \times Acceleration}}$

Substituting their values,

$\sf F= 4060 (-5.55)$

$\sf F = -22553.3N \ or \ -2.3 \times 10^{4} N$

Therefore, the retarding force on the  car is -22553.3N or -2.3 x 10⁴ N