Question

Find the average frictional force needed to stop a car weighing 500 kg in a distance of 25 m if the initial speed is 72 km/h.
Concept of Physics - 1 , HC VERMA , Chapter " Work and Energy"

Answer 1

Here Given :-
mass , m= 500 kg

Initial speed u = 75 km/h
 =72000m / 3600 s
u = 20 m/s.
distance,s = 25 m
final speed v = 0

We know the formula
v² - u² =2as
on putting the value
0² - 20² = 2 × a × 25
a= 8 m/s²
but the acceleration is in the opposite direction then
a = -8 m/s²

So Now the frictional force F =  m × a
F = 500 × 8
F = 4000 N.

Hence the frictional force is 4000 N.


Hope it Helps.

Answer 2

Hi.

Given.
Mass(m) = 500 kg.
Speed = 72 km/hr.
= 72 × 5/18   [Changing into m/s]
=  20 m/s.

Distance covered by it = 25 m.

∵ v² - u² = 2aS
⇒ 0 - (20)² = 2(a)(25)
⇒ -400 = 50a
⇒ a = - 8 m/s².

Now, Frictional Force = m × a
 = 500 × -8
 = - 4000 N.

Here, negative sign shows the force is acting in the opposite direction of the displacement.

∴ Frictional force is 4000 N.


Hope it helps.