# A truck moving with a speed of 54 km/h. Truck driver applied brakes suddenly and brings the truck to rest in 5 s, then the average retarding force on truck, if mass of the truck and driver is 400 kg, will be:

= 54 x 5/18 m/s

= 15 m/s

V = 0 and t = 5 s.

Retardation, a = v – u/t = 0 – 15/5 = – 3 m/s ²

Avg retardation force,

F = 400 x 3

= 1200 N

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(A) 1200 N

The answer which you have given is wrong

Right answer is here

Given, u = 54 km/h

= 54 x 5/18 m/s

= 15 m/s

V = 0 and t = 5 s.

Retardation, a = v – u/t = 0 – 15/5 = – 3 m/s ²

Avg retardation force,

F = 400 x -3

= -1200 N

The answer will come in negative because acceleration is in negative or the retardation is acting

Here the question is asked about “average retarding force” not about “average force”. So, it is already clear that the force is retarding that the acceleration is negative. That is why the answer is 1200 N. We can say that the average retarding force is 1200 N or average force is – 1200 N.