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A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration. Find the force acting on it if its mass is 7 tonnes. For this question, if I solve for acceleration by using the equation a = v - u / t , then I get 1 m/s 2 as the acceleration. But if I use the equation, S = ut + 1/2 at 2 , then I get 2 m/s 2 as the acceleration. Since none of the formulae are wrong, why is the answer different in each case ?

A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration. Find the force acting on it if its mass is 7 tonnes. 
For this question, if I solve for acceleration by using the equation a = v - u / t , then I get 1 m/s2 as the acceleration. But if I use the equation, S = ut + 1/2 at2 , then I get 2 m/s2 as the acceleration. Since none of the formulae are wrong, why is the answer different in each case ? 

Grade:12th pass

1 Answers

Debarati
105 Points
2 years ago
The first formula is not applicable in this case because we don’t know the final velocity (v) here. Since, the truck is going through constant acceleration, you cannot divide 400 by 20 and take 20 as the final velocity. So, 2m/s2 is the correct answer.
I hope you understood.

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