Question

(a) Define kinetic energy of an object. Can kinetic energy of an object be negative? Give reason. 

(b) A car weighing 1200 kg is uniformly accelerated from rest and covers a distance of 40 m in 5 s. Calculate the work, the car engine had to do during this time.

Answer 1

(a) Kinetic energy of an object is defined as the energy possessed by a body by virtue of its motion. 

It is given by, K.E = 1/2 mv² 

No, the kinetic energy of an object cannot be negative since both mass of the object, m and velocity of a substance, v cannot be negative.

(b) Given, 

Mass of the car, m = 1200 kg ,                   Distance covered, s = 40 m 

Time taken, t = 5 s,                                     Initial velocity, v = 0 

Work done, W =? 

Since, W = F × s = ma × s                         [ Since, F = ma ] 

For calculating work done, acceleration (a) is required.

Therefore using, s = ut + 1 /2 at²

40 = 0 × t + 1 /2 a × 5^2​

⟹ = 40×2 /25 = 3.2 m/s ² 

Now, W = F × s = ma × s 

= 1200 × 3.2 × 40 = 153600 J 

The work needed to be done by the car engine = 153600 J