A 1000 kg car is moving along a road at a constant speed. Suddenly. Class 9

A 1000 kg car is moving along a road at a constant speed. Suddenly. Class 9

Question 1.

A 1000 kg car is moving along a road at a constant speed. Suddenly, the driver notices some obstruction ahead and applies the brakes to come to a complete stop. The graphical representation of motion of the car starting from the instant the driver spots the traffic ahead is shown in Fig.

(i) Describe how the car moves between positions A and B.

(ii) Calculate the kinetic energy of the car at A.

(iii) State the work done by the brakes in bringing the car to a halt between B and C.

(iv) What does the kinetic energy of the car transform into?

Answer:

(i) Between A and B: The car moves at constant speed of 35 m s^-1 (uniform motion - this is the driver's reaction time before brakes are applied; no braking force yet).

(ii) KE at A = 1/2 mv² = 1/2 × 1000 × (35)² = 1/2 × 1000 × 1225 = 612500 J = 6.125 × 105J.

(iii) Work done by brakes (B to C): Final KE = 0 (car stops). By work-energy theorem:

Work = ∆KE = 0 - 612500 = - 612500 J. (Negative because braking force is opposite to displacement.)

(iv) Kinetic energy transforms into: Primarily thermal energy (heat) due to friction between brake pads and wheels, and between tyres and road. A small amount also converts into sound energy.