The bullet had a speed of 800 m / s and a mass of 3g and crashed into a board 2.5 cm

The bullet had a speed of 800 m / s and a mass of 3g and crashed into a board 2.5 cm long and then acquired a speed of 200 ms. find the resistance force of the board.

The work of the resistance force of the board A is equal to the decrease in the kinetic energy of the bullet before and after passing the board:

A = A1 – A2 = M (V1 ^ 2 – V2 ^ 2) / 2,

where M is the mass of the bullet, M = 3 g = 0.003 kg;

V1, V2 – bullet speed, respectively, before and after the board; V1 = 800 m / s; V2 = 200 m / s.

On the other hand, work A is equal to the product of the average resistance force F by the thickness of the board s = 2.5 cm = 0.025 m:

A = Fs.

Force F:

F = A / s = M (V1 ^ 2 – V2 ^ 2) / (2 s) = 0.003 × (800 ^ 2 – 200 ^ 2) / (2 × 0.025) = 36000 N = 36 kN.

Answer: 36 kN.