A bullet weighing 10 g, flying at a speed of 800 m / s, pierced a board 8 cm wide

A bullet weighing 10 g, flying at a speed of 800 m / s, pierced a board 8 cm wide. After that, the speed of the bullet decreased to 400 m / s. Find the force of resistance.

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

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

where M is the mass of the bullet, M = 10 g = 0.01 kg;

V1, V2 – bullet speed, respectively, before and after the board; V1 = 800 m / s; V2 = 400 m / s.

On the other hand, work A is equal to the product of the resistance force F and the width of the board s = 8 cm = 0.08 m:

A = Fs.

Resistance force F:

F = A / s = M (V1 ^ 2 – V2 ^ 2) / (2 s) = 0.01 × (800 ^ 2 – 400 ^ 2) / (2 × 0.08) = 30,000 N = 30 kN.

Answer: 30 kN.