Magnitude a resisting force

## Question:

How do you find the magnitude of a resisting force if you have a rifle bullet with at mass of 12 grams traveling with a speed of 400 m/s strikes a large wooden block which it penetrates to a depth of 15 cm. What is the magnitude of the resisting force that acts on the bullet?
Need help!

Thank you;

## Answer:

Here are some things we know. The change in velocity is 400 m/s. The change in momentum is 400*0.012 kgm/s. The average resistance force is the change in momentum divided by the time required to stop the bullet. The distance covered under constant acceleration is 1/2 a* t^2. The average acceleration is the change in velocity divided by the time.
Let's write a as 400/t. then 0.15=1/2(400/t)*t^2 or 0.15=200t or t=0.15/200=7.5e-4 seconds. The resistance force then must be 400*0.12/7.5e-4 N = 6400 N.