# Interactions and Potential Energy

#### CJoy

Homework Statement
The Cubarb parasite, having pursued Defiance Drake this far was not expecting either her sudden acrobatics, or to be shot by her futuristic slug throwing pistol. Nevertheless, Cubarbs have naturally flexible skin that afford them protection from projectiles, depending of course, on the amount of kinetic energy that the projectiles are carrying. The parasite's skin acts as a spring, with constant k=1.8 x 10^6 N/m, and as long as the deflection is less than 3cm (more than that and the skin breaks, and the parasite likely dies. If the bullet has a mass of 50g, and travels at 300 m/s, does the bullet bounce off the creature, or does it penetrate? How do you know?
Homework Equations
Usp=1/2K(total s)^2
Ke=1/2mv^2
Esys=total Ke+total U+total Ethermal=Wext
Emech=total Ke+total U+ total Ethermal
Usps=1/2(1.8x10^6)(0.03)^2=810J
Ke=1/2mv^2=1/2(0.05)(300)^2=2250J
I dont know how to take it farther than this, or if this is the correct way to start the problem. If this is correct, would it be correct to assume that the bullet does penetrate the creature because Ke overcomes Usps?

Related Introductory Physics Homework Help News on Phys.org

#### phinds

Gold Member
If the bullet has a mass of 50g, and travels at 300 m/s, does the bullet bounce off the creature, or does it penetrate?
Uh ... ya think maybe it matters whether the bullet is a hollow point or sharp-nosed or armor-piercing ?

#### jbriggs444

Homework Helper
Usps=1/2(1.8x10^6)(0.03)^2=810J
Ke=1/2mv^2=1/2(0.05)(300)^2=2250J
I dont know how to take it farther than this, or if this is the correct way to start the problem. If this is correct, would it be correct to assume that the bullet does penetrate the creature because Ke overcomes Usps?
Yes, that is pretty clearly the intended approach. I agree with your calculations and your prediction for penetration.

As @phinds suggests, the armored skin specification in the problem statement does not seem very realistic or complete since it ignores the size of a projectile's pointed tip.

"Interactions and Potential Energy"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving