**Question:** Bullet changes it's direction by striking on some surface or object in

A. Coefficient of restitution (COR)
B. Friction
C. Impact force
D. Resistance

**Correct Answer:** B. Friction

**Core Concept:** Friction is a force that opposes the relative motion of two surfaces in contact with each other. It plays a crucial role in determining the trajectory of objects, including bullets, when they collide with surfaces or objects.

**Why the Correct Answer is Right:** When a bullet strikes a surface, the force of friction opposes the bullet's movement, causing it to change direction. This concept is based on the understanding of how forces interact when two surfaces come into contact with each other. In this case, the bullet is the moving object, and the surface it collides with is the stationary object. The force of friction acts perpendicular to the direction of motion, causing a deceleration and change in direction.

**Why Each Wrong Option is Incorrect:**

A. Coefficient of restitution (COR) refers to the ratio of the final velocity to the initial velocity after a collision, not directly related to the change in direction due to friction.
C. Impact force is the force exerted by the bullet on the surface during the collision, not the force responsible for changing the direction.
D. Resistance is a broader term encompassing various opposing forces, but in this context, it is incorrect as it does not specifically address the change in direction due to interaction between the bullet and the surface.

**Clinical Pearl:** Understanding the role of friction in determining the trajectory of projectiles, like bullets, is crucial for various medical scenarios, including ballistics, trauma surgery, and forensic medicine. It helps in analyzing accidents, shootings, and injuries caused by projectiles.