Having hairs ripped out of your scalp can hurt. If enough hair is pulled out at one time you could end up with a bald patch. Whether the hair will grow back depends on why the hairs were pulled out and how much damage the pulling caused to the hair follicles.
Normal hair growth involves a cycle for each individual hair. The hair goes through a period of growth, then one of rest, and then a time when the hair is shed. Your hair will follow this cycle unless the hair follicle is damaged. Ripped-out scalp hair will usually grow back. The determining factor is if the action that ripped out the hair damaged the follicle. Some actions can cause permanent hair loss.
If you wear your hair pulled back into a tight braid or cornrows, you can slowly rip hairs out of the front of your scalp. At first, the hairs will grow back. If you wear this hairstyle for a long time, the follicles of the hairs that have been pulled out repeatedly will be damaged. This leads to a form of hair loss called traction alopecia. If you do not change your hairstyle the hair loss can be permanent.
Trichotillomania is a psychological compulsive/impulse control disorder that affects as much as 4 percent of the population. It is more likely to occur in women than in men. A person who experiences it will feel overwhelming compulsions to pull out her hair. If she receives treatment and is able to control the impulse, the hairs she ripped out of her scalp will grow back. If her hair pulling continues untreated, then the hair follicles will be damaged and the hair will stop growing back.
A man who wears a hairpiece attaches it to his hairline. If he attaches it to the same place again and again, the hairs there will be ripped out of his scalp over and over. If this continues long enough, it has the same affect as a too-tight hair style. He will experience traction alopecia when the hairs stop growing back. If he does not change how he attaches his hairpiece, the new areas of baldness will become permanent.
If some of the hairs on your scalp get caught in a door or a window and are ripped out, they will grow back. If you are in an accident and a larger number of hairs are ripped out and they take part of your scalp with them, then the chance of their regrowth depends on how much damage has been done to your skin. If the wound is bad enough to cause scarring, it is unlikely the hair will grow back.