Why is a knife a wedge?
Pressure is defined as the force per unit area:
That means that if you apply a large force over a small area, the pressure(or force exerted) will be tremendous, which is useful for cutting.
Using this equation you can think about what would hurt the most if it stepped on your foot:
- An elephant with weight 10 000 N and with a foot area of 0.5 square meters.
- Or a woman of weight 700 N with a stiletto heel of an area of 1 square centimeter (0.0001 meters squared).
I'll leave you to find out :)
Anyway- A knife is shaped like a wedge because of the small area it gives, which means we can cut stuff without having to apply a large force- as the pressure can become much higher if we use a small area.