Some Types of Force
There are lots of types of forces, and we will discuss several throughout this semester. Right now, let's introduce three important forces:
- support force, and
- frictional force.
The weight of an object is just the same thing as the force due to gravity acting on the object. If we know the mass of an object, we can find its weight at or near the earth's surface by multiplying its mass by the acceleration due to gravity (g = 9.8 m/s/s), and weight always points downward toward the center of the earth. An object's weight varies depending on location. For instance, an object will not weigh the same on the moon as it does on the surface of the earth because the moon's gravitational pull is not equal in size to that of the earth.
A support force is just the force exerted by a surface on an object which is being supported by the surface. For instance, if you are sitting on a chair, the chair is exerting a support force on you. If you have a table in front of you, the ground exerts a support force on the table, and if your computer is on top of the table, the table exerts a support force on the computer. We could keep this up all day... The support force is often called the "normal force" because the support force is always normal to (straight out of) the surface. So, if your table is on top of a horizontal floor, the floor exerts a support force on the table which points straight up. In fact, a horizontal surface will always exert support forces which point straight up on objects which are resting on it.
The frictional force is the force exerted by objects or substances on each other which are moving or attempting to move relative to one another while in contact with one another. In order for objects to exert frictional forces on each other, they must be in contact with each other (they must be touching), and they must be either moving or attempting to move relative to one another. Frictional forces almost always oppose motion. So, if I am sliding a desk to the right across the floor, the floor will exert a frictional force on the desk opposite the direction the table is moving, or to the left. Frictional forces can be exerted by solid objects on other solid objects, like that exerted by the floor on the desk, and frictional forces can be exerted by fluids (liquids and gases) on other objects. For instance, water can exert a frictional force on you while you are wading, and air can exert a frictional force on your hand as you stick it our of the window of a moving vehicle.