Let’s talk about force. This is an extremely important concept to understand, because forces are acting on you all of the time. Pretty much wherever you are, and whatever you are doing, forces are acting on you. It is almost impossible to get away from forces. Let’s examine some of the forces that you experience every single day:
- Gravity is a force that is constantly pulling you down towards the Earth. Technically, all objects attract all other objects, so even you have some gravity, and you are attracting everything around you towards you, but the force of your gravity is so low that it doesn’t matter. The Earth’s gravity is obviously much larger. It pulls down on you with a force equal to your weight. The moon’s gravity also pulls on you. So does the sun’s gravity. Those are just significantly weaker than the Earth’s gravity, when you are standing on Earth.
- The normal force is one of the most important forces around, but it is not entirely obvious without thinking about it a bit. This force keeps you from sinking down into the ground or through the chair that you are sitting on. When you are just standing around or sitting down or basically being motionless, this force is exactly opposite and equal in strength to gravity. One of the things that has always frustrated me about the X-Men, is Kitty Pride’s super powers. She has the power to walk through walls, basically becoming transparent to physical objects. If she were to do this and have her normal mass, then she would immediately start falling through the floor, since gravity would act on her, but there would be no normal force to stop her from falling.
- Friction keeps you and your car and lots of other things from sliding around all over the place. While friction is extremely important for allowing you to walk and drive your car, it is a force that we constantly battle against. If we could make perfectly frictionless bearings, we could make cars that could get better gas mileage and wouldn’t have to worry so much about parts wearing out. Friction typically causes things to heat up – think of a rope or carpet burn.
- Air resistance or drag is very similar to friction, since it works to slow things down. It is the friction a body moving through a liquid feels. You can actually feel a drag force when you are standing still, since if the wind is blowing by you at strong enough speeds, you will feel a force that is in the same direction as the wind is moving. Drag acts on your car most of the time also, and it becomes the primary force when you go above about 50 MPH. This is why you get worse and worse gas mileage the faster you go.
- Air pressure is another extremely important force that we feel all the time, but never think about. The air around you is thick enough that it is pushing on your skin with about 15 pounds per square inch. If you approximate a person as being 70 inches tall by 6 inches thick by 15 inches wide, you have a total surface area of about 70x15x2 + 70x6x2 + 6x15x2 = 3,120 square inches. This means that you have a total of 46,800 pounds of air pressure pushing on you all of the time! Incredible! Your body is built to expect that much force. So, if that force goes away, it would be like popping a balloon – you explode! Your body can take a few times this much force, which you can feel by going deep under water. If you go too far under water, the pressure becomes too much and bad things happen to your body.
- Electric and magnetic forces are also in everything around you, but you most likely never think of them. These are the forces that allow electricity to flow in wires, cause electric motors to work, allow batteries to charge, and power stations to work. Interestingly, these same forces make surface tension in water, so it doesn’t overflow a cup when you fill it just a bit too much. These forces are actually much stronger than the gravitational force.
- Thrust is a force that you get when you expel exhaust. It is obviously very important for rockets! It is not really a force that you encounter everyday, though, unless you do things like fly in jet airplanes.
See! There are forces everywhere! Most forces that we will talk about in the coming posts can be directly related to the above list of forces.