|MadSci Network: Other|
Becky, If a magnet, being held by a person in a car, is hanging in front of a car and braced so that the magnet remains the same distance from the car (so they will never touch) the car will not move forward. This can be explained by using Newton's laws of motion. According to Newton's 1st law, an object will remain in motion or remain at rest unless acted on by an outside force. Newton's 2nd law states that the net force applied on a body is proportional to the acceleration. Finally, Newton's 3rd law states that for every action there is an equal and opposite reaction. According to these principles, in order for the car to move, there needs to be a net force in (for example) the forward direction (2nd law) otherwise the car will stay at rest (1st law). To see if there is a force on the car, we draw what is called a free body force diagram to illustrate all the forces present in the system (a further explanation of what a force diagram is can be found in any introductory level college physics text book). The force of gravity is balanced by what is called the normal force so that neither the car or the magnet will move vertically. The magnet exerts a horizontal force on the car. However, according to Newton's 3rd law, the car also exerts a force on the magnet. In other words, for the magnet to attract the car, the car also has to attract the magnet. These forces are equal and opposite. In this case, the magnet is being held at a set distance from the car. The object holding the magnet in place also has forces acting horizontally outward, essentially pushing against both the car and magnet with equal and opposite forces. All these forces cancel one another out. Hence this system has no EXTERNAL forces acting on it and the car will not move. You could experiment with this at home. Get a small magnet and some metal object that the magnet can pick up. Attach the magnet and the object together, separated by a short distance so they do not touch, as per your question (i.e. glue a thin toothpick or something to keep them apart). If the magnet is on top, they do not start flying upward, and if the magnet and object are horizontal, they do not begin to move around the room. In order for the magnet to pull something around the room, an outside force (you) need to be pulling the magnet along. Without that, they will just sit there. Note: If there is no object holding the car and magnet apart, they will both move towards each other until they are touching, but the center of mass of the system as a whole will not change. This means that the car/magnet system will not move forward. Erika
Try the links in the MadSci Library for more information on Other.