I took your question out of that old quantum thread, since yours doesn't involve quantum. (posted without checking until Lee returns) That property turns out to be general, regardless of the details of the source of the magnetic field. As you can see in this example, it causes acceleration at right angles to the motion. We invent a different field, one which only causes moving charges to accelerate. Switching back to the frame where the wire is stationary, we have to account for why that moving particle is accelerating toward the wire even though in this frame there's no electric field. From its point of view, the nearby wire is negatively charged, and it will experience a net electric field and accelerate toward the wire. So our charged particle sees a more concentrated line of negative charges. What if it's moving a bit parallel to the wire, say to the right? The negative charge line is more contracted in its frame, since it's moving to the left, and the positive charge line is less contracted. If it's set in motion in any direction perpendicular to the wire, it sees no contraction of either the positive or negative line of charges. It sees no force, since the wire is neutral. That's a simple symmetrical way of describing a current, a source of a magnetic field. Look at a positively charged particle up a bit from the wire, standing still in the wire frame. Now think of an electrically neutral wire, with positive charges moving to the right and negative ones moving to the left. If something is in motion relative to you, it shrinks along the direction of that motion, compared to the dimensions it has according to someone at rest with respect to the object. We start with special relativity, specifically the Lorentz-Fitzgerald contraction effect. in Purcell's book on Electricity and Magnetism. Here's how the argument is often made, e.g. We have to start with some deeper principles. Since the vector cross product is always at right angles to each of the vector factors, the force is perpendicular to v. To give a more explanatory answer, we have to say something about why this force exists with that form. The superficial answer is simply that the Lorentz (magnetic) force is proportional to v× B, where v is the particle velocity and B is the magnetic field. Do be careful not to fall over though.That's quite a deep question. The rougher the surface the more friction is produced, this is why rockets are streamlined! Friction Experimentsįind out why you slip and slide more on smooth surfaces than rougher surfaces with this slipping and sliding activity. The amount of friction between two objects depends on what the objects are made from. There is more friction between the velcro strips than the ribbon. Imagine sliding two strips of ribbon over each other and then think how hard it would be with two velcro strips. Friction allows things to start and stop moving and slows them down. Friction Experiments for Kids What is Friction?įriction tries to stop objects sliding past each other. Learn about simple push and pull forces in this easy activity. Over the years we’ve completed lots of forces experiments on Science Sparks, here are the best ones. Other examples of forces are magnetism, gravity and air pressure. Try rubbing your hands together? Do they get hot? You feel the friction between your hands as heat.Īir resistance and friction take time to slow an object down, if you want an object to stop quickly you need to apply further force, for example a brake on a bike. The reason it slows down is because of two forces, air resistance and friction.Īir resistance is air pushing on a moving object which slows it down.įriction is the force between two objects when you rub them together. If you give a toy car a push what happens? It speeds up and then slows down. For example you could push an empty box easily, but a filled box would be harder, it would need more force to move. Forces can also make objects change direction or shape.Ī lighter object needs less force to move than a heavier object. Suppose the velocity of an object of mass m. If you push a toy car it moves, if you push it harder it moves faster. Newtons second law of motion also gives a relation between force and acceleration. Forces can make object moves or stop, speed them up or slow them down. Forces and Motion – What is a force?Ī force is a push or a pull.
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