![]() ![]() A wooden table works exactly the same way, only much more stiffly. If the weight is suddenly removed, then the plastic will snap back, demonstrating that it was exerting an upward force while it was bent. ![]() If you imagine the table being replaced by something more flexible, such as a light sheet of plastic, then you know the plastic will bend downward as more weight is placed on it. In situations where there may not seem to be any reaction, for example, a book sitting on a table, you have to remember that the table is pushing up on the book even as gravity pulls it down. Whenever you fire a gun, there is a strong "kick" backwards as the bullet shoots forward. An alternative way to think about this law is to say that forces always come in pairs: each pulling with equal strength in opposite directions.Įxamples of the Third Law: If you jump off a small boat to the shore, the boat will move in the opposite direction from your jump. Newton's Third Law states that for every action there is an equal and opposite reaction. The momentum P R of the rifle must be equal and opposite to the momentum P B of the bullet, or m Rv R = m Bv B. We can directly compare mass to weight on the Earth's surface (for example, on a bathroom scale) only because the gravity is more-or-less constant everywhere near the surface of the Earth. The mass of an object never changes, whereas the weight can be almost anything, depending on where the object is. However, the mass of the astronauts has not changed. ![]() Astronauts on the Moon weigh about 1/6 of what they do on the Earth, because the Moon is much smaller than the Earth and so has weaker gravity. Weight is merely the result of gravity pulling on the mass. Mass is something that an object has, and is intrinsic to it. HOWEVER, it is critical to realize that mass and weight are not the same, and in fact, don't have anything specific to do with each other. The most intuitive way to think about mass is in terms of weight: if you double the mass of an object, you will double its weight at the Earth's surface. (Note that force always has a direction.) The mass of an object, m, which appears in this equation is simply a measure of how much matter the object contains. The equation F = ma actually defines what we mean by force. Your instantaneous speed and your average speed become more and more nearly the same as you make the time interval over which you measure the speed smaller and smaller.ġ) All objects in motion in a straight line (or at rest) will remain in motion in a straight line (or remain at rest) unless acted upon by a force.ģ) For every action there is an equal and opposite reaction. Your instantaneous speed is your speed at a given instant, which in your car is conveniently measured by your speedometer. However, unless you live in a very strange neighborhood, it is very unlikely that your speed was exactly 30 miles per hour for the entire journey. For example, if you drive the 15 miles from your home to a friend's home in 30 minutes, then your average speed is 15 miles / 30 minutes = 0.5 miles per minute = 30 miles per hour. You also need to realize that the above formulas for speed and acceleration actually give you the average speed or acceleration over the time interval in question. And since its velocity at the top is zero, this means that its velocity would remain zero forever and the rock would just float there.) (Think about it - if the acceleration was really zero for the rock at the top of its arc, then its velocity would not be changing. But, it is STILL being accelerated downward at 9.8 m/s by the Earth's gravity, because its velocity is CHANGING from zero to not-zero, and it is the CHANGE in velocity that determines acceleration, not the velocity itself. In fact, it is not even necessary for the object to be moving! For example, a rock thrown into the air comes momentarily to a stop when it reaches its maximum height. In other words, it is not necessary for an object to be changing speeds in order to be accelerating. Therefore, its acceleration is also constantly changing, because the direction in which the object is being accelerated is changing. For example, an object moving in a circle at constant speed is NOT moving at constant velocity, because the direction of its motion is constantly changing. Velocity has a direction, therefore acceleration has a direction. In other words, "50 miles/hour" is a speed, "50 miles/hour going N by NW" is a velocity.Īcceleration is the change in the velocity per unit time, or:Ī = / Speed is the distance traveled per time, or s = d / t. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |