 # NEWTONS FIRST LAW Newtons First Law Aristotle in

• Slides: 20 NEWTON’S FIRST LAW Newton’s First Law � Aristotle, in the 4 th century BC, made the mistake of saying that if something is moving, it is because a force is acting. � Galileo said that no force is required to keep an object moving. Newton’s First Law � � Isaac Newton came along soon afterward and stated: “If something is at rest (and you don’t “mess” with it), it will remain at rest. If something is moving (and you don’t mess with it), it will keep moving. ” We refer to this as Newton’s 1 st Law, or the Law of Inertia. Newton’s First Law � So, in the absence of a force: ØSomething moving will continue moving with constant velocity ØSomething at rest will stay at rest Newton’s First Law � Inertia is another word for Mass or Matter. � Zero net force means that all of the forces acting on an object must be balanced (cancel out). � If there is no net force, there is no acceleration (another word for change). When Mr. Hewitt suspends a heavy ball by a string and pulls on a second string hanging below the ball to compare the roles of weight and mass in determining whether the top or bottom string breaks, which string breaks when he pulls slowly? TOP STRING Why? The tension is greater in the (UPPER , LOWER) string, so the property of (mass, weight) is more important in determining which string breaks. Explain more fully here if needed. Which string breaks when he pulls quickly? BOTTOM STRING Why? The property of (Mass , Weight) is more important. Explain more fully here if needed: The top string is protected by the massive ball. When Mr. Hewitt lies on a table with the anvil on his stomach and has a student hit the anvil with a sledgehammer, why is he not hurt? The mass of the anvil creates a tendency for the anvil to stay where it is. The mass protects his stomach. Newton’s First Law If the head of a hammer is loose, and you wish to tighten it by banding the handle against the top of a workbench, why does this work to tighten the hammerhead? The mass of the hammerhead has a large inertia when the handle stops as it hits the table, the hammerhead wants to keep going tightens the hammerhead to the hand. Newton’s First Law � Why are you shorter at night than in the morning? All day long, your inertia (mass) acts on your spinal chord and you get shorter Newton’s First Law In the scaffold system illustration, the key concept is that because the system is not accelerating, the net force on the system must be zero. In other words, “the ups equal the downs. ” Newton’s First Law In the video, Hewitt leaves you at the end with a question to think about: If Harry weighs 200 pounds, but the rope has dry rot and will only support 150 pounds, why would tying one rope to a flagpole make this Harry’s last day on the job? Harry’s weight of 200 lbs. is divided up by 2 ropes. If he ties one end off, all of his weight is redistributed to 1 rope. UH… OOHHH!!! Newton’s First Law If you were in outer space and threw a ball, how much force would be required to keep it moving through the vacuum of space? Zero – a body in motion will stay in motion Law of Inertia Newton’s First Law If you push on something horizontally with 10 N of force and a friend pushes with 8 N of force in the opposite direction, what is the net force? 10 N – 8 N = 2 N to the right -----10 N-----> <-----8 N----- = 2 N Newton’s First Law What is the tension in the arms of a 150 pound person who does pull-ups with both arms? ü 75 lbs. on each arm Newton’s First Law What is the net force on any object in equilibrium? ü Zero Newtons Newton’s First Law (Mass , Weight) is a measure of the actual material in a body, depending only on the number and kind of atoms that compose it. (Mass , Weight) is a measure of the gravitational force that acts on the material and depends on where the object is located. The amount of inertia an object has depends on its (mass , weight). Newton’s First Law Two painters are on a scaffold system similar to the one drawn in the video by Paul Hewitt. If the board has a weight of 150 N and each painter has a weight of 800 N, how much tension is in the two ropes if each painter stands on an opposite end of the board? 800 N + 150 N = 1750 N Divided by 2 (ropes) = 875 N Newton’s First Law The same two painters have rearranged themselves on the scaffold. Both painters are now standing on the right half of the scaffold. This has increased the tension in the right rope to 1300 N. What is the tension in the other rope? Ø Ø Ø All forces balance out to be zero. The weight acting down equals the tension acting up A body at rest tends to stay at rest = 0 acceleration PROBLEM SOLVED q q Right rope = 1300 N down Scaffold weight = 150 N down Total weight down = 1750 N X (left) = # N on the left of the scaffold 1750 N (total) = 1300 N (right) + 150 N (scaffold) + X( left) X (left) = 300 N 