PHYSICS Misconceptions

I wish none of You Infected with the Misconception!!! A quick check………….. ……………. ………. Q: If you’re spinning a yo-yo around your head at a constant 30 revolutions per minute, then It has constant velocity It got constant acceleration which is = acceleration due to gravity It has non-uniform speed You can feel it’s acceleration as a tug on the string. Q: An object is projected into the air at an angle of 30˚ with the vertical with a speed of 14m/s. What is its speed at the highest point of its journey? 14m/s Zero 7m/s 14m/s Q: Friction is an example of Action at a distance force Gravitational force Electromagnetic force None of the given Remember: forces do not cause motion. Forces cause accelerations Newton's first law of motion declares that a force is not needed to keep an object in motion. Why Do People Say Weight instead of Mass? People often use "weight" to mean "mass", and vice versa. Because gravity is pretty much the same everywhere on Earth, we don't notice a difference. if the object were far out in space it would just float around, exerting no force on the scales. The scales would show 0 kg ... but the mass is still 100 kg  Weight or Mass  so sometimes people say "kilogram force" (kgf) or "pound force" (lbf) to show that they are talking about the force that the mass exerts because gravity is pulling down on it. But there is a better measurement ... Newtons Why Scales Show Kilograms or Pounds But scales show Kilograms or Pounds because that is what people understand best ... ... but it is really just an estimate of the mass above them. Scales should really show Newtons, but that might confuse people! What do you think of weight of the Earth? While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus? Trick Question! Each force is the same size. For every action, there is an equal ... (equal!). The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction. Besides, fireflies have guts and bug guts have a tendency to be splatterable. Windshields don't have guts. There you have it. Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ... greater than the acceleration of the bullet. smaller than the acceleration of the bullet. the same size as the acceleration of the bullet. The force on the rifle equals the force on the bullet. Yet, acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass. Remember: acceleration and mass are inversely proportional. Now take a look at the situation below and see if you know, just by looking, which way the block should move with (and then without) friction: Should block 1 go up the plane, or down? There’s no way to know without knowing m1, m2, the angle of the inclined plane, and the coefficient of static friction Three students A,B and C were discussing their physics homework prior to class. They are discussing an object that is being acted upon by two individual forces (both in a vertical direction) ignoring friction and air resistance; the free-body diagram for the particular object is shown at the right. During the discussion, what they say is given below: A: the object under discussion could be moving in horizontal direction B: the object MUST be just at rest C: Forces do not cause acceleration; Forces cause motion A woman of mass 70 kg weighs herself in an elevator. If she wants to weigh less, should she weigh herself when accelerating upward or downward? Explain A fluid flows steadily from left to right in the pipe shown above. The diameter of the pipe is less at point 2 then at point 1, and the fluid density is constant throughout the pipe. How do the velocity of flow and the pressure at points 1 and 2 compare? Wisit our school website for Worksheets www.iisriyadh.com and for my lectures at wiziq www.wiziq.com/wiziqphysics And, also don’t close your window without giving feedback Thank you for being with me for an hour. INTERNATIONAL INDIAN SCHOOL, RIYADH, K.S.A XI_PHYSICS_Design of GulfSahoday 2011,BluePrint and Important Topics. –Md. A. Mumeed, Physics Teacher-I.I.S.R +966507433412 9 © Md. Abdul Mumeed, Senior Secondary Teacher in PHYSICS at I. I. School, Riyadh, K.S.A.