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Apparent weight/ normal force Physics Mr. Maloney Objectives You will be able to … describe how an inanimate object can apply a force apply Newton’s 2nd Law to determine an object’s apparent weight (Normal Force) solve problems involving apparent weight. © 2002 Mike Maloney The “Normal” Force How does the table know how hard to push to hold something up? © 2002 Mike Maloney The “Normal” Force Surfaces are able to apply a surface normal force because they deform, if only slightly. This deformation stretches or compresses molecular bonds, which in turn apply a force trying to get the molecular arrangement back into place. © 2002 Mike Maloney Apparent weight We have already said that Weight (Fg) = mg, but when we talk of weight in everyday life what do we really mean? What happens to your weight in a elevator? Or when you are on a hill? Or when someone pushes on your shoulders? Or when you are carrying a lot of books? Or after you have jumped out of an airplane? The weight we feel, or our APPARENT WEIGHT is another name for the NORMAL FORCE on us. This is also the force that a bathroom scale reads. © 2002 Mike Maloney Normal Force [FN] For example, the normal force of a book sitting on a table is the force exerted on the book by the table. According to Newton’s 2nd law, this is equal to its weight at rest in this situation. The table pushes up on the book. The weight of the book pushes down on the table © 2002 Mike Maloney FN Fg Normal Force [FN] What would happen if we added a mass on top of the book? The normal force would increase to balance out both the book and the mass’ weight. Lets test with a few students The table pushes up harder now. FN Fg, book © 2002 Mike Maloney Fg, mass http://mrmaloney.com/mr_maloney/simulations/jar-files/ejs_mem2_elevator4.jar Normal Force on Elevator Think about this situation … work in groups Picture yourself as a penguin riding on an elevator. What would your apparent weight be under the following six (6) conditions? 1. The Elevator is at Rest. 2. The Elevator is rising at a constant velocity 3. The Elevator is accelerating up 4. The Elevator is accelerating down 5. The Elevator is in free fall 6. The Elevator is accelerating sideways © 2002 Mike Maloney Normal Force [FN ] (CONTD.) We said the normal force on a book on a table is equal to its weight, and if you pushed on it or laid a mass on top of the book the normal force would increase. What would happen to FN if the book were on a hill or a ramp? © 2002 Mike Maloney Normal Force [FN] (CONTD.) Lets go back to our block on an incline. What forces are acting on the block if we don’t include friction? © 2002 Mike Maloney Normal Force [FN] (CONTD.) Right if friction is negligible, only gravity and FN are acting on it. But how big is the normal force? How can we find out? FN Fg © 2002 Mike Maloney Normal Force [FN] (CONTD.) Part of the Weight is acting down the incline {Fgx} Part is acting into the incline {Fgy} Which one acts in the same direction as the normal force? Right, Fgy And how do we find Fgy with respect to the weight? © 2002 Mike Maloney FN 30° Fgy Fg Fgx Normal Force [FN] (CONTD.) From our vector lessons, Fgy = Fg ∙ cos(θ) In this case Fgy = Fg ∙ cos(30) = Fg ∙(0.87) = 0.87 ∙ Fg Which corresponds with our theory that it is only part of the weight. Sim of Block on Hill © 2002 Mike Maloney FN 30° Fgy Fg Fgx Objectives Can you … describe how an inanimate object can apply a force apply Newton’s 2nd Law to determine an object’s apparent weight (Normal Force) solve problems involving apparent weight. © 2002 Mike Maloney