Free-Body Diagrams Worksheet



Name: _________________________________Free-Body Diagrams(modified from csun.edu/~jjv7820/fbd.doc and kellhighschool.files/drawing-free-body-diagrams.doc?)Free body diagrams (FBDs) are simplified representations of an object which includes only the forces acting on the object. The body is free because the diagram shows it without its surroundings (the body is 'free' of its environment). The mass of the object in kilograms should be written inside of the box. FBDs help to eliminate unnecessary information given in a problem, they may include the following forces:Gravity The gravitational force acts on all objects. The acceleration due to gravity (on Earth) is constant and acts downwards, it is approximately g = -9.8 m/s, but the force due to gravity depends on the object’s mass. The gravitational force, is Fg = mg (We will also refer to this as the weight of the object, so we can also say W = mg)Normal 454342513208005762625844550The normal force (FN) is what prevents objects from 'falling' indefinitely. It is always perpendicular (normal) to the surface an object is in contact with. For example, if there is a box on the floor, then we say that the box experiences a normal force by the floor; and because of this force, the box does not fall into the floor. The normal force on the box points upward, perpendicular to the floor. For problems in this course, the normal force opposes gravity, so FN will equal FGFriction Friction is a force that opposes motion, it is caused by the rubbing of surfaces. The magnitude of the friction force is related to the normal force, but whereas the normal force was perpendicular to the surface, the frictional force is parallel. More specifically, its vector always points away from the direction of motion. Some surfaces are “rougher” than others, this is quantified by the coefficient of friction (). The frictional force is given by the product of the normal force and the coefficient of friction, Ff = FN. Applied ForceAn object might experience an external force by a physical push or pull. This could be caused by the wind pushing on a ship, a child pulling on a wagon, or in the case of our first example, a person pushing a box on the floor. The applied force is written as FA. In the problems for this course, this will be equal to the frictional force because we are overcoming that force to get the object to move at a steady pace. So, FA = FFr210502546926500Try to label the four forces in the diagram to the below. Let’s assume that the applied force is going to the left.For now, the first step is to recognize all the forces involved by drawing FBDs. It takes practice to look at a situation and determine all the forces at play – practice is the key! Try the following:A 2.18 kilogram book is at rest on top of a table.A 57 gram egg is free-falling from a nest in a tree (double-check that mass!). Neglect air resistance. A 0.5 kg flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity down and to the right. Consider the frictional force of air resistance (fluid friction). Now, let’s throw some math in here!A rightward force of 7N is applied to a 2.5kg book in order to move it steadily across a desk. Consider frictional forces. What would the coefficient of friction be?DRAW FBD HERE:SOLVE PROBLEM HEREUse the following information to solve #5Coefficient of friction for rubber and dry asphalt: 0.9Coefficient of friction for rubber and wet asphalt: 0.45A 1500 kg car heading west slams on brakes. What would the frictional force be to stop the car on a sunny day?DRAW FBD HERE:SOLVE PROBLEM HEREWhat would the frictional force be to stop the car on a rainy day?DRAW FBD HERE:SOLVE PROBLEM HEREWhich scenario in #5 one would the car be harder to stop? Why?Describe what the Fnet would be for the following three pictures. Remember, FNet is the result of all of the forces combined. For opposing directions, remember that FNet is equal to Winners-Losers ABCA)B)C) Which examples are balanced forces? ___________Unbalanced forces? ___________How do you know?A 20 kg object is at rest. A force of 200 N to the right is required to set the block in motion. What is the coefficient of friction?DRAW FBD HERE:SOLVE PROBLEM HERE:Once the same box is in motion, it will maintain a constant velocity as long as a 150 N force to the left acts upon it. What is the coefficient of friction here?DRAW FBD HERE:SOLVE PROBLEM HERE:Wait…I don’t understand. If it is the same box in #8 and #9, how can I have two different coefficients of friction? ................
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