Unit Dynamics Module Applications of Newton s Three Laws page 1 of 1 Free Body Diagrams When using Newton s second law 1 Focus on one object and consider all the forces acting on it 2 Draw a free body diagram or force diagram 3 Choose a coordinate system A free body diagram is a rough sketch of an object and the forces applied to it Apply Newton s second law to an object resting on a table First focus on the object Next draw a rough sketch of the object use boxes and draw all the forces acting on the object In this case the object experiences a force due to gravity W m g There is also a contact force by the table on the object FOT Finally choose a coordinate system In this case upward is positive Now you can determine the sum of all the forces on the left side of Newton s second law In this case the object is not accelerating so the right side of the equation is zero Consider a coffee filter falling to the ground You notice that it falls at a constant speed What is the force of air drag on the filter You have already focused on an object the coffee filter Next draw a force diagram The filter experiences a force due to gravity W m g It also experiences a force from air drag FOA Since the coffee filter falls at a constant speed the right side of Newton s second law is zero The force of air drag on the filter is equal to its weight Newton s second law is a vector equation so you can use it for two and three dimensional problems In this example two people are pushing a box with forces F2 and F1 and angles and from the horizontal The surface is frictionless You would like to know the acceleration on the box Using Newton s second law write the vector sum of the forces as the product of mass and acceleration F1 F2 m a Then you can write the x and ycomponents of Newton s second law The acceleration on the box is a ax i ay j www thinkwell com info thinkwell com Copyright 2001 Thinkwell Corp All Rights Reserved 1837 doc rev 03 29 2001
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