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Home Minds On Newton's Laws Mission NL7 Newton's Second Law Help Help 4

Mission NL7 Newton's Second Law

The Question

Force diagrams depicting the the magnitudes and directions of the forces acting upon four objects are shown below. In each case, the down force is the force of gravity. Rank these objects in order of their acceleration, from smallest to largest.

Physics Rules

Newton's Second Law
The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

Formula Fix

The relationship between net force (Fnet), mass (m) and acceleration (a) is expressed by the equation
 
a = Fnet/ m

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Minds On Time

The acceleration of an object depends directly upon net force and inversely upon mass. All four objects represented by the diagrams have identical net force. In each diagram, the net force (vector sum of all individual forces) is 20 Newton. Thus, the mass will be the variable of importance in comparing the accelerations of the four objects.

Since acceleration is inversely proportional to the mass, the object with the greatest mass has the smallest acceleration. Inspect each diagram carefully and calculate the mass. If the down force is the force of gravity (as mentioned), then the mass is approximately 1/10-th the down force. (That is the result of using the equation Fgrav = m•g.) Rank the four objects in decreasing order of mass - which will be the same as the increasing order of their acceleration.

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