Physics equations/Static forces
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Most applications of Newton's laws require only a few equations
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Friction and the normal force
[edit  edit source] is the force friction when an object is sliding on a surface, where ("mewsubk") is the kinetic coefficient of friction, and N is the normal force.
 establishes the maximum possible friction (called static friction) that can occur before the object begins to slide. Usually .
Also, air drag often depends on speed, an effect this model fails to capture.
These equations for static and kinetic friction almost always are valid only as approximations.
*** Problem: Generating equations from adding forces
[edit  edit source]Find the x and y components of the three forces on the small grey circle at the center  click to see or hide solution  Solution:

*** Problem: Almost proving Netwon's third law (one dimensional)
[edit  edit source]Consider two carts of mass known masses ( and ) connected by a taught string, with one mass () experiencing and external force of known magnitude. Assume that the collection of objects held together by a taught string obeys where F is the sum of all external forces. But we do not assume that string (in the middle) exerts equal and opposite forces on the two objects. (In this way we prove Newton's third law.)  click to see or hide solution  Solution: We take the acceleration, as well as the two internal forces as unknowns (we have three unknowns because we make no assumption about the equality of the two internal forces transmitted by the string, which we assume to have negligible mass. With three unknowns, we seek three equations. Beginning with Newton's second law (ΣF=ma) applied to and M=m_{1}+m_{2}, we have:
(They all have the same acceleration because the string is assumed taught.)

*** Problem: A freebody diagram on a system with many parts
[edit  edit source]Make free body diagrams for each component of this system. Include attachments to the ceiling.  click to see or hide solution  Solution: 
*** Problem: Motion under the influence of kinetic friction
[edit  edit source]If this object is in motion and the coefficient of kinetic friction is μ, find the acceleration.  click to see or hide solution  Solution: This problem is unsolved. 