![]() ![]() The fact that it acts opposite to the direction in which the other forces would, collectively, accelerate the object explains the statement of property 1. Hence, there must be some other force in the horizontal direction that is directed opposite my push and has exactly the correct magnitude to cancel my push’s effect. Thus the book’s acceleration is zero, and from Newton’s First (or Second) Law we know that the forces on the book must sum to zero. If I push lightly enough the book doesn’t move. Now I push lightly on the book in a direction parallel to the table top. Similar to kinetic friction, μ S is called the static friction coefficient and is determined entirely by the nature of the materials in contact.Īn example should make both these properties clearer. Below we’ll denote this threshold value as ƒ s,=μ S| F N|. Property 2 can be stated in words as the magnitude of the static friction force is always less than or equal to a threshold value given by μ S| F N|. ƒ S acts opposite the direction of intended motion.The static friction force, ƒ S, can be characterized as follows It is important to remember (and interesting to ponder) the fact that kinetic friction coefficients don’t depend on the contact surface area between sliding objects. For example, pine wood sliding on Plexiglas has a fixed value for its kinetic friction coefficient. The kinetic friction coefficient is entirely determined by the materials of the sliding surfaces. Where μ k is called the kinetic friction coefficient and | F N| is the magnitude of the normal force of the surface on the sliding object. The magnitude of the kinetic friction force, ƒ k, on an object is When you send a text book sliding across a desk, the force of kinetic friction on the book is what brings the book to rest. It is always directed opposite the direction of motion. Kinetic friction acts on an object which is sliding along another object. First, we need to distinguish between the two types of friction forces: kinetic friction and static friction. We’ll make the meaning of the term intended motion clearer in a moment. Thus, a friction force on an object is always directed opposite the direction of an object’s motion or intended motion. Frictionįriction is a force that describes the tendency of objects in contact to resist relative motion. The best example of this is the case of a plane with friction. This was convenient but there are inclined plane problems in which we must calculate | F N| before we can find | F net| and a. Notice that in this problem the normal force, F N, cancelled with F G,⊥ so that we never had to worry about calculating the magnitude of F N. ![]() Thus we conclude that | F net|= mgsin θ and that F net is directed down the plane.Ģ: To find the acceleration apply Newton’s Second Law in the direction of F net then Using trigonometry on the triangle with dashed sides we have| F G,∥|= mgsin θ. Thus the magnitude of the net force, | F net| is just the magnitude| F G,∥|. Since F net is along the plane all force components perpendicular to the plane must sum to zero. To determine the magnitude | F net| we use the fact that the vector F net is the vector sum of all forces acting on the block The block’s acceleration must be purely down the plane, for otherwise the block would fall into or jump off the plane, we know these things won’t happen. Notice that dashed arrows have been used to resolve F G into two components F G,⊥ and F G,∥ which are perpendicular and parallel to the plane respectively.ġ: To determine the direction of the net force, F net, notice that Newton’s Second Law, F net=m a, implies that the net force is in the same direction as the acceleration, a. Figure 1 illustrates the situation with a free body diagram for the block. ![]() Solution:The forces acting on the block are gravity, F G, and the normal force, F N. The magnitude and direction of the block’s acceleration in terms of g and θ.The magnitude and direction of the net force on the block in terms of m, g and θ.We’ll discuss the effects of friction in the next section.Įxample 1: A block of mass m is on an incline that makes an angle, θ, with the horizontal. ![]() Here is a representative inclined plane problem which ignores the effects of friction. Motion on an inclined plane is a classic application of Newton’s Second Law and free body diagrams.
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