Information sheet The friction model Nuffield （信息表纳菲尔德的摩擦模型）.pdf

Solve friction problems In this activity you will solve problems involving friction. You will combine the model F ≤ μR with Newton’s Second Law and the constant acceleration equations. Information sheet The friction model Friction acts tangentially along surfaces in contact, in the direction that opposes motion. Before sliding occurs: friction is just sufficient to maintain equilibrium and prevent motion. F FMAX On the point of sliding and when sliding occurs: F = μR where F is the friction force acting between two surfaces in contact, μ is the coefficient of friction, and R is the normal contact force. (Note that sometimes N is used instead of R for the normal contact force.) You can use the friction model to determine whether a body will slide, what force will be required to make a body slide, or to find the coefficient of friction between two surfaces in contact. Example A 5 kg box on a horizontal table is pushed by a horizontal force of 15 N as shown on the right. 5 kg 15 N If the coefficient of friction is 0.4, will the box move? Think about What is the smallest force that could make the box slide along the table? Solution R The sketch shows the forces acting on the box. –2 g g Note that the weight of a box of mass 5 kg is 5 where = 9.8 ms Since the vertical forces are in equilibrium, R = 5g F 15 N Therefore the maximum possible friction is F = μR = 0.4 5g = 19.6 N 5g