King Fahd University of Petroleum & Minerals Mechanical Engineering Dynamics ME 201 BY Dr. Meyassar N. Al-Haddad Lecture # 11 Sir Isaac Newton 1642-1727 Proposed fundamental laws that are the basis of modern mechanics 3 laws of motion law of gravitation Newton’s laws Law of inertia a body in motion will stay in motion and a body at rest will stay at rest unless acted upon by a net external force. Law of force-acceleration A particle acted upon by an unbalanced force F experiences an acceleration a that has the same direction as the force and a magnitude that is directly proportional to the force F ma Law of action-reaction for every action, there is an equal and opposite reaction mg = FN Law of gravitation - all bodies are attracted to one another with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. m1m2 F G 2 r 13.2 The Equation of Motion F ma • Free-Body diagram (Force Diagram) • Kinetic diagram (acceleration Diagram) 13.3 Equation of Motion for a System of Particles F ma Fi f i mi a i Internal forces cancel each other F m a F ma i i i G 13.4 Equations of Motion: Rectangular Coordinates • When the net force is projected to separate coordinate axes the Newton’s second law still holds F ma F i F j F k m( a i a x y z F F F x max y ma y z maz x y j az k) Free Body Diagram Method •Draw each object separately •Draw all the forces acting on that object •Get x and y components of all the forces to calculate the net force •Apply Newton’s second law to get acceleration •Use the acceleration in any motion analysis and establish a Kinetic Diagram ma Normal & Frictional Force Action-Reaction forces - mg = FN F mg Ff FN Static Friction ( ms ) The coefficient of static friction is material dependent. Friction • Static friction – parallel force on the surface when there is no relative motion between the 2 objects • Static friction force can vary from zero to Maximum Static Dynamic Ff = msFN Ff = mkFN Applied external force Kinetic Friction ( mk ) • Kinetic friction – parallel force on the surface when there is relative motion between the 2 objects • Kinetic friction force is always the same F f μ k FN Static • is material dependent. Friction • The coefficient of Kinetic friction Ff = msFN Dynamic Ff = mkFN Applied external force Spring Force • Spring force Fs ks s • k : spring stiffness (N/m) • s : stretched or compressed length s l lo lo l Example 13-4 • • • • m = 2 kg y = 1m smooth a=? ma S 12 0.752 0.75 0.5m mg Fs sin ma 1 θ tan 53.13 0.75 1 Nc Fs cos θ ks cos θ 3x 0.5x cos 53 0.9 N Nc Fs cos 0 ag ks sin θ 9.21m/s 2 m Problem mg y x N mk N a=? F y 0; N mg cos 0 N 40 9.81 cos 20 0 N 369 N F x max mk N mg sin ma 0.25(369) 40 9.81 sin 20 40ax ax 5.66 m / s 2 Example 13-5 m A= 3 kg m B= 5 kg From rest vB= ? In 2 second o aB t Block B Same should be ٍSame Fy may Block A Fy may 2s A s B l 196.2 T 5aB 2 A B 0 981 2T 3a A 2a A aB 0 Problem VA = ? ac 0 = 2ac + aA 2Sc + SA = L aA = -2ac F 0: N 200 * 9.81cos 30 0 N 1699 N F ma : 0.5N 2T mg sin 30 200a y x c c F y maA 125 * 9.81 T 125aA T 1226.25 125aA (2) mg maA (2) (1) 849.5 2(1226.25 125aA) 981 200ac 622 200ac 250aA 200ac 500ac ac 0.888(m / s 2 ) aA 1.777(m / s 2 ) T 1004 N VA 2a A s 4.62(m / s) Review • • • • Example 13.1 Example 13.2 Example 13.3 Example 13.4