VIEWS: 5 PAGES: 27 POSTED ON: 9/1/2012 Public Domain
Mechanics A force is a push or pull. Contact Forces Non-contact Forces such as gravity, magnetic, and electric forces. Newton’s First Law of Motion An object continues in a state of rest or in a state of motion at constant speed along a straight line… unless compelled to change that state by a net force. The mass of an object is a quantitative measure of inertia. The SI Unit of mass is kilogram (kg). It’s Slinky!! Immediately after I release the spring, what does the bottom of the spring do? 1. It begins to fall immediately at the same rate as the rest of the 0% spring. 2. It begins to fall, but at a slower rate than the top of the spring. 3. It remains stationary. 4. It moves upward as the top of the spring moves downward. 1 2 3 4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Immediately after I release the spring, what does the bottom of the spring do? Newton’s Second Law of Motion When a net external force F acts on an object of mass m, the acceleration a that results is directly proportional to the net force and has a magnitude F that is inversely proportional to the mass. The direction of the acceleration is a the same as the direction of the net m force. SI Unit of Force: kg • m/s2 = newton (N) Individual Forces Net Force 4N 10 N 6N The net force on an object is the vector sum of all forces acting on that object. m = 1850 kg F + 275 N + 395 N 560 N = +110 N a F 110 N 0.059 m/s 2 m 1850 kg If the airplane’s mass is 13 300 kg, what is the magnitude of the net force that the catapult and jet engine exert on the plane? F ma (13 300 kg)(31m/s ) 2 = 4.1105 N The Normal Force The Normal Force & Friction Static Frictional Force The magnitude of the static frictional force can have any value from zero up to some maximum value, fsmax, depending on the applied force. In equation form, we write: fs f s m ax The equality holds only when fs attains its maximum value, which is f s m ax s FN where µs is the coefficient of static friction and FN is the magnitude of the normal force. The Normal Force & Friction Static Frictional Force - Example The Force Needed to Start a Sled Moving A sled is resting on a horizontal patch of snow and the coefficient of static friction is 0.350. The sled and its rider have a total mass of 38.0 kg. Determine the horizontal fs F force needed to start the sled barely moving. f s max s FN s mg (0.350)(38.0 kg)(9.80 m/s ) 2 130 N Kinetic Frictional Force The magnitude of the kinetic frictional force is given by f k k FN where k is the coefficient of kinetic friction and FN is the magnitude of the normal force. Kinetic Frictional Force - Example Sled Riding A sled is traveling at 4.00 m/s along a horizontal stretch of snow. The coefficient of kinetic friction is 0.0500. How far does the sled go before stopping? f k k FN k mg ma v v 2 2 v v 2 2 a k g x 0 0 2a 2( k g ) (4.00 m/s) 2 16.3 m 2(0.0500)(9.80 m/s ) 2 Newton’s Third Law of Motion Whenever a body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body. If an astronaut pushes on the 92 kg spacecraft with a force P = +36 N, then, according to Newton’s third law of motion, the spacecraft simultaneously pushes back on the astronaut 11 000 kg with a force –P. Acceleration of the spacecraft P 36 N as 0.0033 m/s 2 ms 11 000 kg Acceleration of the astronaut P 36 N aA 0.39 m/s 2 mA 92 kg Second Law: object tends to remain force or in Third Law: when one body exerts a force on another Newton’s First Law: an a non-zero net externalat reston an object produces second body exerts a force equal in magnitude, but motion at an acceleration onunless a net proportional to the net body, the constant velocity that object force acts on that object oppositely directed on the first body force and inversely proportional to its mass. FN F y FN mg 0 F F F x F ma W = mg Newton’s Second Law: a non-zero net external force on an object First Law: when one body exerts a at rest or Newton’s Third Law:an object tends to remain force on in produces body, the second that object proportional to the net another an acceleration on body exerts a force acts on motion at constant velocity unless a net force equal in that force and inversely proportional to its mass. magnitude, but oppositely directed on the first body object FN F y FN mg 0 fS F F F x F fS 0 f W = mg