PHY 101 1nd Edition Lecture 11 Outline of Last Lecture I 4 4 Newton s Second Law II Gravitational Force III Gravitational Force Mass Weight Outline of Current Lecture IV Newton s Third Law V Application of Newton s Laws VI Objects in Equilibrium Current Lecture 4 4 Newton s Third Law Forces always exist in pairs action and reaction forces o Equal in magnitude o Opposite in direction o Act on different objects o if object 1 and object 2 interact the force exerted by object 1 on object 2 F 12 is equal in magnitude but opposite in direction to the force exerted by object 2 on object 1 F21 F12 F21 4 5 Application of Newton s Laws Draw free body diagram Ex Pulling a crate How many forces acting on the crate neglect friction Equation of the motion of crate o X component max T o Y component may Fn mg 0 No acceleration along the y Objects in Equilibrium These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Objects either at rest or moving with constant velocity are said to be in equilibrium In other words the vector sum of the external forces on an object in equilibrium is zero Example Traffic light weight 100N find the tension force in each of the 3 cables in the y direction T3 mg 0 T3 mg 100N equilibrium condition of the knot in the x direction T1x T2x 0 o eq 1 T1cos 143 T2cos 53 0 in the y direction T1y T2y mg 0 o eq 2 T1sin 143 T2cos 53 mg 0 solve eq 1 for T2 o eq 3 T2 T1cos 143 cos 53 1 33T1 T2 o T1sin 143 1 33T1 sin 53 mg 0 Solve for T1 o T1 60 1N Solve for T2 o T2 1 33T1 79 9 N Example Runaway Car A car with mass m on an icy driveway inclined at 20 A calculate the acceleration of the car neglect friction o Step 1 choose coordinates o X axis down the slope o Y axis normal to the slope o normal perpendicular to the slope o Step 2 free body diagram o Step 3 geometry directions angles Incline Coordinates and Angles Normal Force o Fn mgcos Because net force along the y direction is zero o Sum Fy Fn mgcos 0 X component of the gravitational force o Fx mgsin Normal force has no x component Acceleration of the car along the y direction 0 Net force along the x direction o Fx Fgsin20 mgsin20 mg 100N cos 143 0 799 Acceleration along the x direction o Ax Fx m mgsin20 m gsin20 3 35m s2 B if the length of the driveway is 25 m and the car starts from rest how long does it take to travel to the bottom of the slope o Use kinematics o x 1 2at2 25m o 25 1 2 3 35 t2 3 86s C what is the speed at the bottom o Vx at 3 35 3 86 12 9m s
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