Newtons Third Law Newtons 3 rd Law examples

Newton’s Third Law • • • Newton’s 3 rd Law examples Example 4 -12 – Two blocks on table Example 4 -13 – Atwood’s Machine Block on Inclined Plane Canal Boat ?

Newton’s 3 rd Law • Whenever one object exerts for on a second object, second object exerts and equal and opposite force on first object – Action–reaction law. – Necessarily acts on different objects – Action reaction pairs not confused with 2 nd law pairs • Examples – – – Force on hand on table, table on hand Force of hand on wall…. Force of hammer… Pedestrian walking…. Sled example identify 3 rd and 2 nd law pairs Book on table on earth - identify 3 rd and 2 nd law pairs

nd 2 vs. • 2 nd Law forces act on same object! ? N 40 N 98 N rd 3 Law • 3 rd Law forces act on different objects!

Action-reaction

Example 4 -12 (coupled boxes) • Two boxes connect together • FBD for each box • Tension equal and opposite 3 rd law • Accelerations the same • Solve simultaneous equations for acceleration, tension • Could solve as one block if not interested in tension

Example 4 -12 (coupled boxes) •

Example 4 -12 (equation solving) •

Example 4 -13 Atwood’s Machine • Free body diagram for each block • Tensions same by 3 rd law • Accelerations locked together (- sign) • Equation of motion for each • Solve for acceleration • Solve for tension • Limiting cases

Block on Incline • • FBD Tilted axes Getting the angles Fn = mg cosθ Fx = mg sin θ ax = g sin θ Get v, x at 2 seconds Limiting cases