Newton’s Third Law

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

Newton’s 3rd 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 2nd law pairs

• Examples– Force on hand on table, table on hand– Force of hand on wall….– Force of hammer…– Pedestrian walking….– Sled example identify 3rd and 2nd law pairs– Book on table on earth - identify 3rd and 2nd law pairs

Action-reaction

Again – 3rd Law

• 1st and 2nd Law forces act on same object!

• 3rd Law forces act on different objects!

Again – Trying different fonts

• 1st and 2nd Law forces act on:

Same Object!

• 3rd Law forces act on:

Different Objects!

Example 4-12 (coupled boxes)

• Two boxes connect together

• FBD for each box

• Tension equal and opposite 3rd 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)

• Both boxes

• Lead box

• Trailing box

Example 4-12 (equation solving)

• Two equations

• Substitute (2) in (1)

• Add (1) and (2)

• Same result! (then solve for FT)

Example 4-13 Atwood’s Machine

• Free body diagram for each block

• Tensions same by 3rd 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