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Dynamics: Cause of Motion

Dynamics: Cause of Motion. Sections 7.2, 7.3. Reminders. Lab this week is A6 -NSL: Newton’s Second Law is due Friday at 4 p.m . No reading quiz due before class on Thursday.

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Dynamics: Cause of Motion

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  1. Dynamics: Cause of Motion Sections 7.2, 7.3

  2. Reminders • Lab this week is A6-NSL:Newton’s Second Lawis due Friday at 4 p.m. • No reading quiz due before class on Thursday. • Test #2 on Thursday, October 9, addressing all material since Test #1– but recall that physics builds on prior knowledge. The focus is on Chapters 6 and 7, but will include a bit of Chapter 4 as well.

  3. Experience & Experiment • Experience shows that: • a greater force for a given mass causes greater acceleration • a greater mass for a gives force gives lesser acceleration • Experiment shows that: • acceleration is directly proportional to force • acceleration is inversely proportional to mass

  4. Newton’s 2nd Law, ΣF=ma • The sum of all forces on an object equals mass times acceleration • An object accelerates at a rate of 1.5m/s2 under a sum force of 53N. What is its mass? • How much upward force does the ground apply to someone with a mass of 75kg to counter balance the pull of gravity? Note that g = -9.8m/s2 and that Fnet = ma and that a = g. That is, W = mg

  5. Newton’s Third Law • If object A exerts a force FAB on an object B, then object B exerts a force FBA on object A such that FAB = -FBA. • If an object is not accelerating, then FAB = -FBA and the sum of forces (vectors) equals zero. • An object can accelerate only if the vector sum of forces acting on it does not equal zero. • If Newton’s third law is correct, how is it possible to for anyone to win a tug-of-war?

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