Friday, November 14, 2008 - Blocks 1 and 3

Collected HW problems 3,8,9,14,24 from Applying Newton's Laws worksheet.

Newton's 3rd Law introductory lecture:

Inanimate objects can exert elastic forces: demo with rubber band, laser beam on wall.

No such thing as an isolated force - forces come in pairs (pears)

Newton's 3rd law recipe: A acts on B, B acts on B
Demo with finger in water.

Demo with force sensors. A can't pull harder on B than B pulls on A.

Newton's 3rd Law: If object A exerts a force on object B, then B exerts a force on A that is equal in magnitude and opposite in direction.

Tug-of-War: A and B. The one who wins the tug-of-war is the person who pushes harder against the ground.

Action/Reaction forces never cancel out because they do not act on the same object. Because they do not act on the same object, they never appear in the same free body diagram.

Examples of action reaction pairs. Contrast with forces that are equal in magnitude but opposite in direction but are NOT action reaction pairs.

Action and Reaction for Different Masses: If you drop a ball, why does the ball fall down and not the Earth fall up? Same magnitude force = m A = M a. Ball has much less mass so it has larger acceleration. Earth has huge mass, tiny acceleration.

Magic tube: tube exerts upward force on ball, ball exerts downward force on tube. We can measure this extra force using a force sensor and, if the ball is falling at terminal velocity, determine the weight of the ball.

Hand out RA 4.4 (summary of class notes) due Monday.
Hand out CD 6, due for the last time on Thursday.