9/21/10

Post date: Sep 20, 2010 8:1:36 PM

Bell Ringer: A 1.50 kg fish called Wanda is swimming downstream with a force of 25 N. The water also applies a force downstream on the fish or 65 N. What is Wanda's acceleration during this? Draw the free body diagram (showing two forces pushing the fish along: One from the fish's swimming motion, one from the water pushing on it), write the Fnet = ma equation, substitute the sum of the forces for Fnet, and solve for the acceleration.

• • Solving force problems - drawing a free body diagram

    • Forces that may be included:

      • Weight (w) - the downward force caused by gravity. This is present when the object in question is NOT in deep space where there is no measurable gravity. (w = mg)

      • Friction (Ff) - a force that opposes motion (or goes in the opposite direction as motion). This is present when two surfaces are in contact, such as a block sliding across the floor. (Ff = u * Fn)

      • Normal Force (Fn) - a force that pushes out perpendicularly from a surface, keeping the object from passing through that surface. When standing on the floor, the normal force pushes you upward and exactly balances your weight. When pushing on a wall, the normal force pushes straight out from the wall and exactly balances the force with which you push on the wall.

      • Other forces (Fpull, Fpush, Fengine, Flift, F1, F2, Fa, Fb, etc.) - any other forces specified or implied in the problem, in a direction specified or implied by the problem. For example, when you float in water there is an upward force from the water keeping you from sinking called the buoyant force. When a car moves, there is a force forward caused by the engine.

    • Use free body diagram to write net force equation:

      • Fnet = m*a . In the next line, substitute Fnet with the symbols for all forces that are parallel to motion, added together.

      • Example: A car moving along feels 4 forces: Weight down (w), Normal Force up (Fn), Friction and air resistance backward (Ff), and force from the engine pushing forward (Fengine).

        • Fnet = m*a

        • Fengine - Ff = m*a - don't include up or down forces because the car doesn't move up or down, it moves horizontally.

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• Practice worksheet answers