Conservative and NonConservative Forces Changes in Mechanical Energy

What happens when friction is present? • When friction is present, the work done

A Force is “Non-Conservative” if: • “the work it does on an object that

A Force is “Conservative” if: • “ the work this force does on an

In the presence of friction, the mechanical energy of the system will decrease. •

Example 1 (#)21 A 4 kg particle moves from the origin to position C,

Example 2 (#31) The coefficient of friction between the 3 kg block and the

Example 3(#38) • A 75. 0 kg skysurfer is falling straight down with a

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-Conservative and Non-Conservative Forces -Changes in Mechanical Energy in the Presence of Friction AP Physics C Mrs. Coyle

What happens when friction is present? • When friction is present, the work done by the frictional force W=fd is transferred to heat energy. • The sum of the potential and kinetic energy of the system is not conserved. • Friction is a “non-conservative” force.

A Force is “Non-Conservative” if: • “the work it does on an object that moves between two points depends on the path taken. ” • “the work it does on an object that moves through a round trip is not zero. ” • Example: friction, tension, normal force, propulsion forces.

A Force is “Conservative” if: • “ the work this force does on an object that moves between two points depends only on the position of these two points and not on the path. ” • “the work this force does on an object that moves through a round trip is zero. ” • Example: gravity, force of a spring.

In the presence of friction, the mechanical energy of the system will decrease. • The change in mechanical energy will be negative and will equal the work done by friction. • ΔEmech =Emech final -Emech initial =-fd

Example 1 (#)21 A 4 kg particle moves from the origin to position C, having coordinates x=5 m and y=5 m. One force on the particle is the gravitational force acting in the negative y direction. Using W= F·r = Frcosθ, calculate the work done in going from O to C along a)OAC, b) OBC, c) OC. (See Fig. 1 on next slide. ) Your results should be all identical. Why? (Ans: -196 J)

Fig. 1 B C(5 m, 5 m) O A

Example 2 (#31) The coefficient of friction between the 3 kg block and the surface is 0. 4. The system starts from rest. What is the speed of the 5 kg ball when it has fallen 1. 5 m? Ans: 3. 74 m/s

Example 3(#38) • A 75. 0 kg skysurfer is falling straight down with a terminal speed of 60 m/s. Determine the rate at which the skysurfer-Earth system is losing mechanical energy. (Ans: d. E dt mech =mgv= 44. 1 k. W)