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Extended Work-Energy Theorem

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Extended Work-Energy Theorem We denote the total mechanical energy by E KE PE PEs Since (KE PE PEs ) f (KE PE PEs )i The total mechanical energy is conserved and remains the same at all times 1 1 1 1 mv 2 mgy kx2 mv 2 mgy kx2 2 i i 2 i 2 f f 2 f A block projected up a incline A 0.5-kg block rests on a horizontal, frictionless surface. The block is pressed back against a spring having a constant of k = 625 N/m, compressing the spring by 10.0 cm to point A. Then the block is released. (a) Find the maximum distance d the block travels up the frictionless incline if θ = 30°. (b) How fast is the block going when halfway to its maximum height? A block projected up a incline Point A (initial state): v 0, y 0, x 10cm 0.1m Point B (final state): i i i v f 0, y f h d sin , x f 0 1 kx2 mgy mgd sin 2 i f 1 1 1 1 mv 2 mgy kx2 mv 2 mgy kx2 2 i i 2 i 2 f f 2 f 1 kx2 0.5(625N / m )( 0.1 m )2 d 2 i mg sin (0.5kg )(9.8 m / s2 )sin30 1.28m A block projected up a incline Point A (initial state): vi 0, yi 0, xi 10cm 0.1m Point B (final state): v f ?, y f h / 2 d sin / 2, x f 0 1 2 1 2 h k 2 2 kxi mv f mg ( ) x v gh 2 2 2 m i f h d sin (1.28m)sin 30 0.64m 1 1 1 1 kmv 2 mgy kx2 mv 2 mgy kx2 v 2 x2i gh i 2 i 2 f f 2 f f m i ...... 2.5m / s Types of Forces Conservative forces Work and energy associated with the force can be recovered Examples: Gravity, Spring Force, EM forces Nonconservative forces Power is often dispersed and work done against it cannot easily be recovered Examples: Kinetic friction, air drag forces, normal forces, tension forces, applied forces … Conservative Forces A force is the work of an object moving between two points, if the object is independent of the path between the points The work depends only on the starting and ending positions of the object Any conservative force can have a potential energy function associated with it Work done by gravity Wg PEi PE f mgyi mgy f Work done by spring force 1 1 W PE PE kx2 kx2 s si sf 2 i 2 f Nonconservative Forces If the work you do on an object depends on the path between the final and the starting points of the object, then a power is not conserved. Work depends on the path of motion For a non-conservative force, the potential energy can not be identifiedWork done by a nonconservative force Wnc F d fk d Wotherforces Figure 8.10 It is generally dissipative. The dispersal Physics for Scientists and of energy takes the form of heat or soundEngineers 6th Edition, Thomson Brooks/Cole © 2004; Chapter 8 Physics for Scientists and Engineers 6th Edition, Thomson Brooks/Cole © 2004; Extended Work-Energy Theorem The work-energy theorem can be written as: Wnet KE f KE i KE Wnet Wnc Wc Wnc represents the work done by nonconservative forces Wc represents the work done by conservative forces Any work done by conservative forces can be accounted for by changes in potential energy Wc PEi PE f Gravity work Wg PEi PE f mgyi mgy f Spring force work 1 1 W PE PE kx2 kx2 s i f 2 i 2 f Extended Work-Energy Theorem Any work done by conservative forces can be accounted for by changes in potential energy Wc PEi PE f (PE f PEi ) PE Wnc KE PE (KE f KEi ) (PE f PEi ) Wnc (KE f PE f ) (KEi PEi ) Mechanical energy include kinetic and potential energy 1 1 E KE PE KE PE PE mv2 mgy kx2 g s 2 2 Wnc E f Ei Conservation of Mechanical Energy A block of mass m = 0.80 kg slides across a horizontal frictionless counter with a speed of v = 0.50 m/s. It runs into and compresses a spring of spring constant k = 1500 N/m. When the block is momentarily stopped by the spring, by what distance d is the spring compressed? Wnc (KE f PE f ) (KEi PEi ) 1 1 1 1 1 2 1 2 mv 2 mgy kx2 mv 2 mgy kx2 0 0 kd mv 0 0 2 f f 2 f 2 i i 2 i 2 2 1 1 0 0 kd2 mv 2 0 0 2 2 m d v2 1.15cm k Physics for Scientists and Engineers 6th Edition, Thomson Brooks/Cole © 2004; Changes in Mechanical Energy for conservative forces A 3-kg crate slides down a ramp. The ramp is 1 m in length and inclined at an angle of 30° as shown. The crate stats from rest at the top. The surface friction can be negligible. Use energy methods to determine the speed of the crate at the bottom of the ramp. 1 1 1 1 fd W ( mv 2 mgy kx2 ) ( mv 2 mgy kx2 ) otherforces 2 f f 2 f 2 i i 2 i 1 1 1 1 ( mv 2 mgy kx2 ) ( mv 2 mgy kx2 ) 2 f f 2 f 2 i i 2 i d 1m, yi d sin 30 0.5m,vi 0 y 0,v ? Figure 8.11 f f Physics for Scientists and Engineers 6th Edition, 1 2 ( mv f 0 0) (0 mgyi 0) Thomson Brooks/Cole © 2 2004; Chapter 8 v f 2gyi 3.1m/ s Physics for Scientists and Engineers 6th Edition, Thomson Brooks/Cole © 2004; Changes in Mechanical Energy for Non-conservative forces A 3-kg crate slides down a ramp. The ramp is 1 m in length and inclined at an angle of 30° as shown. The crate stats from rest at the top. The surface in contact have a coefficient of kinetic friction of 0.15. Use energy methods to determine the speed of the crate at the bottom of the ramp. 1 1 1 1 fd W ( mv 2 mgy kx2 ) ( mv 2 mgy kx2 ) otherforces 2 f f 2 f 2 i i 2 i 1 Nd 0 ( mv 2 0 0) (0 mgy 0) k 2 f i k 0.15,d 1m, yi d sin 30 0.5m, N ? N mg cos 0 Figure 8.11 1 dmg cos mv 2 mgy Physics for Scientists and k 2 f i Engineers 6th Edition, Thomson Brooks/Cole © v f 2g(yi k d cos) 2.7m/ s 2004; Chapter 8 Changes in Mechanical Energy for Non-conservative forces A 3-kg crate slides down a ramp. The ramp is 1 m in length and inclined at an angle of 30° as shown. The crate stats from rest at the top. The surface in contact have a coefficient of kinetic friction of 0.15. How far does the crate slide on the horizontal floor if it continues to experience a friction force.1 1 1 1 fd W ( mv 2 mgy kx2 ) ( mv 2 mgy kx2 ) otherforces 2 f f 2 f 2 i i 2 i 1 Nx 0 (0 0 0) ( mv 2 0 0) k 2 i 0.15,v 2.7m/ s, N ? k i Figure 8.11 N mg 0 Physics for Scientists and Engineers 6th Edition, 1 Thomson Brooks/Cole © mgx mv 2 k 2 i 2004; Chapter 8 v2 x i 2.5m 2k g Block-Spring Collision A block having a mass of 0.8 kg is given an initial velocity vA = 1.2 m/s to the right and collides with a spring whose mass is negligible and whose force constant is k = 50 N/m as shown in figure. Assuming the surface to be frictionless, calculate the maximum compression of the spring after the collision. 1 1 1 1 mv 2 mgy kx2 mv 2 mgy kx2 2 f 1 2f 2 f 2 1 i 2 i 2 i Figure 8.14 mv max 0 0 mv A 0 0 Physics for Scientists and 2 2 Engineers 6th Edition, Thomson Brooks/Cole © m 0.8kg 2004; Chapter 8 x v (1.2m/ s) 0.15m max k A 50 N / m Block-Spring Collision A block having a mass of 0.8 kg is given an initial velocity vA = 1.2 m/s to the right and collides with a spring whose mass is negligible and whose force constant is k = 50 N/m as shown in figure. Suppose a constant force of kinetic1 2 friction acts1 2 between1 2 the block1 and2 the fd Wotherforces ( mv f mgy f kxf ) ( mv i mgyi kxi ) surface, with µk = 0.5,2 what is the maximum2 2 compression2 xc in the spring. 1 1 Nd 0 (0 0 kx2 ) ( mv 2 0 0) k 2 c 2 A N mg and d xc 1 1 kx2 mv 2 mgx 2 c 2 A k c 2 25 xc 3.9xc 0.58 0 xc 0.093m Energy Review Kinetic Energy Associated with movement of members of a system Potential Energy Determined by the configuration of the system Gravitational and Elastic Internal Energy Related to the temperature of the system Conservation of Energy Energy is conserved This means that energy cannot be created nor destroyed If the total amount of energy in a system changes, it can only be due to the fact that energy has crossed the boundary of the system by some method of energy transfer Practical Case E = K + U = 0 The total amount of energy in the system is constant.
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