Physics 231. Topic 5: Energy and Work. Alex Brown October 2, MSU Physics 231 Fall

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1 Physics 231 Topic 5: Energy and Work Alex Brown October 2, 2015 MSU Physics 231 Fall

2 What s up? (Friday Sept 26) 1) The correction exam is now open. The exam grades will be sent out after that on Wednesday Oct 7. 2) Homework 04 is due Tuesday Oct 13 th and covers Chapters 5 and 6. It is a little longer that usual so you may want to start early. MSU Physics 231 Fall

3 MSU Physics 231 Fall

4 Key Concepts: Work and Energy Work and Energy Work and it s association with forces Constant forces (driving up a hill) Variable forces (stretching a spring) Kinetic Energy Work Energy Theorem Relationship to velocity Potential Energy Conservation of Mechanical Energy Power Covers chapter 5 in Rex & Wolfson MSU Physics 231 Fall

5 What is Energy? Motion kinetic energy Ability to produce motion potential energy How to we transfer energy? MSU Physics 231 Fall

6 Work and Energy Work: Transfer of energy Quantitatively: The work W done by a constant force on an object is the product of the force along the direction of displacement and the magnitude of displacement. W = (Fcos ) x = F x x Units: N m = Joule 1 calorie = J Fcos F 1 Calorie = 4184 J = 1 kcal x MSU Physics 231 Fall

7 Non-constant force/angle W=(Fcos ) x: what if Fcos is not constant while covering x? x F Fcos Example: what if or F changes while pulling the block? Area= A=(Fcos ) x Fcos W= ( A)=total area x x x The work done is the area under the graph of Fcos vs x MSU Physics 231 Fall

8 Example 4 m A person drags a block over a floor with a force parallel to the floor. Force 4N 2N After 4 meters, the floor turns rough and instead of a force of 2N a force of 4N must be applied. 0 The force-distance diagram shows the situation. 4 8 m distance How much work did the person do over 8 meter? a) 0 J b) 16 J c) 20 J d) 24 J e) 32 J Work: area under F-x diagram: 4x2 + 4x4 = 24 J MSU Physics 231 Fall

9 Work and Energy Positive work is done when the angle is less than 90 degrees, energy goes into the object 1) energy can be stored (potential energy increases) 2) motion can be created (kinetic energy increases) MSU Physics 231 Fall

10 Work and Energy Negative work when the angle between the force and the displacement is more the 90 degrees, energy is removed: 1) stored energy can be decreased (potential energy decreases) 2) motion can be reduced (kinetic energy decreases) Special case for friction force, the angle is 180 degrees; potential or kinetic energy is removed and heat is created No work when the angle between the force and the displacement is equal to 90 degrees MSU Physics 231 Fall

11 Work and Energy Sled is pulled across a surface at constant speed. Where does the energy go in this case? Answer: two forces are acting; the energy goes into friction (the ground/sled heat up!) MSU Physics 231 Fall

12 Clicker Question: Tension and Work A ball tied to a string is being whirled around in a circle. What can you say about the work done by tension? a) tension does no work at all b) tension does negative work c) tension does positive work No work is done because the force acts in a perpendicular direction to the displacement. Using the definition of work W = F (Δs) cos because = 90 º, then W = 0. T v MSU Physics 231 Fall

13 Power: The rate of energy transfer Work (the amount of energy transfer) is independent of time. W=(Fcos ) x total over all time To measure how fast we transfer the energy we define: Power = P = (Work/time) = (W/ t) (J/s=Watt) P = (Fcos ) x/ t = (Fcos ) v average 1 Watt = horsepower MSU Physics 231 Fall

14 A Runner While running, a person dissipates about 0.60 J of mechanical energy per step per kg of body mass. A) If a 60 kg person develops a power of 70 Watt during a race (distance = L), how fast is she running (1 step=1.5 m)? B) What is the force the person exerts on the road? W=F x P=W/ t=fv A) Work per step: (0.60 J/kg) (60 kg) = 36 J Work during race: (36 J) [( L ) / ( step-length )] = (36 J) ( L) / (1.5 m) = (24L) J/m Power = W/ t = 24 L/ t = 24 v = 70 Watts v =2.9 m/s B) F=P/v so F=24 N MSU Physics 231 Fall

15 Net Work When many forces are acting the net work done by all of them is the sum of each term W net = (F x1 + F x2..) x = = F x,net x= (F net cos ) x MSU Physics 231 Fall

16 Question A worker pushes a wheelbarrow with a force of 50 N over a distance of 10 m. A frictional force acts on the wheelbarrow in the opposite direction, with a magnitude of 30 N. What net work is done on the wheelbarrow? a) 0 b) 100 J c) 200 J d) 300 J e) 500 J W net = F x,net x = (50-30) (10) = 200 J MSU Physics 231 Fall

17 Example h=100m A toy rocket of 5.0 kg, after the initial acceleration stage, the speed is constant and travels 100 m in 2 seconds. A) What is the work done by the engine? B) What is the power of the engine? A) W = (Fcos ) h = m rocket g h = (5.0 kg) (9.81 m/s 2 ) (100 m) = 4905 J Force by engine must balance gravity! B) P=W/ t = 4905/2=2453 Watt (=3.3 horsepower) or P=(Fcos )v = mg(h/t) = /2=2453 Watt MSU Physics 231 Fall

18 Clicker Question: Force and Work A box is being pulled up a rough incline by a rope connected to a pulley. How many forces are doing non zero work on the box? W = (Fcos ) x a) one force b) two forces c) three forces d) four forces e) no forces are doing work Any force not perpendicular to the motion will do work: N does no work T does positive work f k does negative work T N Mg sin mg sin does negative work mg MSU Physics 231 Fall f k

19 Potential Energy Potential energy (PE): energy associated with the position of an object within some system. Gravitational potential energy: Consider the work done by the gravity in case of a falling object: W = (Fcos ) x W gravity = F g cos(0 o ) h = mg h = mg h i mg h f = PE i -PE f The system is the gravitational field of the earth. PE = mgh Since we are usually interested in the change in gravitational potential energy, we can choose the ground level (h=0) in a convenient way. MSU Physics 231 Fall

20 (A) v f v 0 at (B) x v t o 1 at 2 2 (C) x v t f 1 at 2 2 (D) 1 2 x v v ) t f ( 0 vt (E) x 2 1 ( v v 2a f 2 0 ) MSU Physics 231 Fall

21 Kinetic energy: Consider an object that changes speed only t=0 V O x=100m t=2s V f W = Fcos x = (ma) x used Newton s second law (E) x = (v f2 -v 02 )/2a W=½m(v f2 -v 02 ) = ½mv f 2 -½mv 0 2 Kinetic energy: KE=½mv 2 When work is done on an object and the only change is its speed: The work done is equal to the change in KE: W = KE final -KE initial = KE f -KE i MSU Physics 231 Fall

22 Conservation of energy Mechanical energy = Potential Energy + Kinetic energy ME = PE + KE Mechanical energy is conserved if: the system is closed (no energy can enter or leave) the forces are conservative (see soon) We re not talking about this! Heat, chemical energy (e.g battery or fuel in an engine) Are sources or sinks of internal energy (not mechanical). MSU Physics 231 Fall

23 Example of closed system An object (0.2 kg) is dropped from a height of 35 m. Assuming no friction, what is the velocity when it reaches the ground? t=0 s h=35 m v=0 m/s t>0 s v at h=0? At launch: ME = mgh + ½mv = J At ground: ME = mgh + ½mv 2 =0 + ½ 0.2 v 2 = 0.1 v 2 J Conservation of ME: J = 0.1 v 2 J v = 26.2 m/s MSU Physics 231 Fall

24 Clicker Quiz! A energy time In the absence of friction, which energy-time diagram is correct? potential energy total energy kinetic energy B C energy energy time time MSU Physics 231 Fall

25 Where is the kinetic energy 1) highest? 2) lowest? Parabolic Motion t=0 t=1 t=2 t=3 t=5 A B C D E MSU Physics 231 Fall

26 Where is the potential energy 1) highest? 2) lowest? Parabolic Motion t=0 t=1 t=2 t=3 t=5 A B C D E MSU Physics 231 Fall

27 A swing If released from rest, what is the velocity of the ball at the lowest point? 30 o L=5m (PE+KE) = constant h PE release =mgh (h=5-5cos(30 o )) =6.57m J KE release =0 PE bottom =0 KE bottom =½mv 2 ½mv 2 =6.57m so v=3.6 m/s MSU Physics 231 Fall

28 A B h Ball on a track end h end In which case has the ball the highest velocity at the end? A) Case A B) Case B C) Same speed In which case does it take the longest time to get to the end? A) Case A B) Case B C) Same time MSU Physics 231 Fall

29 Conservation of mechanical energy Mechanical energy = potential energy + kinetic energy In a closed system, mechanical energy is conserved * V=100 m/s ME = PE+KE = mgh + ½mv 2 = constant h=100m M=5 kg What about the accelerating rocket? At launch: ME = 5*9.81*0 + ½5*0 2 = 0 J At 100 m height: ME = 5*9.81*100 + ½5*100 2 = J V O =0 We did not consider Fuel burning -Another source of energy that is not mechanical energy MSU Physics 231 Fall

30 MSU Physics 231 Fall

31 Roller coaster KE PE TME NC KE PE TME NC With friction KE PE TME NC KE PE TME NC KE PE TME NC MSU Physics 231 Fall

32 Conservative Forces A force is conservative if the work done by the force when Moving an object from A to B does not depend on the path taken from A to B. Example: work done by gravitational force Using the stairs: W g = mgh f -mgh i = mg(h f -h i ) h=10m Using the elevator: W g = mgh f -mgh i = mg(h f -h i ) The path taken (longer or shorter) does not matter: only the displacement does! MSU Physics 231 Fall

33 Non-Conservative Forces A force is non-conservative if the work done by the force when moving an object from A to B depends on the path taken from A to B. object on rough surface top view Example: Friction You have to perform more work against friction if you take the long path, compared to the short path. The friction force changes kinetic energy into heat. (In this example difference forces are applied so that the KE is the same at the end.) MSU Physics 231 Fall

34 Conservation of mechanical energy only holds if the system is closed AND all forces are conservative ME i -ME f =(PE+KE) i -(PE+KE) f =0 if all forces are conservative Example: throwing a snowball from a building neglecting air resistance ME i -ME f =(PE+KE) i -(PE+KE) f =E nc if some forces are nonconservative. E nc =positive energy dissipated by non-conservative forces (in the books notation E nc = -W f ) Example: throwing a snowball from a building taking into account air resistance MSU Physics 231 Fall

35 Overview Equations of kinematics x(t)=xo + Vo t + ½at 2 v(t)=vo + at Conservation of mechanical energy ME i = ME f E nc =0 Closed system Newton s second Law F=ma Work-energy Theorem E nc =ME i -ME f Work W=(Fcos ) x MSU Physics 231 Fall

36 Clicker Quiz! A energy time When there is friction, which mechanical energy-time diagram is correct? potential energy total energy kinetic energy B C energy energy time time MSU Physics 231 Fall

37 Question Old faithful geyser in Yellowstone park shoots water hourly to a height of 40 m. With what velocity does the water leave the ground? a) 7.0 m/s b) 14 m/s c) 20 m/s d) 28 m/s e) don t know Step 1: Step 2: Step 3: Step 4: KE i + PE i = KE f + PE f KE = ½ mv 2 PE = mgh At ground level: ME = ½mv 2 + mgh = ½mv = ½mv 2 At highest point: ME = ½mv 2 + mgh = 0 + m*9.81*40 = 392m Conservation of energy: ½mv 2 = 392m v 2 = 2x392 so v=28 m/s MSU Physics 231 Fall

38 Conservation of mechanical energy 5 kg M1 4.0 m M2 3 kg = v 2 v= 3.13 m/s What is the speed of m 1 and m 2 when they pass each other? ME = (PE 1 +PE 2 +KE 1 +KE 2 )=constant At time of release: PE 1i = m 1 gh 1i = 5.00*9.81*4.00 = J PE 2i = m 2 gh 2i = 3.00*9.81*0.00 = 0.00 J KE 1i = ½m 1 v 2 i = 0.5*5.00*(0.) 2 = 0.00 J KE 2i = ½m 1 v 2 i =0.5*3.00*(0.) 2 = 0.00 J Total = J At time of passing: PE 1f = m 1 gh 1f = 5.00*9.81*2.00 = 98.0 J PE 2f = m 2 gh 2f = 3.00*9.81*2.00 = 58.8 J KE 1f = ½m 1 v 2 = 0.5*5.00*(v) 2 = 2.5v 2 J KE 2f = ½m 2 v 2 = 0.5*3.00*(v) 2 = 1.5v 2 J Total = v 2 J MSU Physics 231 Fall

39 Friction (non-conservative) The pulley is now not frictionless. The friction force equals 5 N. What is the speed of the objects when they pass? ME i = ME f + E nc E nc = f friction x = 5.00*2.00 = 10.0 J 5 kg M1 4.0 m M2 3 kg = v 2 v=2.7 m/s Without Friction: v= 3.13 m/s MSU Physics 231 Fall

40 Question! 5 kg M1 4.0 m 3 kg M2 TRUE TRUE FALSE In the absence of friction, when m 1 starts to move down: 1) potential energy is transferred from m 1 to m 2 2) potential energy is transformed into kinetic energy 3) m 1 and m 2 have the same kinetic energy MSU Physics 231 Fall

41 Question A ball rolls down a slope as shown in the figure. The starting velocity is 0 m/s. There is some friction between the ball and the slope. Which of the following is true? h a) The kinetic energy of the ball at the bottom of the slope equals the potential energy at the top of the slope b) The kinetic energy of the ball at the bottom of the slope is smaller than the potential energy at the top of the slope c) The kinetic energy of the ball at the bottom of the slope is larger than the potential energy at the top of the slope MSU Physics 231 Fall

42 Quiz A ball rolls down a slope and back up a more shallow slope as shown in the figure. The starting velocity is 0 m/s. There is some friction between the ball and the slope. Which is true? h1 h2 a) The maximum height reached on the right (h2) is the same as the height the ball started at on the left (h1) b) The maximum height reached on the right (h2) is smaller than the height the ball started at on the left (h1) c) The maximum height reached on the right (h2) is larger than the height the ball started at on the left (h1) MSU Physics 231 Fall

43 Question A ball of 1 kg rolls up a ramp, with initial velocity of 6 m/s. It reaches a maximum height of 1 m (i.e., the velocity is 0 m/s at that point). How much energy is dissipated by friction? a) 0. b) 8.2 J c) 9.8 J d) 18 J e) 27.8 J ME i ME f = E nc (KE i +PE i ) - (KE f +PE f )= E nc kinetic energy: ½mv 2 potential energy: mgh g=9.81 m/s 2 Initial: ME = ½mv 2 (kinetic only) = ½x1x6 2 = 18 J Final: ME = mgh (potential only) = 1x9.8x1 = 9.8 J E nc = = 8.2 J MSU Physics 231 Fall

44 Question An outfielder who is 2M tall throws a baseball of 0.15 kg at a speed of 40 m/s and angle of 30 degrees with the field. What is the kinetic energy of the baseball at the highest point, ignoring friction? a) 0 J b) 30 J c) 90 J d) 120 J e) don t know Two components of velocity at start: v ox = v o cos(30 o ) = 34.6 m/s v oy = v o sin(30 o ) = 20 m/s At highest point: only horizontal velocity v x = v ox = 34.6 m/s v y = 0 m/s kinetic energy: ½mv 2 = ½(0.15)(34.6) 2 = 90 J MSU Physics 231 Fall

45 Question An outfielder who is 2m tall throws a baseball of 0.15 kg at a speed of 40 m/s and angle of 30 degrees with the field. How high does the ball go at its highest point, ignoring friction? Initial: KE + PE = ½mv 2 + mgh = ½ (0.15)(40) 2 + (0.15)(9.81)(2) = At highest point: KE + PE = 90 + mgh = h = h h = 22.4 m ie, the ball travels 20.4 m higher than the player s height MSU Physics 231 Fall

46 Work and Energy Work: W = Fcos( ) x Energy transfer The work done is the same as the area under the graph of Fcos versus x Power: P = W/ t Rate of energy transfer Potential energy (PE) Energy associated with position. Gravitational PE: mgh Energy associated with position in grav. field. Kinetic energy KE: ½mv 2 Energy associated with motion Conservative force: Work done does not depend on path Non-conservative force: Mechanical energy ME: Work done does depend on path ME = KE + PE Conserved if only conservative forces are present ME i = ME f Not conserved in the presence of non-conservative forces ME i = ME f + E nc MSU Physics 231 Fall

47 Question: Free Fall 1 2 Two stones, one twice the mass of the other, are dropped from a cliff. Just before hitting the ground, what is the VELOCITY of the heavy stone compared to the light one? a) quarter as much b) half as much c) the same d) twice as much e) four times as much All freely falling objects fall with the same acceleration (g=9.81 m/s 2 ). Because the acceleration is the same for both, and the distance is the same for both, then the final velocities will be the same for both stones. MSU Physics 231 Fall

48 Quiz: Free Fall 2 Two stones, one twice the mass of the other, are dropped from a cliff. Just before hitting the ground, what is the KINETIC ENERGY of the heavy stone compared to the light one? a) quarter as much b) half as much c) the same d) twice as much e) four times as much KE i + PE i = KE f + PE f Consider the work done by gravity to make the stone fall distance d: KE = W net = F d cos KE = mgd Thus, the stone with the greater mass has the greater KE, which is twice as big for the heavy stone. MSU Physics 231 Fall