Work, Energy, Power Chapter 7 in a nutshell Work is Force times Distance. The change in Kinetic Energy is equal to the work. Power is Work per unit time. W F d W K P W / T New Concept: Kinetic Energy K = m V2 Work causes a change in kinetic energy, so the units are the same.
Units of Kinetic energy: Joule But, from the definition of kinetic energy, the units are also, 1 Joule = 1 Kg-m2/s2 Table 7-1 Typical Values of Work Activity Equivalent work (J) Annual U.S. energy use 8 x 1019 Mt. St. Helens eruption
1018 Burning one gallon of gas 108 Human food intake/day Melting an ice cube 107 104 Lighting a 100-W bulb for 1 minute
6000 Heartbeat 0.5 Turning page of a book 103 Hop of a flea 107 Breaking a bond in DNA
1020 Work is force times distancebut! Only the force component in the direction of motion counts! Units of work: Joule Force in direction of motion is what matters F Fx = F cos D
W = D * F cos Figure 7-3 Force at an Angle to Direction of Motion: Another Look Ive got work to do. F = Mg W=F*y=MgH F = Mg y This is the work done by the
person lifting the box. How about the reverse?. F = Mg W=F*y = - MgH Work can be positive or negative. Lifting a box is positive work. Lowering the box is negative work. H F = Mg
Positive or Negative? Work done by gravity. Fg = - Mg y = - H Work done by force of gravity is positive when an object is dropped. Problem 7-77 W = 50J, given v, F, x. What is theta? What is M?
W F (cos )d W cos Fd W cos 1 Fd 50.0 J (45.0 N)(1.50 m) 42.2 cos 1 1
1 W K m(vf 2 vi 2 ) mvf 2 2 2 m 2W vf 2 2(50.0 J)
m s 2.60 2 14.8 kg Figure 7-4 Positive, Negative, and Zero Work Raindrops are falling on my head. W F y
( Mg ) ( H ) MgH V1 F = - Mg y = -H W K 1 1 Mv22 Mv12 2 2
V2 1 MgH M (v22 v12 ) 2 Special case: Object dropped from rest. Mass M dropped from height H. What is speed just before hitting the ground? (Neglect friction of air) -Mg W=MgH H
W=K=1/2 M v2 v 2 gH Thought provoking: The inclined plane. Distance = h Force = mg W = mgh Distance = L = h/sin Force = mg sin W = mgh The work is the same in both cases. This is an example of
conservation of energy, which we will see much more of in the future. How about the speed? F=Mg sin H If Block fell straight down through height H, its speed would be: v 2 gH Use F=Ma to find speed down the ramp. V=at
L=1/2 a t2 so 2L v a a 2aL But a = g sin And L = H/sin So v 2 gH
Same magnitude of speed, but DIFFERENT DIRECTION! Figure 7-6 Graphical Representation of the Work Done by a Constant Force Figure 7-7 Work Done by a Non-Constant Force Figure 7-8a Work Done by a Continuously Varying Force Figure 7-8b Work Done by a Continuously Varying Force
Figure 7-8c Work Done by a Continuously Varying Force When force is not constant. The area under a force-distance curve is the work, W. Force The average force times the distance gives the work. This is the same as the area under the curve. Distance
Back to our Spring Flings. F = K x F = -K x Force F=Kx How much work is done to move the block? Be careful with the signs of the forces! The force to move the block must be
equal and opposite to the force of the spring on the block. W=FX W = K X2 Distance X Work done to move a mass on a spring a distance x: W 12 Kx 2
Conceptual Checkpoints What is work, in physics terms? What is kinetic energy? What is power? Work is force in the direction of motion. Kinetic energy is Mv2 Power is Work (or Energy) per Unit time. Work and the Weightlifter A weightlifter performs three steps. What is the work done for each of the steps? Assume the weight is 100 Kg, the height of the lift is 1 M, and the acceleration of gravity is 10 M/s^2.
Work done against gravity to lift an object a height H is W = MgH W1: The weight is lifted up to the height of 1 M. W2: The weight is held at 1M for 5 seconds. W3: The weight is lowered down to the ground. Cheryl Hawarth, USA gold medal. Weightlifting Work. W1: The weight is lifted up to the height of 1 M. W2: The weight is held at 1M for 5 seconds. W3: The weight is lowered down to the ground. The work done W1, W2, W3 is:
1. 1000J, 5000J, 1000J 2. 1000J, 0J, -1000J 3. 1000J, 5000J, -1000J The energy is kinetic, relatively speaking. V = + 5 m/s V = -5 m/s Train moves to the right with speed V. A baseball is thrown to the left with the same speed. What is the kinetic energy
of the baseball? Its mass is 1 kg. What is the kinetic energy? V = + 5 m/s K = M V2 M = 1 kg V = -5 m/s 1. 2. 3. 4. 12.5 J
5J Zero Either zero or 12.5J, depends on your frame of reference. The Power to overcome Friction. Work is Force times distance. Power is Work per unit time. AN IMPORTANT SPECIAL CASE Block moves with STEADY SPEED under influence of applied force that EXACTLY balances friction, so V is CONSTANT. Force of friction
F Under these conditions, P W F X F v T T Frictional Force, Ff = N
THIS IS A SPECIAL CASE IN WHICH AN APPLIED FORCE RESULTS IN CONSTANT SPEED. IT OCCURS BECAUSE THE NET FORCE IS ZERO (SUM OF ALL APPLIED FORCES). How about this case? Total work is just the sum of the work for each spring. 1 1 W K1 x 2 K 2 x 2 2 2 1 K1 K 2 x 2 2
And this one? This is harder. The two springs DO NOT stretch the same distance (they have different K values). x x1 x2 F1 k1 x1 F2 k 2 x2 x But F1 = F2 (why?), so let it be called F. F1 F2
k1 k 2 1 1 x F k1 k 2 So, can create an effective spring constant, Keff, keff 1 W keff x 2
Then, we can apply the usual formula: 2 1 1 1 1 1 1 k1 k 2 k k 2 1
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