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Announcements. Test 1 next Thursday, 5–7 PM Covers material in Chapters 2–5 Review session Monday 5:30–7 PM, CR 103 Bring your questions. A. 100 N. 100 N. D. 0 m. 10 m. B. 100 N. 100 N. 100 m. 100 m. E. 10 N. C. 100 m. Group Work. - PowerPoint PPT Presentation
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Announcements
• Test 1 next Thursday, 5–7 PM
• Covers material in Chapters 2–5
• Review session – Monday 5:30–7 PM, CR 103– Bring your questions
Group Work
1. Rank the following scenarios from least work done to most work done.
10 m100 N
A.
100 m10 N
B.
100 m100 N
C.
100 N
100 mD.
100 N
0 mE.
CPS Question
Work is
A. Force time.
B. Force distance
C. Force / distance.
D. Force / time.
E. Mass velocity
Group Work
2. Estimate the work done by the strong man in the video. Show your work! Justify your estimates of force and distance.
Work is a Scalar
Source: Griffith, The Physics of Everyday Phenomena
component of force in direction of motion
Dot Product Geometrically
• Product of the projection of one vector onto the other
• “Overlap”
b cos
a cos
a
b
ab cos
Dot Product Properties
• Positive if || < /2• Negative if || > /2
• Zero if vectors perpendicular (|| = /2)• Maximum magnitude if parallel or anti-
parallel
+–
CPS Question
The piglet has a choice of three frictionless slides to descend. Along which slide is the greatest net force exerted on the piglet?
A B C
D. The net force is the same for all.
CPS Question
The piglet has a choice of three frictionless slides to descend. Along which slide would the piglet slide the longest distance?
A B C
D. The distance is the same for all.
CPS Question
The piglet has a choice of three frictionless slides to descend. Along which slide would the piglet finish soonest?
A B C
D. The time is the same for all.
Group CPS Question
The piglet has a choice of three frictionless slides to descend. Along which slide would gravity do the most work on the piglet?
A B CD. Same work for all. E. Need more information.
Example Problem 6.57
A luggage handler at the Laramie Airport pulls a 20-kg suitcase up a ramp inclined at 25° above the horizontal with a force of 140 N parallel to the ramp. The coefficient of kinetic friction between the ramp and the box is k = 0.30. the suitcase travels 3.80 m along the ramp. Finda. the work done on the suitcase by the handlerb. the work done on the suitcase by gravityc. the work done on the suitcase by the normal forced. the work done on the suitcase by frictione. the total work done on the suitcasef. the final speed of the suitcase, if its initial speed was zero
CPS Question
To accelerate an object from 10 to 20 m/s requires
A. more work than to accelerate from 0 to 10 m/s.
B. the same amount of work as to accelerate from 0 to 10 m/s.
C. less work than to accelerate from 0 to 10 m/s.
Work of Acceleration• To accelerate to speed v with constant force F
t
v
speed
time
mvtF = m (slope) =
12 d = vt
• Work = F·d
• Work = = mv212vt1
2mvt
slope = a = Fm
vt=
area = d
Another Perspective
• So, for the 0–10 vs. 10–20 m/s case:
• If same force, then same time– a’s and v’s are equal, so t’s are equal
• Average speeds are 5 vs. 15 m/s
• The 10–20 m/s case travels 3 as far
Kinetic Energy
the work to bring a motionless object to speed
KE = 12 mv2
equivalent to
the work a moving object does in stopping
Which has more kinetic energy?
A.
B.
C.
D.
10 kg 10 m/s
5 kg 10 m/s
10 kg 20 m/s
40 m/s5 kg
CPS Question
CPS Question
The piglet has a choice of three frictionless slides to descend. Along which slide would the piglet finish with the highest speed?
A B C
D. The final speed is the same for all.