Physics 197 Lecture Notes - Lecture 9: Kinetic Energy, Tennis Ball, Point Particle

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Physics 197 Lecture Notes - Lecture 8: Circular Motion, Free Body Diagram, Rolling Resistance

Physics 197 Lecture Notes - Lecture 9: Kinetic Energy, Tennis Ball, Point Particle

Physics 197 Lecture Notes - Lecture 10: Pogo Stick

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Kinetic energy: kinetic energy is the energy associated with motion, scalar, unit: joules (j, (cid:1831)=(cid:2869)(cid:2870)(cid:2870) Work-kinetic energy theorem: cause: work, work is the force along of direction of displacement, effect: change in kinetic energy, an object"s speed changes. If fnet causes an object to speed up, the work done is positive. If fnet causes an object to slow down, the work done is negative. If fnet does not displace the object, then no work is done: ex: you are swinging a tennis ball on a string in a vertical circle. What forces do work: tension is perpendicular to displacement so does no work, gravity is parallel to displacement at some points, so does work! Dot product: multiply vectors for a scalar result, 2 vector input, takes component of one vector parallel to the other vector and multiplies, (cid:1827) (cid:1828)= (cid:1827) (cid:1828) cos, work equation: =(cid:1832) =(cid:1832)cos, only valid for constant force.

Related Questions

A 270 g block is dropped onto a relaxed vertical spring that has aspring constant of k = 2.1 N/cm. The block becomes attached to thespring and compresses the spring 11 cm before momentarilystopping.
(a) While the spring is being compressed, what work is done on theblock by the gravitational force on it?
1Your answer is correct. J

(b) What work is done on the block by the spring force while thespring is being compressed?
2Your answer is incorrect. J

(c) What is the speed of the block just before it hits the spring?(Assume that friction is negligible.)
3Your answer is incorrect. m/s

(d) If the speed at impact is doubled, what is the maximumcompression of the spring?
4 m

Work done by a constant force is the dot product of the force andthe displacement. Work done by a variable force is calculated withan integral. The net work done on an object is equal to the changein the object's kinetic energy. If the object stops, its kineticenergy is zero.

Goal Apply conservation of angular momentum to a simplesystem.

Problem A student sits on a pivoted stool while holding a pair ofweights. (See Figure) The stool is free to rotate about a verticalaxis with negligible friction. The moment of inertia of student,weights, and stool is 2.25 kg Â· m2. The student is set in rotationwith arms outstretched, making one complete turn every 1.26 s, armsoutstretched.

Question If the student suddenly releases the weights, how does hisown angular momentum, rotational kinetic energy, and angular speedchange? (Select all that apply.)

His rotational angular speed decreases.
His rotational angular speed remains the same.
His angular momentum remains the same.
His rotational kinetic energy remains the same.
His angular momentum increases.
His angular momentum decreases.
His rotational angular speed increases.
His rotational kinetic energy increases.
His rotational kinetic energy decreases.

A star with an initial radius of 1.00 108 m and period of 30.0 dayscollapses suddenly to a radius of 1.00 104 m.

(a) Find the period of rotation after collapse.

(b) Find the work done by gravity during the collapse if the massof the star is 2.00 1030 kg.

The only force doing work on the system is gravitation, so the workdone by gravitation is the change in kinetic energy of the star.J

(c) What is the speed of an indestructible person standing on theequator of the collapsed star? (Neglect any relativistic or thermaleffects, and assume the star is spherical before and after itcollapses).

HINTS
* Do you expect the initial angular speed to be larger or smallerthan the final angular speed? How is angular speed related toperiod of rotation?

* Is the initial moment of inertia larger or smaller than the finalmoment of inertia?

* Is angular momentum conserved?

* As the star collapses, do you expect the period of rotation tobecome smaller or larger?

1. An object is dropped from a height.Once it is moving,which ofthe following statements are true,at least at one point?
a)its velocity is more than its accelerations
b)its velocity is less than its acceleration
c)its velocity is the same as its acceleration
d)its velocity is never equal to its acceleration

2. Which type of quantity is characterized by both magnitude anddirection?
a)scalar
b)vector
c)trigonometric
d)algebraic variable

3. A hiker walks 200m west and then walks 100m north. In whatdirection is her resulting displacement?
a)north
b)west
c)northwest
d)none of the answer is correct

4. Vector 1 is 7 units long and is at 70 degree. Vector 2 is 5units long and is at 225 degree. Vector 3 is a 3 units long and isat 150 degree. Which vector has equal magnitude components?
a)Vector 1
b)vector2
c)vector 3
d)none of the vectors has equal magnitude components??????

5. Which of the following is an example of the type of force thatacts at a distance?
a)gravitational
b)magnetic
c)electrical
d)all of the above

6. A crate weight W is being pushed across a horizontal surface bya force directed at an angle of 20 degrees below the horizontal.What is the magnitude of the normal force on the crate?
a)it is less then W
b)it equals W
c)it is more than W
d)none of the above since the coefficient of kinetic friction isnot given

7. The net force on an object is in the positive x-direction.Consider the following statements.
(i) The object can be moving in the negative x-direction.
(ii) The object can be speeding up.
(iii) The object can be slowing down.
(iv) The object can be moving in the positive y-direction.

Which of the statements are true?
a. (i) and (ii)
b. (ii) and (iii)
c. (iii) and (iv)
d. Choose this answer if all the statements are true.

8.The work done by static friction can be:
a. positive.
b. negative.
c. zero.
d. Any of the above.

9.The rate at which work is done is equivalent to which of thefollowing?
a. increase in potential energy
b. thermal energy
c. potential energy
d. power

10.The dimensional equivalent of the quantity impulse in terms ofthe fundamental quantities (mass, length,
time) is which of the following?
a. MLT-1
b. ML2T-2
c. MLT
d. MLT-2

11. The dimensional equivalence of the quantity "momentum" in termsof the fundamental quantities (mass,
length, time) is:
a. MLT-1.
b. ML2T-2.
c. MLT.
d. MLT-2.

12.In order for a satellite to be geosynchronous, its orbitmust:
a. go over the North and South Poles.
b. be over the equator.
c. be over a single longitude.
d. emit television signals.

13. The quantity "angular momentum" (in terms of the fundamentalquantities of mass, length, and time) is equivalent
to:
a. MLT-2.
b. ML2T-1.
c. ML2T-3.
d. ML3T.
d. MLT-2.

Physics 197 Lecture Notes - Lecture 9: Kinetic Energy, Tennis Ball, Point Particle

Physics 197 Lecture Notes - Lecture 8: Circular Motion, Free Body Diagram, Rolling Resistance

Physics 197 Lecture Notes - Lecture 9: Kinetic Energy, Tennis Ball, Point Particle

Physics 197 Lecture Notes - Lecture 10: Pogo Stick

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