PHYS 1051 Lecture Notes - Lecture 9: Net Force

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copperfly323

6 Jul 2018

School

Department

Physics

Course

PHYS 1051

Professor

Ciferno

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Ch. 7 Work Notes

Work

Necessary for work to occur:

oForce

oDisplacement

*Note: the force or at least a component of it must be along the line of

displacement

oThe work done is the product of Force and Displacement

oW=FD

Work is a scalar quantity

oNegative signs do not mean direction of the work

oPositive work on an object means that energy is added to the object and it

speeds up

oNegative work on an object means that energy is removed or taken from the

object and it slows down

Energy

Fnet does the net work

Kinetic Energy- ½ mv^2

oThe energy an object has because of its motion

Work-Kinetic Energy Theorem- the net work done in an object is equal to its change in

kinetic energy

Work done by a varying force

Springs

K= spring constant

Units: n/m

Indicates the strength of a spring

The force applied by the spring

F=Kx ----on the spring

F=-kx ----- by the spring (Hooke’s Law)

The area bounded by an FD graph yields a value for work

Linear Springs

W=1/2kx^2

oWork done by a spring

oX= elongation or compression of spring

W=1/2Fx

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Document Summary

Necessary for work to occur: force, displacement. *note: the force or at least a component of it must be along the line of displacement: the work done is the product of force and displacement, w=fd. Kinetic energy- mv^2: the energy an object has because of its motion. Work-kinetic energy theorem- the net work done in an object is equal to its change in kinetic energy. The area bounded by an fd graph yields a value for work. W=1/2kx^2: work done by a spring, x= elongation or compression of spring. Work that slows things down is negative. Freefall- only force is gravity: since gravity is the only force, it is the net force and it does the net work. Mgh= gravitational potential energy-energy associated with position in a gravitational field. As negative work is done, potential energy is increasing and vice versa. H is measured from an arbitrary reference point. U will be the same regardless of chosen reference point.

Related Questions

1. Can work be done on a system if there is no motion?

A) Yes, if an outside force is provided B) Yes, since motion is only relative C) No, since a system which is not moving has no energy D) No, because of the way work is defined

2. The quantity 1/2 mv 2 is

A) the power supplied to the object by the force B) the work done on the object by the force C) the kinetic energy of the object. D) the potential energy of the object.

3. If you push twice as hard against a stationary brick wall, the amount of work you do

A) doubles. B) is cut in half C) remains constant but non-zero D) remains constant at zero

4. If the net work done on an object is positive, then the object's kinetic energy

A) increases. B) decreases C) remains the same D) is zero

5 A force of 20 N moves an object in the direction of the force. The work done when the object moves from 2.0 to 4.0 m is o A) 20 J OB) 40 J oC) 60 J D) 80 J

whitemosquito79

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.

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.

PHYS 1051 Lecture Notes - Lecture 9: Net Force

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