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PHYS 122 Lecture Notes - Lecture 2: Net Force

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coppermacaw594
26 Jan 2018
School
Drexel University
Department
Physics
Course
PHYS 122
Professor
Christina Love

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Document Summary

The net work acting on an object is equal to the change in kinetic energy. Work done by the net force on a particle equals the change in the particle"s kinetic energy. Total work done on particle = work done by net force. The work done by the new force is called the total work w(tot) Average power during time interval t > p(av) = w/ t (work done during time interval / duration of time interval) The si unit of power is the watt (1 w = 1 j/s), but another familiar unit is the horsepower (1 hp = 746 w) The total mechanical energy of a system is the sum of its kinetic and potential energy. A quantity that always has the same value is called a conserved quantity. When only the force of gravity does work on a system, the total mechanical energy of that system is conserved.

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Related textbook solutions

Physics: Principles with Applications
7th Edition, 2013
Giancoli
ISBN: 9780321625922

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Related Questions

A 1500 kg caracceleratesuniformly from rest to a speed of 18m/s in 4.59 s.



A) Find the WORK done by the NET FORCE on the car during thistimeinterval. (Answer in units of J.)
acceleration=2.349 m/s^2
force=3570.69 N
distance=25.289 m
work=90300.91 J



B) Find the average power DELIVERED by the NET FORCE to the carinthis time interval. (Answer in units of w.)
power=19461.403 J/s


C) What is the instantaneous power delivered by the NET FORCE tothecar at 2 s? (Answer in units of w.)
i cant figure this one out! please explain it to me.
The kinetic energy Kof an object of mass mmoving at a speed vis defined as K=(1/2)mv^2. It seems reasonable to say that the speed of an object--and, therefore, its kinetic energy--can be changed by performing work on the object. In this problem, we will explore the mathematical relationship between the work done on an object and the change in the kinetic energy of that object.

Let us now consider the situation quantitatively. Let the mass of the sled be  m  and the magnitude of the net force acting on the sled beF_net. The sled starts from rest.

Consider an interval of time during which the sled covers a distance sand the speed of the sled increases from v_1 to v_2. We will use this information to find the relationship between the work done by the net force (otherwise known as the network) and the change in the kinetic energy of the sled.
H)
Find the net force F_net acting on the sled.
Express your answer in terms of someor all of the variables m,s,v_1, and v_2.
I)
Find the net work W_net done on the sled.
Express your answer in terms of someor all of the variables F_net and s.
The kinetic energy Kof an object of mass mmoving at a speed vis defined as K=(1/2)mv^2. It seems reasonable to say that the speed of anobject--and, therefore, its kinetic energy--can be changed byperforming work on the object. In this problem, we willexplore the mathematical relationship between the work done on anobject and the change in the kinetic energy of that object.

Let us now consider the situationquantitatively. Let the mass of the sled be m and the magnitude of the net force acting on the sled beF_net. The sled starts from rest.

Consider an interval of time duringwhich the sled covers a distance sand the speed of the sled increases from v_1 to v_2. We will use this information to find the relationshipbetween the work done by the net force (otherwise known as thenet work) and the change in the kinetic energy of thesled.
H)
Find the net force F_net acting on the sled.
Express your answer in terms of someor all of the variables m,s,v_1, and v_2.
I)
Find the net work W_net done on the sled.
Express your answer in terms of someor all of the variables F_net and s.

Part A.

Blocks A, B, and C are pushed across a frictionless table by a hand that exerts a constant horizontal force. Block A has mass 2M block B has mass 3M and block C has mass M.

a) Draw separate free-body diagrams for each of the three blocks. Clearly label the forces. in the spaces below, draw vectors to represent the net force on each block.
b) Draw the vectors with correct relative magnitudes.

Part B.

Blocks A and B are at rest on a level, frictionless surface. Block B is attached to an ideal massless spring of constant k, which is initially at its equilibrium length. Block B has greater mass than block A (mB > mA).

During the interval from t1 to t2 a hand pushes block A to the left with a constant force of magnitude F0, compressing the spring a distance as shown. At time t2, both blocks again have a speed of zero.

Consider system A consisting of block A and system BS consisting of block B and the spring.

c) During the interval from t1 to t2, is the absolute value of the net work done on system BS by external forces, greater than, less than, or equal to that done on system A by external forces?

 

 

At some later time, the hand is holding the blocks at rest with the spring compressed a distance d0. At time t3 the hand releases the blocks. At time t4, the spring returns to its equilibrium length and both blocks have speed vf, as shown.

Consider the interval from t3 to t4:

d) Is the change in kinetic energy of system BS greater than, less than, or equal to that of system A?
e) Is the change in total energy of system BS positive, negative, or zero?