Mechanical Energy: A to B, Forces -70J/+50J

[nrc_8706]

an object moves from A to B only two forces act on it. one force is conservative and does -70J of work, the other force is nonconservative and does +50J of work.

kinetic energy of the object decreases and mechanical energy decreases, right?

 

[Astronuc]

Mechanical energy can be either kinetic energy (energy of motion) or potential energy (stored energy of position).

See - http://www.glenbrook.k12.il.us/gbssci/phys/Class/energy/u5l1d.html

So when you ask about mechanical energy, do you mean mechanical potential energy?

 

[nrc_8706]

energy

a 2.47kg block is pushed 1.7m up a vertical wall with constant speed by a constant force of magnitude F applied at an angle of 63.3 with the horizontal. acceleration of gravity is 9.8m/s^2

if the coefficient of kinetic friction between the block and wall is 0.553, find the work done by F.

sum of all forces on the x-axis=0 sum of all forces on the y-axis=,a
N=-Fsin(angle) Fcos(angle)-mg-Friction force=ma

CORRECT?

 

[nrc_8706]

the choices only mention kinetic and mechanical

 

[Astronuc]

a 2.47kg block is pushed 1.7m up a vertical wall with constant speed by a constant force of magnitude F applied at an angle of 63.3 with the horizontal. acceleration of gravity is 9.8m/s^2
if the coefficient of kinetic friction between the block and wall is 0.553, find the work done by F.
sum of all forces on the x-axis=0 sum of all forces on the y-axis=,a
N=-Fsin(angle) Fcos(angle)-mg-Friction force=ma
CORRECT?

The angle is with respect to the horizontal.

The vertical force, F_y must balance the friction force, which is proportional to F_x, and the weight of the block, mg.

Then the work is simply the force applied over distance. F_y is constant because the block is pushed at constant speed, i.e. no acceleration.

 

[nrc_8706]

for the first question, the answer that i suggested is not correct. how come?

 

[Astronuc]

In the first question, I am trying to understand if any potential energy is involved.

Generally, if the work done is positive, then mechanical energy increases, and if work is negative, mechanical work decreases.

The total mechanical energy (or mechanical energy) is the sum of kinetic and potential energies.

Deceleration would imply negative work and this coincides with a reduction in kinetic energy, where as acceleration implies positive work being done increases kinetic energy.

A nonconservative or dissipative force reduces kinetic energy.

I am trying to think of a + nonconservative work.