Physics - Module 9: Conservation of Energy
In both figures, a particle of mass m is released from rest at a height, h. In figure (a), the particle is dropped straight downward and in figure (b) the particle is released from rest down a frictionless ramp. Which particle, the one in figure (a) or (b), will have more kinetic energy at the bottom?
Both particles will have the same kinetic energy at the bottom. -non-conservative forces doing work = mechanical energy of the system is conserved for both (a) & (b).
A block slides along a rough surface and comes to a stop. What can you conclude about the frictional force exerted on the block?
The frictional force does negative work on the block and decreases its kinetic energy. -The frictional force acting on the block points in the opposite direction to the displacement of the block, -The work-energy theorem: the work done is equal to the change in kinetic energy. A negative change in kinetic energy means that the kinetic energy decreases.
When you lift a book upward off of a table, what is true about the work done on the book by the force of gravity?
The work done by the force of gravity is (-) negative and proportional (α) to the upward displacement of the book. -The work done by gravity is negative because the force of gravity, which points downward, is in the opposite direction of the displacement, which is upward. The work done by the force of gravity is proportional to the displacement of the book because the force of gravity is approximately constant everywhere near the surface of Earth.
You observe two identical balls of putty heading directly toward each other at equal speeds. What can you say about their total kinetic energy?
They have twice the kinetic energy of either ball by itself. -same mass + equal speeds = they have equal amounts of kinetic energy. - kinetic energy is a scalar (not a vector), the direction that they are moving doesn't matter.
The mechanical energy of a system is conserved during a certain process only if __________. a. non-conservative forces are not exerted on the system during that process b. the mechanical energy of each part of the system stays exactly the same during that process c. non-conservative forces do zero total work on the system during that process d. no external forces are exerted on the system at all during that process
a. non-conservative forces do zero total work on the system during that process -The mechanical energy of a system is defined as the sum of kinetic energy and potential energy of the system. Non-conservative forces (such as a normal force) can be exerted on the system, but as long as they don't do any total work on the system the mechanical energy will still be conserved.
In both figures, a particle of mass m, is released from rest at a height, h. In figure (a), the particle is dropped straight downward and in figure (b) the particle is released from rest and slides down a ramp with a rough surface. Which particle, the one in figure (a) or (b), will have more kinetic energy at the bottom? (Figure (a) shows a particle of mass m on the edge of a verticle cliff of height h. Figure (b) shows a particle of mass m at the top of a straight, shallow slope of height h.)
The particle in figure (a) will have more kinetic energy than the particle in figure (b) at the bottom. -(a), non-conservative force is doing work = mechanical energy is conserved. kinetic energy at the bottom will = gravitational potential energy it started with (mgh). -(b), friction—a non-conservative force—acts on the particle and decreases its total mechanical energy, converting some mechanical energy into thermal energy. = less kinetic
A variety of forces are applied to an object such that the net force does positive work on that object. What can you conclude about the speed of the object?
When the total work done on the object is positive, the object's speed will increase. W=∆K -This is because the object's speed increases if its kinetic energy increases, which must happen when the total work done is positive. -when total work done is (+) positive, the object's kinetic energy and speed will never decrease or remain constant.
Which of the following forces is considered a conservative force? a. The force of tension from a rope b. The force due to a spring c. The force of someone pushing an object d. The force of friction due to a surface
b. The force due to a spring - The work done by the force due to a spring is path independent. - The forces of tension, friction, and a person pushing can change the mechanical energy of a system and the work done by these forces is path dependent, so they are non-conservative forces.
