Category Archive Conservation of energy conceptual questions


Conservation of energy conceptual questions

In physics and chemistrythe law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be conserved over time. For instance, chemical energy is converted to kinetic energy when a stick of dynamite explodes.

If one adds up all forms of energy that were released in the explosion, such as the kinetic energy and potential energy of the pieces, as well as heat and sound, one will get the exact decrease of chemical energy in the combustion of the dynamite. Theoretically, this implies that any object with mass can itself be converted to pure energy, and vice versa, though this is believed to be possible only under the most extreme of physical conditions, such as likely existed in the universe very shortly after the Big Bang.

Conservation of energy can be rigorously proven by Noether's theorem as a consequence of continuous time translation symmetry ; that is, from the fact that the laws of physics do not change over time. A consequence of the law of conservation of energy is that a perpetual motion machine of the first kind cannot exist, that is to say, no system without an external energy supply can deliver an unlimited amount of energy to its surroundings.

Examples include curved spacetimes in general relativity [3] or time crystals in condensed matter physics. Ancient philosophers as far back as Thales of Miletus c. However, there is no particular reason to identify their theories with what we know today as "mass-energy" for example, Thales thought it was water.

Empedocles — BCE wrote that in his universal system, composed of four roots earth, air, water, fire"nothing comes to be or perishes"; [8] instead, these elements suffer continual rearrangement. Epicurus c. InSimon Stevinus was able to solve a number of problems in statics based on the principle that perpetual motion was impossible.

Application and Practice Questions

InGalileo published his analysis of several situations—including the celebrated "interrupted pendulum"—which can be described in modern language as conservatively converting potential energy to kinetic energy and back again.

Essentially, he pointed out that the height a moving body rises is equal to the height from which it falls, and used this observation to infer the idea of inertia. The remarkable aspect of this observation is that the height to which a moving body ascends on a frictionless surface does not depend on the shape of the surface. InChristiaan Huygens published his laws of collision. Among the quantities he listed as being invariant before and after the collision of bodies were both the sum of their linear momenta as well as the sum of their kinetic energies.

However, the difference between elastic and inelastic collision was not understood at the time. This led to the dispute among later researchers as to which of these conserved quantities was the more fundamental.

In his Horologium Oscillatoriumhe gave a much clearer statement regarding the height of ascent of a moving body, and connected this idea with the impossibility of a perpetual motion. Huygens' study of the dynamics of pendulum motion was based on a single principle: that the center of gravity of a heavy object cannot lift itself. The fact that kinetic energy is scalar, unlike linear momentum which is a vector, and hence easier to work with did not escape the attention of Gottfried Wilhelm Leibniz.

It was Leibniz during — who first attempted a mathematical formulation of the kind of energy which is connected with motion kinetic energy. Using Huygens' work on collision, Leibniz noticed that in many mechanical systems of several massesm i each with velocity v i. He called this quantity the vis viva or living force of the system. The principle represents an accurate statement of the approximate conservation of kinetic energy in situations where there is no friction.

Many physicists at that time, such as Newton, held that the conservation of momentumwhich holds even in systems with friction, as defined by the momentum :. It was later shown that both quantities are conserved simultaneously, given the proper conditions such as an elastic collision. InIsaac Newton published his Principiawhich was organized around the concept of force and momentum. However, the researchers were quick to recognize that the principles set out in the book, while fine for point masses, were not sufficient to tackle the motions of rigid and fluid bodies.

Some other principles were also required. The law of conservation of vis viva was championed by the father and son duo, Johann and Daniel Bernoulli. The former enunciated the principle of virtual work as used in statics in its full generality inwhile the latter based his Hydrodynamicapublished inon this single conservation principle.

Daniel's study of loss of vis viva of flowing water led him to formulate the Bernoulli's principlewhich relates the loss to be proportional to the change in hydrodynamic pressure. Daniel also formulated the notion of work and efficiency for hydraulic machines; and he gave a kinetic theory of gases, and linked the kinetic energy of gas molecules with the temperature of the gas.A rollercoaster cart with a total mass of 1,kg is at the top of a hill moving at.

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Assuming no friction and neglecting air resistance, what is the speed of the cart when it reaches the bottom of the hill, 30m below?

From the problem statement, we know that the cart has both initial potential and kinetic energy. We can assume that there is no final potential energy. At their highest jump, each friend is 2m above the trampoline. The trampoline has a threshold of N, above which it breaks. Do the friends succeed in breaking their trampoline? No; they exert a force of. Yes; they exert a force of. According to the problem statement, we can treat the friends as a single mass of kg.

We can use the expression for conservation of energy to calculate the velocity as they hit the trampoline. The statement says that the friends decelerate from this velocity to zero over a period of 0. Therefore, we can write:.

Neglect air resistance and assume. There are two ways to solve this problem. The first uses the concept of conservation of energy, and the second uses kinematics.

Without that height, we would have to do method 2. Note that this is one of the big five kineamtics equations with which you should be familiar. Plugging in our values, we get:. We also know how far the ball travels horizontally, so we can find out long it takes to cover that distance:. The velocity is negative because the ball is now traveling back downward.

We can now combine the component velocities into the final velocity of the ball:. Each time the ball strikes the ground, ten percent of the ball's kinetic energy is lost in the collision. What is the maximum height the ball will reach after hitting the ground four times? To find the total energy after four bounces, we multiply the initial potential energy by.

Based on conservation of energy, we can find the height that the ball travels with this remaining kinetic energy. Using conservation of energy, find out how much work gravity does on the ball when it travels from the bottom to the maximum height.

The energies involved in this problem are kinetic and potential energy.

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Conservation of energy shows that the initial energies will be equal to the final energies.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics.

It only takes a minute to sign up. I was having some conceptual difficulty reconciling my intuitive understanding of kinematics with conservation of energy, so I made up a short problem that tested my intuitions:. Suppose I define an initial point to be 20m above the ground. An object leaves this initial point along Path A, which is straight down. Another object leaves the initial point along Path B, which is parallel to the ground. What is the speed of each object as it hits the ground?

This result clearly defies conservation of energy. Both objects start at the same initial height, and so have the same potential energy.

They both end their paths at the same height, and so end with the same potential energy. But they both have different velocities, and so different kinetic energies. The thing is, this is exactly what my intuitions would predict.

How much an object accelerates in total is just a function of the time it spends accelerating. The object traveling along Path A has a fast downwards velocity, so it only has a short time to accelerate. Its final speed is just the sum of its initial speed and however much it accelerates in that time. The object traveling along Path B has no initial downwards velocity, and so has plenty of time to accelerate. The total final speed of this object is the vector sum of its initial velocity and however much it gained while accelerating downwards, which is a much larger number than the total acceleration of the first object.

So what's going on? The calculations break conservation of energy, so I must be doing something wrong. And even if there's a flaw in my calculations and the numbers actually do come out the same, my intuition still says otherwise. The two particles have the same energy. You went wrong in your application of the quadratic formula.

Correct, but the kinetic energy of a particle is not dependent on how much it has accelerated. It only depends on the instantaneous velocity of the particle. Here the final velocity in the y-direction for particle A will be higher than that of particle B. The final velocity in the x-direction for particle B will be higher than that of particle A equally so, in fact. Calculating the final speed of both would yield the same answer, and so the kinetic energy will remain equal. Sign up to join this community.

The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Kinematics contradicting conservation of energy? Ask Question. Asked 4 years, 1 month ago. Active 4 years, 1 month ago.

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Search titles only. Search Advanced search…. Log in. Support PF! Buy your school textbooks, materials and every day products Here! JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding. Conservation of energy: Conceptual question. Thread starter Neha Siddhartha Start date May 24, Tags conceptual understanding conservation of energy gravitation potential energy kinetic energy.

Neha Siddhartha. Twin A takes 20 seconds to climb the hill, while twin B takes 40 seconds. Their gain in potential energy is the same, as they are going up the same hill so h, or height, must be the sa me.

However, I do not understand how to explain it using the equations. Twin A would have 4x the kinetic energy of Twin B, so the kinetic E applied does not equal the potential E gained. So, should I instead think of the total energy of each biker? I think that with a greater velocity they will encounter greater friction.

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Is this correct? My professor said friction is just random or nondirectional kinetic energy. How can I explain this mathematically with just the equations given? Neha Siddhartha said:. Insights Author.

conservation of energy conceptual questions

Gold Member. Hint: twin "B" must be taking the long way around, as he travels twice as far as twin "A". What's that suggest as far as friction is concerned not air but tire-to-ground? Homework Helper. CWatters Science Advisor.

Neha - Think about the work done in each case. For example what is the equation for the work done against gravity? I think you might be over complicating the answer. I disagree with this hint. The phrase "constant speed" indicates that each twin stays at the same speed she started with.To browse Academia. Skip to main content.

Work Energy Theorem - Kinetic Energy, Work, Force, Displacement, Acceleration, Kinematics & Physics

Log In Sign Up. After the system is released from rest, which of the following statements are true? U is the gravitational potential energy and K is the kinetic energy of the system. Determine the Concept Because the pulley is frictionless, mechanical energy is conserved as this system evolves from one state to another. Neglect effects due to air resistance. Which statement below is true? Determine the Concept Choose the zero of gravitational potential energy to be at ground level.

The two stones have the same initial energy because they are thrown from the same height with the same initial speeds. Therefore, they will have the same total energy at all times during their fall. When they strike the ground, their gravitational potential energies will be zero and their kinetic energies will be equal. Thus, their speeds at impact will be equal.

Forces that are external to a system can do work on the system to change its energy. In order for some object to do work, it must exert a force over some distance. The mechanical energy gained by the jumper comes from the internal energy of the jumper, not from the floor.

conservation of energy conceptual questions

The frictional force that accelerates a sprinter increases the total mechanical energy of the sprinter. Because the work required to stretch a spring a given distance varies as the square of that distance, the work is the same regardless of whether the spring is stretched or compressed. Discuss the energy changes that occur as you use the boards to slow your motion to a stop. Determine the Concept The boards do not do any work on you.

Your loss of kinetic energy is converted into thermal energy of your body. Conservation of Energy a True. Sal chooses a short, steep trail, while Joe, who weighs the same as Sal, chooses a long, gently sloped trail. At the top, they get into an argument about who gained more potential energy.

Which of the following is true: a Sal gains more gravitational potential energy than Joe. The definition of work is not limited to displacements caused by conservative forces. Consider the work done by the gravitational force on an object in freefall. The work done may change the kinetic energy of the system. If that is so, then which of the following are necessarily true?

Picture the Problem Because the constant friction force is responsible for a constant acceleration, we can apply the constant-acceleration equations to the analysis of these statements.

We can also apply the work-energy theorem with friction to obtain expressions for the kinetic energy of the car and the rate at which it is changing. Because none of the above are d is correct. The kinetic energy of the rock remains constant, but the gravitational potential energy is continually changing. Is the total work done on the rock equal to zero during all time intervals? Does the force by the rod on the rock ever have a nonzero tangential component?

Determine the Concept No. From the work-kinetic energy theorem, no total work is being done on the rock, as its kinetic energy is constant. However, the rod must exert a tangential force on the rock to keep the speed constant.

The effect of this force is to cancel the component of the force of gravity that is tangential to the trajectory of the rock.Lesson 2 has thus far focused on how to analyze motion situations using the work and energy relationship.

The relationship could be summarized by the following statements:. There is a relationship between work and mechanical energy change. Whenever work is done upon an object by an external or nonconservative forcethere will be a change in the total mechanical energy of the object.

If only internal forces are doing work no work done by external forcesthere is no change in total mechanical energy; the total mechanical energy is said to be "conserved. Now an effort will be made to apply this relationship to a variety of motion scenarios in order to test our understanding.

Use your understanding of the work-energy theorem to answer the following questions. Then click the button to view the answers. Consider the falling and rolling motion of the ball in the following two resistance-free situations. In one situation, the ball falls off the top of the platform to the floor. In the other situation, the ball rolls from the top of the platform along the staircase-like pathway to the floor.

For each situation, indicate what types of forces are doing work upon the ball. Indicate whether the energy of the ball is conserved and explain why. Finally, fill in the blanks for the 2-kg ball. Since it is an internal or conservative force, the total mechanical energy is conserved. Thus, the J of original mechanical energy is present at each position. And so the kinetic energy at the bottom of the hill is J G and J.

The answers given here for the speed values are presuming that all the kinetic energy of the ball is in the form of translational kinetic energy. In actuality, some of the kinetic energy would be in the form of rotational kinetic energy. Thus, the actual speed values would be slightly less than those indicated.

Rotational kinetic energy is not discussed here at The Physics Classroom Tutorial. If frictional forces and air resistance were acting upon the falling ball in 1 would the kinetic energy of the ball just prior to striking the ground be more, less, or equal to the value predicted in 1?

The kinetic energy would be less in a situation that involves friction. Friction would do negative work and thus remove mechanical energy from the falling ball. The answer is D. The total mechanical energy i. The answer is B. The PE is a minimum when the height is a minimum.

Position B is the lowest position in the diagram. The answer is C. Since the total mechanical energy is conserved, kinetic energy and thus, speed will be greatest when the potential energy is smallest.

Conservation of energy

Point B is the only point that is lower than point C. The reasoning would follow that point B is the point with the smallest PE, the greatest KE, and the greatest speed. Therefore, the object will have less kinetic energy at point C than at point B only. Many drivers' education books provide tables that relate a car's braking distance to the speed of the car see table below. Utilize what you have learned about the stopping distance-velocity relationship to complete the table.

The car skids m.

conservation of energy conceptual questions

Thus, there must be a nine-fold increase in the stopping distance.A ball with mass of 2kg is dropped from the top of a building this is 30m high. What is the approximate velocity of the ball when it is 10m above the ground?

Use conservation of energy. The gravitational potential energy lost as the ball drops from 30m to 10m equals the kinetic energy gained. Change in gravitational potential energy can be found using the difference in mgh.

So Joules are converted from gravitational potential to kinetic energy, allowing us to solve for the velocity, v. Consider a spring undergoing simple harmonic motion.

Conservation of energy: Conceptual question

Kinetic energy is highest when the spring is moving the fastest. Conversely, potential energy is highest when the spring is most compressed, and momentarily stationary. When the force resulting from the compression causes the spring to extend, potential energy decreases as velocity increases. A pendulum with a mass of kg reaches a maximum height of 2.

What is its velocity at the bottommost point in its path? Two children are playing with sleds on a snow-covered hill.

conservation of energy conceptual questions

Sam weighs 50kg, and his sled weighs 10kg. Sally weighs 40kg, and her sled weighs 12kg. When they arrive, they climb up the hill using boots. Halfway up the meter hill, Sally slips and rolls back down to the bottom. Sam continues climbing, and eventually Sally joins him at the top. Sam goes down the hill first, claiming that he will reach a higher velocity. If Sally had gone first, Sam says they could collide. Sally goes down the hill first, claiming that she will experience lower friction and thus reach a higher velocity.

If Sam had gone first, Sally says they could collide. At the bottom of a neighboring hill, a neighbor watches Sally and Sam come down the hill. From the moment the neighbor begins watching, to just after they both come to a stop, who has dissipated more heat in the form of friction? Assume all friction is lost as heat.

Sally has greater kinetic energy in this example than does Sam. From the moment when the neighbor begins watching we can calculate the kinetic energy.

Once stopped, all of the kinetic energy will have been dissipated. At the bottom of the oscillation potential energy is at a maximum and kinetic energy is at a minimum.

At the bottom of the oscillation potential energy is at a maximum and kinetic energy is at a maximum. At the bottom of the oscillation potential energy is at a minimum and kinetic energy is at a minimum. At the bottom of the oscillation potential energy is at a minimum and kinetic energy is at a maximum. Energy must be conserved through the motion of a pendulum. Let point 1 represent the bottom of the oscillation and point 2 represent the top.

At point 2, the velocity is zero; thus, the kinetic energy is zero and all of the system energy is potential energy. At the highest point in the swing, potential energy is at a maximum, and at the lowest point in the swing, kinetic energy is at a maximum. In this case, the boulder starts with zero kinetic energy and ends with both kinetic and potential energy.

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