13 Work and Energy

13 Work and Energy

• An object has energy by moving.
• Potential energy can be created by interactions with other objects.
• Work is done when a force acts over a distance.
• When work is done on an object, the energy can change.
• The chapter shows you how to use different forms of energy to make predictions about the behavior of objects.

• An object's center of mass is moving.

• The concept of multi-object systems storing energy depends on how the objects are arranged in the system.

• The energy can be stored in the molecule.

• The cart pictured above has a higher potential for energy than the motion detector because it is higher up.
• The cart would speed up if it were released from rest.
• The cart's maximum speed could be determined using the kinematics equations.
• Since the track is curved, the cart's acceleration will change.
• The studies in Chapter 10 are invalid when acceleration changes.
• The methods of energy saving must be used.

• A system is a collection of objects.
• You might have talked about the potential of an object in class.
• The exam development committee doesn't like that language.
• An object can have energy if it moves.
• A single object can't have potential energy.

• The result of an interaction between objects is potential energy.

• I'm going to talk about objects with potential energy.
• It's okay with me if you talk about a block on a spring with potential energy because the spring is compressed.

• You'll get everything right on the exam.

• The work done when a force is in the same direction as the object's motion is considered to be a positive quantity, while the work done when a force is in the opposite direction is considered to be a negative quantity.

• A string applies a 10-N force to the right of a box, dragging it across the floor for a distance of 50 cm.

• The work done by each force acting and the net work done on the box are calculated.

• Start by drawing a diagram of the box.

• Always sketch the direction of the force and displacement.

• Consider each force in its own way.
• The force acts to the right.
• The box is 50 cm to the right.

• The work done by the ten sion is 10 times less than the displacement because the force is in the same direction.

• Consider the force.
• Since the box moves at constant speed, the left force is equal to the right force.

• If a force is exerted on a system of objects and the system's center of mass moves parallel to the force, work was done.

• The equation is displacement times parallel force, which may be a more convenient expression.

• The work done by friction is -5 J.

• The force of the Earth does not work on the box since no component of the weight is parallel to the displacement.

• The normal force does not work on the box since no component of the force is parallel to the displacement.

• Break the force into components if it acts at an angle to the displacement.
• The component does not work.
• The component parallel to the displacement can be used to get the work done.

• The net force can be determined using a free-body diagram.

• If you want to find the work done by any force, you should take the component of the net force that's parallel to the displacement and add it to the total.

• Determine the work done by each force.
• Add the work done by each force as well as the negative signs.

• A conservative force doesn't change the mechanical energy of a system.

• gravity and springs are the only conservative forces that you need to deal with on the AP Physics 1 exam.
• When a spring works on an object, energy is stored in the spring and can be recovered and converted back to energy.
• The total potential and energy of the object-spring system is constant.

• A nonconservative force can change the mechanical energy of a system.

• Friction raises the object's temperature by raising the internal energy of the object.
• The propeller of an airplane is a nonconservative force that increases the airplane's mechanical energy.

• Net work must include work done by conserva tive and nonconservative forces.

• It's easier to separate conservative and nonconservative forces.

• An archer pulls an arrow of mass 0.10 kg attached to a bow string back 30 cm by using a force that increases uniformly with distance from 0 N to 200 N.

• The AP exam could ask a lot of questions.
• The problem should be categorized before you start calculating.
• There are three ways to approach a mechanics problem.
• momentum is unlikely to be useful because there is nocolli sion.
• A changing force means that the equations are not valid.
• Only the work-energy theorem is useful.

• The force of the string varies "uniformly", which means that the force gets bigger as the distance gets bigger, just like a spring.
• Just like a spring, treat the bowstring.

• Since the force of the archer on the string is changing, this equation for work is not easy to apply.
• The entire work-energy theorem is what you should use.

• If you want to find the work done by the archer in pulling back the bow string, you have to use the time from when he starts pulling until the maximum extension.
• The mechanical energy of the string-arrow system is changed by the work done by the archer.
• When the string is extended 0.30m, the archer's maximum force is 200 N.

• The work done by the archer is 1/2.

• If you want your chemistry teacher to have heart palpitations, use two significant figures.

• This calculation could be asked for by the AP exam.
• The spring constant is a property of the spring.
• The archer has to do more work.

• We can't use kinematics with different net force or acceleration.
• The work-energy theorem can be used again if the archer releases the bow string and the arrow is released.

• The equilibrium position is defined by the number 0.

• When all the forces acting conservative, mechanical energy is conserved.
• It is possible to change the energy from potential to back, but the total cal energy must remain the same.
• There are two calculation that we did.
• The 30 J of potential energy is converted to energy.
• 24 m/s is the speed.

• The mass would hang at permanent rest.

• The same amount of work has to be done on both you and the mountain.
• The bowstring's conservative force is due to the mechanical energy of the bowstring-arrow system.

• For people who didn't grow up in America, 1 m/s is less than 4 km/hr.

• This is not as fast as a tennis player's serve, for those who are uncomfortable with the World's Greatest Sport.

• Work talks about walking up and driving up for several hours.
• The difference is power.

• In units of joules/second, also known as watt, power is measured.
• A car engine can give out hundreds of horsepower, equivalent to 100 kilowatts, while you can only put out a few hundred watt of power.

• Chapter 18 has a stone resting on top of a mass on a spring.

• 40 J is the potential energy stored in the spring-earth-stone system.

• A stone thrown straight up into the smooth incline has 6 J rela air a distance greater than the distance to the base of the incline.

• When the cart is 15 cm in front of the moving detector at the line marked on the track, it will be its reading.
• The faster cart is released from rest at the position shown.

• Justify your answer.

• The result won't be affected by the energy at the bottom.
• The same speed is used for both carts.

• It will be 3 J.

• The 40 J of potential is the amount of energy that can be lost by the stone.

• 3 J of energy is what the now has.

• The square O is under the 6 J.

• You should get 3.5 m/s if we compressed the spring 1.4 times.

• "Unexponented" is a word that could be used.

• That's an expression in equations.

• The time it takes the line.
• Student A does the front of the cart, not the middle or back.

• Student B uses a meterstick, but it's not 20 J of work.
• Power is divided by the time it takes to do the work.
• First, measure the amount of work, whoever takes less vertical distance from the desk to the line time to do the work develops more power.
• The track is 15 cm in front of the detector.

• The answer is choice A.

• If you use a bubble level and do the same work, you can get a more accurate vertical height of 40 cm.

• When it does work, a "conservative" force converts potential energy to other forms of mechanical energy.
• A conservative force doesn't change the mechanical energy of a system.

• NC is done by a non conservative force.

• When the force on an object is not constant, energy-saving methods must be used.
• Curved tracks, springs, and pendulums are the most common of these situations.

• The force Student A applies on the box is straight up, at least while the box is moving.
• When the box starts moving horizontally, no force is needed to keep it moving.