"Kinetic Energy and Kinetic Energy Theorem" Law of Conservation of Mechanical Energy PPT

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"Kinetic Energy and Kinetic Energy Theorem" Law of Conservation of Mechanical Energy PPT

Part 1: Essential knowledge and foundation of literacy

1. Kinetic energy

[Thinking] The picture shown is a chariot used to attack the city gate in ancient wars. The chariot is equipped with a log with a large mass. There are many soldiers pushing the chariot so that the log hits the city gate at a very high speed, which is easy. Breaking down the city gate. When the mass of the log is very large and the speed is very high, what energy is used to increase the energy of the log?

Hint: kinetic energy.

1. The energy an object possesses due to motion is called _____.

2. The expression of kinetic energy: Ek=______.

3. Kinetic energy is a scalar quantity, _____ quantity (optional "state" or "process").

4. Unit: Joule (J).

2. Kinetic Energy Theorem

[Thinking] As shown in the figure, F is the total external force on the block, and the total work W done by the force F on the displacement is l.

(1) How does the kinetic energy of the block change?

(2) Is the sum of the work W done by the total external force and the change in kinetic energy ΔEk equal?

Tips: (1) Increase.

(2)Equal.

1. Derivation process:

2. Content: What a force does to an object in a process is equal to what the object does here

In a process___________.

3. Expression: W=

Kinetic energy and kinetic energy theorem PPT, part 2 content: Key capabilities and literacy formation

1. Nature of kinetic energy

1. The "three properties" of kinetic energy:

(1) Relativity: Choose different reference systems, the speed of objects is different, and the kinetic energy is also different. Generally, the ground is used as the reference system.

(2) Scalar property: kinetic energy is a scalar quantity and has no direction.

(3) State quantity: Kinetic energy is a physical quantity that characterizes the motion state of an object, corresponding to the object's motion state (or speed at a certain moment).

2. Three relationships between kinetic energy and speed:

(1) Numerical relationship: Ek= mv2, for the same object, the greater the speed v, the greater the kinetic energy Ek.

(2) Instantaneous relationship: Both kinetic energy and speed are state quantities, and they have an instantaneous correspondence.

(3) Change relationship: kinetic energy is a scalar quantity and speed is a vector quantity. When the kinetic energy changes, the speed (size) of the object must have changed. When the speed changes, it may only be a change in the direction of the speed, and the kinetic energy of the object may not change.

【Think·Discussion】

As shown in the figure, the mass of ball A is greater than the mass of ball B. The ball rolls down the slope. After the carton stationary on the ground is touched, it slides a certain distance and stops. What factors do you think the kinetic energy of an object may be related to? (Physical concepts)

Tip: The kinetic energy of an object is related to its mass and speed.

[Typical example demonstration]

Which of the following statements about kinetic energy is correct ()

A. The kinetic energy of the car must be smaller than the kinetic energy of the train

B. The kinetic energy of a car may be greater than the kinetic energy of a train

C. Cars and trains with the same speed must have the same kinetic energy

D. Two cars with the same mass must have the same kinetic energy

[Analysis] Choose B. Since the speed relationship between the car and the train is not clear, the relationship between the kinetic energy cannot be determined, so A is wrong; although the mass of the car is small, if the speed of the car is greater than the speed of the train, the kinetic energy of the car may be greater than the kinetic energy of the train. , so B is correct; cars and trains with the same speed do not have the same kinetic energy because they have different masses, so C is wrong; two cars with the same mass do not necessarily have the same speed, so they do not necessarily have the same kinetic energy, so D mistake.

【Literacy Training】

1. For an object of a certain mass, which of the following statements is correct ()

A. The kinetic energy does not change and the speed must not change.

B. When the speed changes, the kinetic energy will change.

C. The speed remains unchanged, but the kinetic energy may change.

D. When kinetic energy changes, speed must change

[Analysis] Choose D. If the kinetic energy does not change, the speed must not change, but the direction can change, such as uniform circular motion, so A is wrong; the speed direction changes, the magnitude does not change, and the kinetic energy does not change, such as uniform circular motion, so B is wrong; the speed does not change, kinetic energy must remain unchanged, so C is wrong; if the kinetic energy changes, the speed must change, so D is correct.

2. An iron ball and a wooden ball of equal volume move uniformly in a straight line at the same speed on a horizontal ground. The relationship between their kinetic energies is ()

A. The kinetic energy of the iron ball is large. B. The kinetic energy of the wooden ball is large.

C. The kinetic energy of the two balls is the same D. Unable to determine

【Compensation training】

Among the following statements about kinetic energy, which one is correct ()

A. If the kinetic energy of an object remains unchanged, its speed must also remain unchanged.

B. If the speed of an object remains unchanged, its kinetic energy will also remain unchanged.

C. When an object moves at variable speed under the action of the combined external force, its kinetic energy will change.

D. The kinetic energy of the object remains unchanged, and the net external force it experiences must be zero.

[Analysis] Choose B. In uniform circular motion, the kinetic energy does not change and the speed changes all the time. A is wrong; the speed does not change, which means that the size and direction do not change, so the kinetic energy does not change. B is correct; the object moves at a variable speed under the action of the combined external force, such as uniform speed In circular motion, the kinetic energy does not change, so C is wrong; the kinetic energy of the object does not change, which means that the net external force on the object does no work or the algebraic sum of the work done is zero, so the net external force is not necessarily zero, D is wrong.

2. The meaning of kinetic energy theorem

1. The meaning of W: the algebraic sum of the work done by all external forces including the gravity of the object.

2. The relationship between W and ΔEk: The work done by the resultant force is the cause of the change in the kinetic energy of the object.

(1) The resultant force does positive work on the object, that is, W > 0, ΔEk > 0, indicating that the kinetic energy of the object increases.

(2) The resultant force does negative work on the object, that is, W < 0, ΔEk < 0, indicating that the kinetic energy of the object is reduced.

(3) If the resultant force does no work on the object, the kinetic energy remains unchanged.

3. The essence of the kinetic energy theorem: a concrete manifestation of the functional relationship. The change in the kinetic energy of an object can be measured by the work done by the combined external force.

[Typical example demonstration]

A car with mass m, driven by a horizontal constant force F, moves from rest at the bottom of the hillside A to the top of the hillside B with a height h, and obtains a speed v, and the horizontal distance AB is s. Which of the following statements is correct ()

A. The work done by gravity on the car is mgh

B. The work done by the combined force on the car is mv2+mgh

C. The work done by the thrust on the car is Fs

D. The work done by the resistance force on the car is Fs- mv2-mgh

【Literacy Training】

1. Regarding the following relationship between the resultant work of a moving object and changes in kinetic energy and speed, which one is correct ()

A. When an object moves at a variable speed, the total external force must not be zero and the kinetic energy must change.

B. If the work done by the net external force on the object is zero, then the net external force must be zero.

C. When the net external force of an object does work, its speed must change.

D. The kinetic energy of the object remains unchanged, and the net external force it experiences must be zero.

[Analysis] Choose C. When an object moves at a variable speed, the speed must change, and the net external force must not be zero, but the speed does not necessarily change, so the kinetic energy does not necessarily change, so A is wrong; if the work done by the net external force on the object is zero, the net external force may not be zero, maybe This is because the direction of the net external force is always perpendicular to the direction of the speed. For example, in uniform circular motion, the net external force is not zero, so B is wrong. The net external force of an object does work. According to the kinetic energy theorem, it is known that the kinetic energy of the object must change, and its speed must change. , so C is correct; the kinetic energy of the object remains unchanged, and the work done by the net external force must be zero, but the net external force is not necessarily zero, so D is wrong.

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Update Time: 2024-10-03

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