Physics 8 Lesson 27: Conservation of energy in mechanical and thermal phenomena
1. Theoretical summary
1.1. Transfer of mechanical and thermal energy from one object to another:
Describe the transfer of mechanical and thermal energy in the following phenomena:
– The steel ball rolling from the inclined chute collides with the wood, causing the wood to move.
Drop a piece of heated aluminum into a cup of cold water.
– The bullet from the barrel flew out, fell down, cooled down and sank.
1.2. Conversion between forms of mechanical energy, between mechanical and thermal energy:
⇒ Mechanical energy can be converted from one form to another.
1.3. Conservation of energy in mechanical and thermal phenomena:
– Law of conservation and transformation of energy
Question 1: Why in the phenomenon of pendulum oscillation, the pendulum only oscillates for a short time and then stops at equilibrium? What form of energy is the mechanical energy of the pendulum converted into?
In the phenomenon of pendulum oscillation, the pendulum only oscillates for a short time and then stops at equilibrium because part of their mechanical energy has been converted into heat energy that heats the pendulum and the surrounding air. around.
Verse 2: Why in the phenomenon of the ball hitting the wooden bar after the collision only moves a short distance and then stops. Where has our potential gone?
After the impact, the ball only moves a short distance and then stops because part of their mechanical energy converts it into heat that heats the ball, the stick, the slide and the surrounding air.
⇒ Their energy decreases gradually.
3.1. Essay exercises
Question 1: What is mechanical and thermal energy?
Verse 2: Observe the case where the ball hits the ground, it bounces. During the bounce, how does its potential and kinetic energy change?
Question 3: Drop an object from height h to the ground. Please tell me how the mechanical energy was changed during the fall?
Question 4: Pull the pendulum from the equilibrium position so that the ball of the pendulum is in position A and then let go of the hand to allow the pendulum to oscillate. Neglect air friction. Which of the following statements is not correct?
3.2. Multiple choice exercises
Question 1: Which of the following statements is most complete with regard to energy conversion?
A. Kinetic energy can be converted into mechanical energy.
B. Potential energy can be converted into kinetic energy
C. Kinetic and potential energy can be converted to each other, but mechanical energy is conserved.
D. Kinetic energy can only be converted into potential energy.
Verse 2: Which of the following is a case where potential energy is converted to kinetic energy?
A. Arrows are fired from bows.
B. The water on the high dam flows down.
C. The ball rolls from the top of the slope to the bottom.
D. All three cases above
Question 3: An object falls from position A to the ground. Neglect air resistance. When the object falls to position B, the kinetic energy of the object is 1/2 of its potential energy. The kinetic energy of an object continues to increase by an amount of 100 J, which is equal to the potential energy. The potential energy of the object at position A is:
A. 50 J B. 100 J C. 200 J D. 600 J
Question 4: Pulling a wire wrapped around an aluminum tube filled with water, sealing the button, people see that the water in the tube heats up and boils, the steam pushes the button out along with a layer of white smoke created by very small water particles. In the above experiment, when did the conversion of mechanical energy into heat occur?
A. Pull back and forth on the rope
B. Hot water
C. Steam makes the button pop
D. Water vapor condenses into small water droplets.
Through the lecture The conservation of energy in these mechanical and thermal phenomena, students need to complete some of the objectives given by the lesson such as:
Understand the characteristics of the transfer of mechanical energy from one object to another and the conversion between forms of mechanical energy, between mechanical and thermal energy.
State the laws of conservation and transformation of energy.
Know how to analyze physical phenomena.