"Special Reinforcement Calculation of Work Two Ways to Start a Locomotive" Excellent Courseware PPT on the Law of Conservation of Mechanical Energy

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"Special Reinforcement Calculation of Work Two Ways to Start a Locomotive" Excellent Courseware PPT on the Law of Conservation of Mechanical Energy

Part One: Learning Objectives

1. Master the applicable conditions of the work formula and the general method of finding work with a variable force.

2. Further master the calculation methods of instantaneous power and average power.

3. Be able to analyze the changes in acceleration and speed during the constant power start and uniform acceleration start of the locomotive.

Two ways to start a successful computer car PPT, the second part of the content: 01 Exploring key points to improve literacy

1. Calculation of work

1. Calculation of work done by constant force

The formula of work W=Flcos α is only applicable to work done by constant force, that is, F is the constant force, and l is the displacement of the object relative to the ground. The flow chart is as follows:

2. Calculation of work done by variable force

(1) Convert work done by variable force into work done by constant force.

When the magnitude of the force remains constant and its direction is always the same or opposite to the direction of motion, the work of such a force is equal to the product of the force and the distance.

(2) When the power P of variable force to do work is constant, such as a locomotive starting with constant power, W = Pt can be used to calculate the work.

(3) Use the average force to find the work: If the force F changes linearly with the displacement x, you can use the average force within a period of displacement to find the work. For example, when a spring with a stiffness coefficient k is stretched by ＝kx2·x＝12kx2.

(4) Use the F-x image to find work

Example 1: There is a curved channel AB on the horizontal plane, consisting of two semicircles with radii R and R respectively. As shown in Figure 2, a smooth ball is pulled from point A to point B with a pulling force of constant magnitude F. , if the direction of the pulling force F is consistent with the direction of movement of the ball at all times, then the work done by the pulling force in this process is

A. Zero

B.FR

C.πFR

D.2πFR

Example 2: An object placed on a rough horizontal ground is subjected to horizontal tension. The image of its speed and time within 0 to 6 s and the image of the power of horizontal tension and time are shown in Figure 3A and B. Which of the following statements is correct? yes

A. The displacement of the object within 0 to 6 s is 20 m

B. The work done by the pulling force within 0 to 6 s is 100 J

C. The magnitude of sliding friction is 5 N

D. The work done by the sliding friction force within 0 to 6 s is -50 J

2. Two ways to start a locomotive

Example 3: The rated power of a car moving on a level road is 100 kW and the mass is 10 t. Assume that the resistance is constant and is 0.1 times the weight of the car (g is 10 m/s2), then:

(1) If the car starts from rest with constant rated power, how does the car's acceleration change?

(2) When the acceleration of the car is 2 m/s2, what is the speed?

(3) What is the maximum speed that the car can reach during motion?

Example 4 (Mid-term of the first semester of the high school affiliated to Shanxi University in 2019) The rated power of the car engine is 60 kW and the mass of the car is 4 tons. When it is driving on a long straight road with a slope α (sin α=0.02), such as As shown in Figure 4, the friction force is 0.1 times the weight of the vehicle (g is taken as 10 m/s2). Find: (Results retain three significant figures)

(1) The maximum speed vm that the car can reach;

(2) If the car starts from rest and makes a uniformly accelerated linear motion with an acceleration of 0.6 m/s2, how long can this process last?

(3) When the car starts from rest and accelerates uniformly with an acceleration of 0.6 m/s2 and reaches the maximum speed, how much work does the car do?

Method summary

How to find several physical quantities in locomotive starting problem

To analyze the locomotive starting problem, two core equations must be grasped: Newton's second law equation F-Ff=ma connects force and acceleration, and P=Fv connects force and speed.

(1) How to find the maximum speed vm of the locomotive. The speed of the locomotive is maximum when it finally moves forward at a constant speed. At this time, the traction force F is equal to the resistance Ff, so vm=PF=PFf.

(2) How to find the duration of uniform acceleration start. Traction force F = ma + Ff, final speed of uniform acceleration vm′ = P ma + Ff, time t = vm′a.

(3) How to find the instantaneous acceleration. Find the traction force according to F=Pv, then the acceleration a=F-Ffm.

Example 5 (End of the first semester of high school in Hebi City, 2018) A car is driving at a constant speed v0 on a straight road, and the engine power is P0. When it is about to enter the downtown area, the driver reduces the accelerator, causing the car's power to be immediately reduced by half and maintained The power continues to drive. Which of the following four images correctly represents the car's speed versus time from the time the driver reduces the accelerator?

PPT on the two starting methods of a successful computer car, the third part: 02 in-class drill, implementation point by point

1. (Calculation of work) An object moves on a rough horizontal surface. First slide the object to the right for a distance l, and then slide the object to the left a distance l until it returns to the starting point. The friction force experienced back and forth is Ff, then The work done by friction during the whole process is

A.0 B.－2Ffl C.－Ffl D.Unable to determine

2. (Start with constant power) (End of the first semester of 2018·Yichang No. 1 Middle School) A car with mass m starts from rest on a straight road with constant power P, and finally makes uniform linear motion at a certain speed. This process , the resistance force experienced by the car is always Ff, and the acceleration due to gravity is g, then

A. The maximum speed of the car is PFf B. The maximum speed of the car is Pmg

C. The traction force of the car remains unchanged. D. The car is moving in a straight line at a constant speed.

3. (Image problem during locomotive startup) (Multiple choice) (End of 2018·Yuncheng Hedong First Intermediate School) A car is driving on a straight road. The traction of the car remains unchanged during the startup phase, and then continues to drive at rated power. After At time t0, its speed increases from zero to the maximum value vm. If the resistance Ff experienced by the car is a constant force, regarding the change of the speed v, traction force F and power P of the car with time t, which of the following options is correct?

4. (Image problem during motorcycle startup) A car starts from a standstill on a level road, performs uniformly accelerated linear motion within the first 5 s, reaches the rated power at the end of 5 s, and then maintains the rated power motion. Its v-t image As shown in Figure 5. It is known that the mass of the car is m=2×103 kg, the resistance experienced by the car is the weight of the car, and g is 10 m/s2, then

A. The resistance the car experiences in the first 5 s is 200 N

B. The traction force in the first 5 s is 6×103 N

C. The rated power of the car is 40 kW

D. The maximum speed of the car is 20 m/s

Keywords: Free download of PPT courseware for high school physics compulsory course 2 of the People's Education Press, two ways to start a work computer car PPT download, law of conservation of mechanical energy PPT download, .PPT format;

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Update Time: 2024-10-08

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