There are a few points I don t know much about physics junior high school , and I am patiently taug

1.Strictly speaking, there is no direct relationship between speed and force.

Force can change the state of motion of an object, and the state of motion of an object is generally related to velocity. That's the direct relationship.

f=ma, on a certain object, the force determines the magnitude of the acceleration, and the acceleration is a sign of how fast or slow the velocity increases. The force determines how quickly and slowly the velocity increases. That's it.

2.The inertia you are talking about is a property of the object itself. That is to say, if an object is placed there, it has inertia.

It is not to say that an object only talks about its inertia when it is in motion. Therefore, the velocity is different when it is different, and the properties of the object depend only on the object itself, and have nothing to do with the velocity given to it by the outside world.

3.The answer is c

Because the train is moving at a uniform speed, and the ball is stationary with respect to the train, then it means that the ball is also moving at a uniform speed, and the speed is the same as that of the train. The train stops instantly, the ball doesn't stop instantly. Therefore, according to the inertia, when the train stops instantly, the speed of the two balls has not had time to change, and it is still the speed of the original train, so C is chosen.

4.Cycling is achieved because people do their own work. When the wind is downwind, the air flow gives a force to the person and the car, so that the acceleration is generated, and the speed naturally increases.

On the contrary, when it is reversed, the air flow gives a reverse force to the person and the car, which is the resistance, which produces a reverse acceleration, so it is necessary to slow down.

I don't know if I made it clear. In fact, the physics books have made it relatively clear. Especially when it comes to conceptual issues, read the book well, sometimes you will find that you have missed a lot of knowledge in the book.

It is very helpful to have time to take a good look at the information books that explain a lot. That's how it came back when physics was not good. ^-

1 Force can change the motion state of an object, and the change of motion state of an object includes a change in direction and a change in velocity or both, so force can change the motion speed of an object.

2 For the same object, such as the car you said, it is difficult to brake when the speed is high, not because the greater the speed, the greater the inertia, the speed has nothing to do with inertia. It is because the traction force of the car is greater when the speed is large, so the forward force to the car is greater, so it will be relatively difficult to brake.

3c Because the ball is stationary with respect to the train, when the train suddenly stops moving, the ball still maintains its meta-velocity moving forward due to inertia.

4. Because the downwind overcomes friction, while the upwind overcomes the resistance and friction of the wind.

5) Power supply voltage = 3V, rheostat selection R1 (10, 1A), fixed resistance r = 25.

According to the series circuit u1 u = r1 r and u = 3-u1, u1 u = 10 25 can be stacked

u1/（3-u1）=10/25

25u1=10（3-u1）

25u1=30-10u1

25u1+10u1=30

35u1=30

u1=6/7v

It can be seen that when the fixed value resistance is 25, and the rheostat is selected as Yuchai R1, when the sliding plate P of the rheostat moves to the leftmost end (that is, the resistance value of the access circuit is the maximum value of 10), the voltage at both ends of the group carefully set the value of the resistor R is still 3-6 7 = 2 and 1 7 volts, that is to say, no matter how to adjust the rheostat R1, the voltage at both ends of the fixed value resistor can not reach the experimental value less than or equal to 2 volts collapse and dismantle the base.

So (5) choose R1.

If the shot is not clear, re-shoot it and there will be no shadows.

The role of the first step is to use the edge of the table as a fulcrum, the left of the fulcrum is the gravity acting on the center of gravity, and the right is the hook yard gravity acting on the right end.

The second step is to measure the strength arms of both.

The third step is moment balancing.

Your question: Because g2 is the gravity of the wooden ruler and G1 is the gravity of the hook yard, take a closer look at the final gravity of the wooden ruler

S on, R1 and R2 in series.

Let the voltage at both ends of the circuit be U

u1=[r1 (r1+r2)]u=2v(1), where r2=10 (2).

S is closed, R2 is short-circuited, and the voltage at both ends of R1 is the voltage at both ends of the circuit, i.e., U=U2=6V(3).

Substituting (2) into (1) gives r1=5 (4).

r1 power consumption p=u 2 r1=

Current 10 minutes workmanship w=pt=

When S is closed, R2 is short-circuited, and the voltmeter measures the supply voltage, i.e. U=6V.

S is open, R1 and R2 are connected in series. The voltage U1 at both ends of R1 = 2V, and the voltage U at both ends of R2 is equal to the supply voltage minus the voltage at both ends of R1. i.e. u2=u - u1=6v -2v=4v

The current in R1 is equal to the current in R2, i.e. i1=I2=U2 R2=4V 10 = So, R1=U1 I1=2V

When S is closed, R2 is short-circuited, the voltage at both ends of R1 is equal to the power supply voltage (6V), and the current through R1 is I=U R1=6V 5 = The work done by the current through R110min is: W=UIT=6V*