A train starts from the station at rated power and moves along the straight track. After traveling for 5min, the speed reaches 30m / s. if the resistance of the train is constant, the distance traveled by the train during this period can be judged () A. It must be greater than 4.5km B. May be equal to 4.5km C. It must be less than 4.5km D. Insufficient conditions to determine

A train starts from the station at rated power and moves along the straight track. After traveling for 5min, the speed reaches 30m / s. if the resistance of the train is constant, the distance traveled by the train during this period can be judged () A. It must be greater than 4.5km B. May be equal to 4.5km C. It must be less than 4.5km D. Insufficient conditions to determine

According to the movement process of the train, draw the speed time image of the train, as shown in the figure,
According to the displacement represented by the area surrounded by the velocity time image curve and the coordinate axis, if the initial velocity of the object is zero,
For uniformly accelerated linear motion with a speed of 30m / s, the displacement is equal to the area of the triangle, i.e. x = 1
two × thirty × 300=4500m；
At this time, the actual curve forming area is greater than 4500m
So a is correct
Therefore: a

In a stationary train, a stationary ball is placed on a horizontal table. If the ball suddenly rolls forward, it means the train () or ()

In a stationary train, a stationary ball is placed on a horizontal table. If the ball suddenly rolls forward, it indicates that the speed of the train slows down. Because the speed of the ball is the same as that of the train at the beginning, and because the train slows down, the speed of the ball is still the previous speed, so the ball should roll forward at the same speed as that of the train later

A small ball is tied to the spring on the smooth horizontal plane in the carriage moving along the straight track, and the spring is in a natural state, as shown in the figure. When passengers see that the length of the spring becomes longer, it may be correct to judge the motion state of the train () A. The train is moving to the right and its speed is increasing B. The train is moving to the right and its speed is decreasing C. The train is moving to the left and its speed is increasing D. The train is moving to the left and its speed is decreasing

According to Newton's second law, it is known that the acceleration of the ball is to the left, and the train may accelerate to the left or decelerate to the right. Therefore, ad is wrong and BC is correct
Therefore, BC is selected

In a train car, there is a smooth horizontal table with a small ball on the table. The passengers nearby observed that if the train moves in a straight line at a uniform speed on the horizontal rail, the small ball will remain stationary on the table. If the train moves in an accelerated straight line, the small ball will start to move backward from stationary. This shows that () A. Newton's first law is established with the car moving at a uniform speed as the reference system B. Newton's first law is established by taking the accelerating carriage as the reference system C. Newton's first law holds whether the car moving at a constant speed is taken as the reference system or the car moving at an accelerated speed is taken as the reference system D. Newton's first law does not hold whether the car moving at a constant speed is taken as the reference system or the car moving at an accelerated speed is taken as the reference system

Newton's law of motion is applicable to inertial reference frame. The train with uniform acceleration is a non inertial reference frame, and Newton's first law does not hold
So choose a

There is a horizontal and smooth table top in the carriage of the train. A small ball is placed on the table top. When the train starts from standstill on the straight track, what is the reason

The ball doesn't follow the table
It will eventually fall to the ground
But you haven't written all your questions. Please ask if your answer is wrong

There is a very mysterious physical phenomenon. Put a glass of water on a horizontal and smooth desktop, shake it left and right, and ask about the movement state of the glass of water in the future Is it moving back and forth? If so, what force does it do

Are you talking about the shaking direction of water? It's hard to say. It depends on whether you shake at a constant speed or at a variable speed
As for the water and the water cup, they move in the direction of release at a uniform speed and are in a state of balance (only affected by gravity and support force), and the wood can do work effectively