7. The object with mass m is pulled by a string through a smooth small hole, and makes a uniform circular motion on a smooth horizontal plane. When the tension is a certain value f, it rotates When the tensile force is a certain value f, the rotation radius is R. when the tensile force gradually increases to 6F, the object still moves in a uniform circular motion. At this time, the radius is R / 2, then the work done by the tensile force on the object is: () A、0 B、FR C、3FR D、5FR/2

7. The object with mass m is pulled by a string through a smooth small hole, and makes a uniform circular motion on a smooth horizontal plane. When the tension is a certain value f, it rotates When the tensile force is a certain value f, the rotation radius is R. when the tensile force gradually increases to 6F, the object still moves in a uniform circular motion. At this time, the radius is R / 2, then the work done by the tensile force on the object is: () A、0 B、FR C、3FR D、5FR/2

B、FR
Suppose that the initial kinetic energy of the object is E1, the velocity is V1, the final kinetic energy is E2, and the velocity is V2, and the pulling force provides centripetal force
From F = MV1 ^ 2 / R: V1 ^ 2 = FR / M
From 6F = MV2 ^ 2 / (R / 2): V2 ^ 2 = 3fr / M
So:
E1=1/2MV1^2=FR/2
E2=1/2MV2^2=3FR/2
According to the law of conservation of mechanical energy
E1 + W = E2
So:
W = fr

As shown in the figure, there is a smooth small hole o in the center of a 2L long light rod, and two small balls with a fixed mass of M and 2m at both ends. The smooth iron nail passes through the small hole and is vertically nailed on the vertical wall. The light rod is statically released from the horizontal position and turned to the vertical position. In the process of rotation, the resistance of air is ignored. The following statement is correct () A. In the vertical position, the velocity of both balls is 2gL B. When the rod is in the vertical position, the force on the m ball is upward, and the size is 2 3mg C. When the rod is in the vertical position, the force of the iron nail on the rod is upward, with the size of 11 3mg D. Due to ignoring all friction resistance, according to the conservation of mechanical energy, the rod must be able to make a complete circular motion around the nail

A. When the light rod is released from the horizontal position from the static position to the vertical position, the speed of the two balls is equal, set as V, according to the conservation of mechanical energy of the system
   2mgL-mgL=1
2 • 3mv2, solution, V=
two
3GL. So a is wrong
B. When the rod is turned to the vertical position, for the m ball: mg-N = MV2
50. The solution shows that n = 1
3mg, so B error;
C. When the rod is turned to the vertical position, for the 2m ball: n '- 2mg = 2mv2
50. The solution shows that n ′ = 10
3mg, so the force of the iron nail on the rod is upward, and the magnitude is n + n ′ = 11
3mg, so C is correct;
D. According to the analysis of the law of conservation of mechanical energy, when the system turns clockwise to the horizontal position, the speed is zero, and then turns counterclockwise, it is impossible to make a complete circular motion. Therefore, D error
So choose C

High school physics problems about friction Two objects a and B are stacked on the ground (a is up, B is down, and a is less than B). When force F acts on two objects a and B respectively, a and B are relatively stationary. Try to analyze the friction force on a and B No picture, please forgive me

When f is applied to object a, a is subjected to the friction force given to it by B in the opposite direction to F. B is subjected to the friction force given to it by a, and F
The friction in the same direction is affected by the friction in the opposite direction of F given by the ground
When f is applied to object B, a is not subject to friction. B is subject to friction from the ground in the opposite direction to F
It can be seen from the overall stress analysis that the ground gives friction to B, and then isolate a and B

If you pull an object and it doesn't move, is friction the pull of the object? When an object moves, is the friction the maximum static friction regardless of the amount of tension? Why is the maximum static friction slightly greater than sliding friction? Why is the magnitude of static friction independent of pressure, but proportional to the maximum static friction? When pulling an object, is the sliding friction related to the pulling force? Why is it proportional to pressure? Not pull?

The object has no motion, and the static friction and the tension applied to the object are a pair of equilibrium forces with equal size and opposite direction, but the friction can not be said to be "tension";
When an object moves, no matter how much the tension is, the friction force on the object is always equal to the friction coefficient multiplied by the pressure on its supporting surface. Generally, this friction force is less than its maximum static friction force;
The maximum static friction is equal to the size of the applied tension when the object will move but not move under the tension. Note that at this time, the speed of the object is zero but the acceleration is not zero, that is, the applied tension is greater than the real friction force on the object. (the friction force received by the object cannot be measured directly, but its size and direction can only be analyzed according to the force analysis);
The static friction force and the pulling force on the object are a pair of equilibrium forces, which are only related to the pulling force. As for the saying that "it is directly proportional to the maximum static friction force", I have heard for the first time;
The interactive friction force has nothing to do with the tensile force, but only with the roughness of the contact surface and the positive pressure of the object on the contact surface;
There is less "dynamic friction" between "say" and "and", which is determined by its definition. That's what his definition says, and that's the actual situation
Students, you should read more books. These basic definitions must be understood and clarified!

How does the direction of circular motion acceleration change

An object in circular motion must produce an acceleration under the action of centripetal force F. according to Newton's law of motion, the direction of this acceleration is the same as that of centripetal force, which is called centripetal acceleration
That is, point to the direction of centripetal force at any time

The three side length ratio of a triangle is 3:4:5. It is known that the circumference of the triangle is 48 cm. What is the longest side length?

20CM