The following is correct about the relationship between the work done by the external force and the

A is correct, b, c, d can be illustrated by uniform circular motion is wrong.

A is correct, b, c, d are wrong.

Velocity is a vector quantity, which has both magnitude and direction, if the external force only changes the direction of velocity and does not change its magnitude, the velocity changes, but the kinetic energy does not change, (kinetic energy is a scalar quantity, only magnitude, no direction.) ）

At this time, although it is a variable speed movement, the kinetic energy remains unchanged, so C is wrong;

At this time, the kinetic energy does not change, if the other forms of energy (potential energy, etc.) also do the same, then the work done by the external force on the object is zero, but the combined external force is not zero, because there needs to be an external force to change the direction of the velocity of the object, so b and d are wrong.

A is true and B is false because w=f*x

c and d are incorrect, such as uniform circular motion.

A is correct, as can be seen from the formula w=fs, when f=0 w is zero.

B error, such as uniform circular motion, C error, such as uniform circular motion is variable speed motion, but the kinetic energy is unchanged, D error, such as uniform circular motion kinetic energy is unchanged, but the force is not zero.

Answer A: According to the definition of work, item A is true and item B is false; The vertical upward throwing motion is a kind of linear motion with uniform variable speed, and its kinetic energy is equal when it passes through the same position in the ascending and descending stages, so the c term is wrong; The kinetic energy does not change, which can only mean that the velocity is unchanged, but the direction of the velocity is not necessarily the same, so the resultant external force is not necessarily zero, so the D term is wrong

a. If the resultant external force du of the object is zero, and zhi is obtained according to w=fs, then the total work done by the external force on the object is set to zero; option a is correct; Expert.

b. If the work done by the resultant external force is zero, but the resultant external force is not necessarily zero It is possible that the resultant external force of the object is perpendicular to the direction of motion and does not do work, option b is wrong;

c. The object moving at variable speed may change the direction of velocity and the magnitude of the velocity does not change Therefore, the kinetic energy of the object moving at variable speed may not change, and option c is wrong;

d. The kinetic energy of the object remains unchanged, and it can only be said that the combined external force does not do work, but the combined external force is not necessarily zero

A right, understood by the kinetic energy theorem and the definition of work.

b is false, when the resultant external force is not 0 but the direction is perpendicular to the displacement, the work done is 0c wrong, it should be the algebra of the work done by the external force on the object, which is equal to the final kinetic energy of the object - the initial kinetic energy d is wrong, the kinetic energy of the object remains unchanged, the work of the combined external force = 0, but it is possible that the variable force does positive work for a period of time, and negative work for another period of time, but the total work = 0

b is false, it may be that the distance is 0, that is, when the object will not move under the action of an external force.

C is wrong, the speed change may mean that the direction changes, the speed does not change, and the kinetic energy remains unchanged at this time. For example, a uniform circular motion.

d is wrong, the speed does not change, the direction may change, and the external force is not 0

A is correct, and everything else can be vetoed with a uniform circular motion.

A right, understood by the kinetic energy theorem and the definition of work.

b is false, when the combined external force is not 0 but the square and vertical circle direction is perpendicular to the displacement, the work done is 0c wrong, which should be the algebra sum of the work done by the external force on the object, which is equal to the kinetic energy of the object at the end of the roughening - the initial kinetic energy.

d is false, the kinetic energy of the object remains unchanged, and the work of the combined external force = 0, but it is possible that the residual bridge collapse of the variable force does positive work for a period of time, and does negative work for another period of time, but the total work = 0

a, for example, the work of an object moving in a uniform circular motion is zero, but the work of the resultant force (centripetal force) is not zero.

b False, as in the previous example, the kinetic energy of a uniform circular moving object is constant, but the resultant external force is not zero.

c False, ditto, uniform circular motion is variable velocity motion, but the kinetic energy of the object does not change.

d is correct, from the work done formula w=fs, it can be seen that when the resultant external force f is zero, the work done by the resultant external force on the object must be zero, so the kinetic energy of the object does not change.

A false. According to w=f*s, however, if the external force is not 0, the work is also 0 when the displacement is 0.

B false. Counterexample of uniform circular motion.

C false. Counterexample of uniform circular motion. The kinetic energy does not change.

d right. Kinetic energy e=1 2*m*v 2, because it is the same object, so m does not change, and the resultant external force is 0, then v does not change, so the kinetic energy does not change.

Answer A: According to the definition of work, item A is true and item B is false; The throwing motion of the vertical upper bench is a kind of linear motion with uniform variable speed, and its kinetic energy is equal when it passes through the same position in the ascending and descending stages, so the c term is wrong; If the kinetic energy does not change, it can only mean that the velocity is unchanged, but the direction of the velocity is not necessarily the same, so the spring force of the outer ruler is not necessarily zero, so the D term is wrong

Answer A The combined external force is zero, according to the formula, the work done by the external force on the object must be zero If the work done by the external force on the object is zero, if it is perpendicular to the direction of motion, then the magnitude of the external force can not be zero Velocity is a vector, and if the velocity changes, the magnitude of the velocity does not necessarily change, so the kinetic energy of the object can remain unchanged If the kinetic energy of the object does not change, the magnitude of the velocity does not change, but the direction of velocity may change, so the resultant external force on the object may not be zero

a. The kinetic energy of the object does not change, and the external force is combined.

DU not necessarily.

zhi is zero, such as uniform circular motion, object dao

The kinetic energy of the body remains the same, but it is a foreign genus.

The force is not zero, so a is wrong;

b. If the work done by the resultant external force on the object is zero, it can be seen from the formula w=flcos that when =90°, even if the resultant external force is not zero, the work of the resultant external force is also zero, if the object moves in a uniform circular motion, the work of the resultant external force is zero, but the resultant external force is not zero, so b is wrong;

c. The object moves at variable speed under the action of external force, and the kinetic energy does not necessarily change, such as uniform circular motion, so C is wrong;

d. If the resultant external force on the object is zero, it can be seen from the formula for calculating the work that the work done by the external force on the object must be zero, so d is correct;

Therefore, d

Answer ab - the external force first does positive work, and then does negative work, the total work is zero, but the resultant force is not zero.

c - uniform circular motion, kinetic energy unchanged.

D - the same as b, the total work is zero, and the resultant external force is not necessarily zero.

A right, understood by the kinetic energy theorem and the definition of work.

b is false, when the resultant external force is not 0 but the direction is perpendicular to the displacement, the work done is 0c wrong, it should be the algebra of the work done by the external force on the object, which is equal to the final kinetic energy of the object - the initial kinetic energy d is wrong, the kinetic energy of the object remains unchanged, the work of the combined external force = 0, but it is possible that the variable force does positive work for a period of time, and negative work for another period of time, but the total work = 0