8 Throw a ball vertically, and the ball falls back to its original place, and the magnitude of the a

Throw a ball vertically, the ball falls back to its original place, it is known that the magnitude of the air resistance is constant, and the distance between the ascent process and the descent process is equal, but the force on the ascent process is: the vertical downward gravity and the vertical downward air resistance.

The descent process is: vertical downward gravity and vertical upward air resistance.

So the acceleration of the upward motion is greater than the acceleration of the downward motion.

a The work done by overcoming gravity during ascent is greater than the work done by gravity during descent Error, because the work done by gravity w=mgh m, g, h is equal, so the work done by gravity is equal due to gravity. b right.

c The average power of overcoming gravity to do work in the ascending process is greater than the average power of gravity in the descending process, the average power of gravity to do work = w t w is equal, and because the acceleration is not the same, because the ascent process does a uniform deceleration motion, the end velocity is 0, and the descending process does a uniform acceleration motion, and the initial velocity is 0, according to h=1 2gt T up t down, so the average power of overcoming gravity to do work in the ascending process is greater than the average power of gravity in the descent process c True d False!!

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Ascending and descending, is the work done by overcoming gravity and the work done by gravity equal, when the time of the ascending and descending process is unequal, for the ascending process: the initial velocity v1, mg+f=ma1, the displacement is h, and the final velocity is zero; Descent process: the initial velocity is zero, mg-f=ma2, the displacement is h, and the final velocity is v2.

From h=1 2at 2 or h=v+v0 2*t, then t1 will increase the power p1 more than the falling power p2

The work done by gravity is only related to the initial and final positions, and the displacement of rising and falling is equal, so A is wrong and B is right; The average power is equal to the force multiplied by the average velocity, and the average velocity of the ascent process is greater than the falling process, so C is true and D is false.

According to the symmetry, it can be seen that the time for the ball to fall is t=the velocity of the ball when it leaves the hand is equal to the velocity in the falling Hui Sakura, which is v=gt=10 and the maximum height of the ball is h=1

gtm = so the answer to the rent is: 5;

BC should be selected to analyze the exercise process as follows:

The ball in the ascending section is subjected to gravity and air resistance, and the direction is downward, a1=(mg+f) m The ball in the descending section is subjected to gravity downward and the air resistance is upward, a2=(mg-f) m, so the acceleration of the ascending section is greater than that of the descending section, and the distance of the ascending and descending segments is equal, so the ascending time t1 and the descending time t2

The magnitude of the work done by gravity and overcoming gravity is equal to mgh, which depends only on h, and h at both ends is the same, so the work done to overcome gravity during ascent is equal to the work done by gravity during descent.

The power is equal to the work divided by the time, and it can be concluded that the power of the ascending section is large and the power of the descending section is small.

So, choose BC.

b The work done by gravity or by overcoming gravity is unchanged.

The work done to overcome gravity during ascent is equal to the work done by gravity during descent is equal to mgh, and b is right.

Because of the effect of air resistance, the velocity of the ball when it falls back is smaller than the velocity when it is thrown up, that is to say, the average speed of descent is smaller than that of rising, and the time of ascent is smaller than that of descent, so the average power of overcoming gravity and doing work during ascent is greater than the average power of gravity during descent, and c is also correct. I choose BC

For the sake of study convenience, let's assume air resistance.

is a fixed value. And we know that resistance is always in the opposite direction of movement.

Then it can be seen that the net force experienced by the ball during the ascent is a force that is greater than the gravitational force and the direction is vertically downward, while the net force experienced during the descent is a force of magnitude less than the gravitational force and the direction is vertically downward, so the acceleration in the upward process.

It is greater than the acceleration in the downward process, so the descent process takes longer.

BC test question analysis: the work done by gravity is related to the height difference of the initial position, so the work done by overcoming gravity during the ascent is equal to the work done by gravity during the descent process, A is wrong, B is correct; The average velocity in the ascending process is greater than the average velocity in the descending process, so the time taken to ascend is less than the time taken to descend, so the average power to overcome gravity and do chaotic collapse work in the ascending process is greater than the average power of gravity in the descending process.

Therefore, BC is selected next to the source

Comments: The work done by gravity is only related to the position of the beginning and the end, and has nothing to do with the path passed; The direction of the resistance during the ascent and descent is different, and the time of the object's motion is also different

The air resistance is downward during the throwing process, and the average acceleration is a1=g+f1 m (f1 is the average air resistance when rising).

When descending, the air resistance is upward, and the average acceleration is a2=g-f2 m (f2 is the average air resistance during descent).

Obvious A1>A2.

The maximum position velocity is 0, and in the case of the same displacement, the greater the average acceleration and the less time it takes, the average velocity s t is about large, so the average velocity is larger when rising.

bc correct. w=mgh (so A is wrong, B is right).

The speed of the rising leakage grinding process through any position is larger than the speed of the falling process through this position, and the average speed of the rising process is larger, and the distance is h, so the time between the return buckets of the rising process is shorter, and p = w t (so c is right, d is wrong).

The work done by gravity is only related to the height difference, obviously A is wrong, B is right.

So doubt the correctness of your answers.

A can pass directly, B is not equal because it is in the opposite direction, and C and D are related to the length of work work.

It's C!

It is necessary to consider that the air resistance wf=fs(f v s=h) does the work of the resistance during the obvious rise (set to wf resistance).

Therefore, the work done to overcome gravity in the process of ascending is mgh-wf liter; The work done by gravity in this life is mgh

The work done to overcome gravity during ascending is less than the work done by gravity during descending.

So a b is all wrong.

The ascent and descent are the same for the ball.

The resistance of the two processes is always doing negative work, and the total energy of the ball is constantly decreasing.

The velocity of the ball during the ascent is greater than the velocity during the descent.

Therefore, the resistance of the ball during the ascent is greater than the resistance during the descent.

The power is great, the power is great.

The drag is proportional to the speed of the ball, that is to say, the drag = the speed of the ball * the coefficient, and the coefficient is unknown.

Two downward forces are applied to the ascent and opposite forces are received when the descent is received.

Choose because the work done by overcoming gravity is equal to mgh, and the height is the same twice, so choose B

2.Because there is resistance to do negative work, the magnitude of the rising velocity at the same height is greater than the descending velocity, that is, the average velocity of the ascent is greater than the average velocity of the descent, and the time taken to ascend is less than the time taken to descend, p=w t, w is the same, the smaller t, the greater p.

2.From the point of view of motion, the rising a1 = a+g, the falling a2 = g-a, a1 > a2, so after the same height t1p2

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