If an object of mass m moves with constant velocity v, the net force on the object is（）

An object is thrown up with an initial speed, the air resistance is, where m is the mass of the object and k is a constant, what are (1) the maximum height the object can reach, and (2)the magnitude of the velocity when the object returns the Earth？ (Assume the gravitational acceleration is a constant)

A．0.20m/s

B．0.40m/s

C．0.80m/s

D．1.60m/s

As shown in the figure, the masses of the tow objects A and B are both m=3.0kg, object A moves with acceleration=1.0, what is the frictional force between object B and the table？(ignore the masses of the pulley and the connecting rope)

A small spherewith mass m moves from the same height to the ground along three different paths. It is assumed that the initial velocity is zero and all frictions can be neglected. If the velocities of the sphere when it reaches the ground are,and，respectively, then

A、

B、

C、

D、

A small sphere with mass m moves along a smooth semi-circular path. The initial velocitypoints downward. Which of the following statements are correct？

A、The momentum is not conserved during the motion.

B、The angular momentum about pointis conserved during the motion.

C、The angular momentum about pointis not conserved during the motion.

D、The momentum is conserved during the motion.

A、

B、

C、

D、

A small sphere with mass m moves along a smooth semi-circular path. The initial velocitypoints downward. Which of the following statements are correct？

A、The momenta of the sphere are the same at pointand point.

B、The momenta of the sphere are different at pointand point.

C、The kinetic energy of the sphere is the same at pointand point.

D、The kinetic energy of the sphere is different at pointand point.

A net forceacts on object A, and a net forceacts on object B. The mass of object B is twice the mass of object A, and the acceleration of object B is twice that of object A. Is it true thatis four times of？

A mass point of 10 kg moves on the x axis under the force F=120t+40, where the unit of F and t are N and s respectively, at t = 0 it is at x = 0.5 m and its velocity is. What are the position and velocity of the mas point at any time？

A、0

B、1.4 m/s

C、2.0 m/s

D、3.4 m/s