物理功、能和功率 DSE、CE 選擇題

1.

Ball moving along a smooth curved rail through points P, Q, R, S and T

A ball moves along a smooth curved rail and passed $P$ at a certain speed as shown in the figure above. Neglecting air resistance and friction, which of the following statements about the motion of the ball are correct ?

(1)The ball has maximum kinetic energy at

R

.

(2)The speed of the ball at

S

is the same as that at

Q

.

(3)The ball can never reach

T

.

2.

Graph of kinetic energy against t squared, a straight line through the origin reaching 15000 J at t squared = 5

A car of mass $1500$ kg is accelerating from rest along a straight road. The above figure shows the variation of its kinetic energy (K.E.) with the square of time $t$ . What is the acceleration of the car ?

3.

Platform at P above a swimming pool; Q at the water surface and R at maximum depth

In the above figure, a mass is released from rest at $P$ from a high platform into a swimming pool. After a while, the mass reaches the water surface at $Q$ and enters the water. Finally, it reaches a maximum depth at $R$ . Which of the following descriptions about the motion of the mass is/are correct ?

(1)From

P

to

Q

, the acceleration of the mass is increasing.

(2)From

Q

to

R

, the net force acting on the mass is pointing upward.

(3)From

P

to

R

, the gravitational potential energy loss of the mass equals to its kinetic energy gain.

4.

Horizontal force F applied to a block on a smooth surface; graph of F against s rising linearly from 0 to 8 N at 2 m then constant to 5 m

Horizontal force $F$ is applied to a block of mass $4$ kg which is initially at rest on a horizontal smooth surface. The variation of the magnitude of $F$ with displacement $s$ is shown in the graph. The direction of $F$ remains unchanged throughout the motion. What is the work done on the block by $F$ from $s = 2$ m to $5$ m ?

5.

Questions 6 and 7 : A car is travelling along a straight horizontal path at $16\ \mathrm{m\,s^{-1}}$ initially. The driver sees an obstacle and applies the brake. The velocity-time graph of his car is shown below.

Velocity-time graph: constant 16 m/s until 1.2 s, then decreasing linearly to zero at 4.5 s

What is the magnitude of the deceleration of the car during braking ?

6.

Questions 6 and 7 : A car is travelling along a straight horizontal path at $16\ \mathrm{m\,s^{-1}}$ initially. The driver sees an obstacle and applies the brake. The velocity-time graph of his car is shown below.

Which of the following graphs best represents the variation of the kinetic energy (K.E.) of the car with distance $s$ travelled ?

Four graphs A to D of KE against distance s

7.

A block of mass $5$ kg is projected up along a smooth inclined plane of inclination $\theta$ as shown in the figure above. The graph shows the variation of kinetic energy of the block with the distance travelled $s$ up along the plane. Find the angle $\theta$ . Take $g$ to be $10\ \mathrm{m\,s^{-2}}$ .

Block on an incline of angle theta, and graph of kinetic energy against s falling linearly from 24 J at s = 0 to zero at s = 2.4 m

8.

A ball of mass $0.5$ kg resting on a horizontal ground is kicked by a player. The ball travels along a straight path and come to rest after travelling a horizontal distance of $10$ m. If the average resistive force acting on the ball during the motion is $3$ N, what is its initial speed ?

9.

Force F pushing a block on a horizontal smooth surface

A constant force $F$ is applied to an object which is initially at rest on a horizontal smooth surface. Which of the graphs below best represents the variation of the power $P$ developed by the force $F$ with time $t$ ?

Four graphs A to D of power P against time t

10.

A tram of mass $1500$ kg is being pulled by a steel cable. It goes up a $30^\circ$ slope with a uniform speed. The average resistive force along the tram is $200$ N. What is the energy required for the tram to move $100$ m up along the slope ?

Tram pulled up a 30 degree slope by a cable