物理牛頓運動定律 DSE、CE 選擇題

1.

Questions 3 and 4 : A man of mass $50$ kg stands still on a balance in a lift which is initially moving downwards. Between $t=0$ and $6$ s, the reading $R$ on the balance varies with time as shown in the graph below. Take $g$ to be $10\ \mathrm{m\,s^{-2}}$ .

Graph of balance reading R / N against t / s: 400 N from 0 to 3 s, then 500 N from 3 s to 6 s

Which of the following statements is/are correct ?

(1)Between

t=0

and

3

s, the net force acting on the man is

400

N.

(2)At

t=3

s, the lift starts to move upwards.

(3)Between

t=3

s and

6

s, the lift is moving at a constant velocity.

2.

Questions 3 and 4 : A man of mass $50$ kg stands still on a balance in a lift which is initially moving downwards. Between $t=0$ and $6$ s, the reading $R$ on the balance varies with time as shown in the graph below. Take $g$ to be $10\ \mathrm{m\,s^{-2}}$ .

What is the acceleration of the lift between $t=0$ and $3$ s ?

3.

Block on a horizontal surface pushed by a horizontal force F

On a horizontal surface, horizontal force $F$ is exerted on a block as shown in the figure above. When $F=25$ N, the block remains at rest. Which of the following statements must be correct ?

4.

Photograph of an ant hanging upside down from the ceiling holding a block in its jaws

The photo above shows an ant hanging upside down from the ceiling. The ant is holding a $500$ mg block using its jaws. At the ceiling, the ant and the block are at rest. Which of the following statements is/are correct ?

(1)The force acting on the ant by the ceiling points upwards.

(2)The force acting on the block by the ant and the weight of the block are an action and reaction pair.

(3)The net force acting on the ant is zero.

5.

Three forces are acting on particle $P$ . In which of the following diagrams is the net force on $P$ not zero ?

Four grid diagrams A to D, each showing three forces of 6 N, 8 N and 10 N acting on a point P

6.

In the figure shown, horizontal force $F$ is applied to a block which is hung by a string under the ceiling. At equilibrium, the string makes an angle $\theta$ with the vertical and the tension in the string is $T$ . The weight of the block is

A block hung by a string from the ceiling, pulled by horizontal force F, the string making angle theta with the vertical and tension T

7.

Velocity-time graph rising linearly to a peak at t1 then falling linearly to zero

A man pushes a supermarket trolley with a constant horizontal force for $t_1$ seconds, and then releases the trolley. The velocity-time graph of the trolley is shown above. Assume that the friction between the trolley and the ground is constant, which of the following graphs (in dotted lines) best represents the variation of the velocity of the trolley with time if the man repeats the action with a stronger force ?

Four velocity-time graphs A to D, each with the original solid line and a dotted line

8.

Spring balance attached to a wall by a light string, horizontal force F pulling the balance

A spring balance is connected to the wall by a light string. A horizontal force $F$ is acting on the spring balance as shown below. The spring balance shows a reading of $5$ N. Assume that the mass of the balance can be neglected, what is the magnitude of the tension in the string ?

9.

A man is standing in a lift. The lift is initially at rest. When the lift starts to move upward, the man feels heavier. Which of the following statements is/are correct ?

(1)The gravitational force acting on the man increases.

(2)The gravitational force acting on the lift increases.

(3)The force acting on the man by the floor of the lift increases.

10.

Block on a rough horizontal surface pulled by string S1 (100 N) to the left and string S2 (30 N) to the right

A block of mass $4$ kg remains at rest on a horizontal rough surface with strings $S_1$ and $S_2$ of tensions $100$ N and $30$ N respectively pulling it. If $S_1$ is broken, what is the magnitude of the net force acting on the block ?