Question

1. you throw a ball up. which one of the following is correct as the ball reaches its maximum height

a. ( v=\text{maximum}, a = 0 )
b. ( v = 0, a = 9.8 mathrm{~m} / mathrm{s} ) down
c. ( v = 0, a = 9.8 mathrm{~m} / mathrm{s} ) up
d. ( v=\text{maximum}, a = 9.8 mathrm{~m} / mathrm{s} ) down

1. a bird, accelerating from rest at constant rate, experiences a displacement of ( 28 mathrm{~m} ) in ( 11 mathrm{~s} ). what is the final

velocity after ( 11 mathrm{~s} ) ?
a. ( 5.1 mathrm{~m} / mathrm{s} )
b. ( 3.2 mathrm{~m} / mathrm{s} )
c. ( 1.8 mathrm{~m} / mathrm{s} )
d. ( 17.4 mathrm{~m} / mathrm{s} )

1. if a projectile is fired straight up at a speed of ( 30 mathrm{~m} / mathrm{s} ), the total time to return to its starting point is about

a. 60 seconds.
b. 6 seconds.
c. 3 seconds.
d. 30 seconds.

1. a train travels 6 meters in the first second of travel, 6 meters again during the second second of travel, and 6

meters again during the third second. its acceleration is
a. ( 0 mathrm{~m} / mathrm{s}^{2} ).
b. ( 6 mathrm{~m} / mathrm{s}^{2} ).
c. ( 18 mathrm{~m} / mathrm{s}^{2} ).
d. ( 12 mathrm{~m} / mathrm{s}^{2} ).

1. in the absence of air resistance, objects fall at constant

a. displacement
b. acceleration.
c. velocity.
d. speed.

1. if you drop a feather and a coin at the same time in a tube filled with air, which will reach the bottom of the

tube first?
a. the feather
b. both at same time.
c. the coin

1. a ball tossed vertically upward rises, reaches its highest point, and then falls back to its starting point. during

this time the acceleration of the ball is always
a. directed upward.
c. opposite its velocity.
b. directed downward.
d. in the direction of motion.

1. suppose a car is moving in a straight line and steadily increases its speed. it moves from ( 35 mathrm{~km} / mathrm{h} ) to ( 40 mathrm{~km} / mathrm{h} )

the first second and from ( 40 mathrm{~km} / mathrm{h} ) to ( 45 mathrm{~km} / mathrm{h} ) the next second. what is the cars acceleration?
a. ( 35 mathrm{~km} / mathrm{h} / mathrm{s} )
b. ( 5 mathrm{~km} / mathrm{h} / mathrm{s} )
c. ( 40 mathrm{~km} / mathrm{h} / mathrm{s} )
d. ( 10 mathrm{~km} / mathrm{h} / mathrm{s} )

1. a car accelerates at ( 2 mathrm{~m} / mathrm{s}^{2} ). assuming the car starts from rest, how much time does it need to accelerate to a

speed of ( 20 mathrm{~m} / mathrm{s} ) ?
a. 20 seconds
b. 10 seconds
c. 40 seconds
d. 2 seconds

1. a freely falling object starts from rest. after falling for 6 seconds, it will have a speed of about

a. ( 60 mathrm{~m} / mathrm{s} ).
b. ( 300 mathrm{~m} / mathrm{s} ).
c. ( 6 mathrm{~m} / mathrm{s} ).
d. ( 30 mathrm{~m} / mathrm{s} ).

1. when a cars velocity is positive and its acceleration is negative, what is happening to the cars motion?

a. the car travels at constant speed.
c. the car slows down.
b. the car speeds up.
d. the car remains at rest.

Explanation:

10.

Step1: 分析竖直上抛运动

竖直上抛运动，到达最高点时，速度\(v = 0\)，加速度为重力加速度\(a = g=9.8m/s^{2}\)，方向向下。

Step1: 用位移公式\(x = v_{0}t+\frac{1}{2}at^{2}\)（\(v_{0}=0\)）

\(x=\frac{1}{2}at^{2}\)，\(a=\frac{2x}{t^{2}}\)，再用\(v = v_{0}+at\)（\(v_{0}=0\)），\(v = at\)。

Step2: 代入数据

\(x = 28m\)，\(t = 11s\)，\(a=\frac{2\times28}{11^{2}}\approx0.46m/s^{2}\)，\(v=0.46\times11\approx5.1m/s\)。

Step1: 竖直上抛运动对称性

上升时间\(t_{1}=\frac{v_{0}}{g}\)（\(v_{0}=30m/s\)，\(g = 10m/s^{2}\)），总时间\(t = 2t_{1}\)。

Step2: 计算

\(t_{1}=\frac{30}{10}=3s\)，\(t = 6s\)。

Answer:

d. \(v = 0,a = 9.8m/s^{2}\) down

11.