Learning Objective
●理解数学模型如何应用到力学中
●理解假设 (Assumptions)如何在力学模型中利用
●知道7个标准单位以及在derived quantities中的使用。(下面前三个在M1必须掌握)
physical quantity | symbol |
---|---|
meter | m |
kilogram | kg |
second | s |
ampere | A |
mole | mol |
candela | cd |
是米(m),千克(kg),秒(s),安培(A),开尔文(K),摩尔(mol),坎德拉(cd)。
Prior Knowledge Check(预备知识)
1.
5
x
2
?
21
x
+
4
=
0
5x^2-21x+4=0
5x2?21x+4=0
2.
3.30km/h to cm/s
4.0.003806
Example1
The motion of a basketball as it leaves a player’s hand and passes through the net can be modelled using the equation
h
=
2
+
1.1
x
?
0.1
x
2
h=2+1.1x-0.1x^2
h=2+1.1x?0.1x2, where
h
h
h m is the height of the basketball above the groupd and
x
x
x m is the horizontal distance travelled. "
a Find the height of the basketball:
i whenitis released ii at a horizontal distance of 0.5 m.
b Use the model to predict the height of the basketball when it is at a horizontal distance of 15 m
from the player.
c Comment on the validity of this prediction.
Solution
a首先绘制这个图
import numpy as np
import matplotlib.pyplot as plt
# 定义x的范围
x = np.linspace(0, 15, 400)
# 定义h函数
h = 2 + 1.1*x - 0.1*x**2
# 绘制h函数
plt.figure(figsize=(8,6))
plt.plot(x, h, label='h(x)')
plt.title('h(x) = 2 + 1.1x - 0.1x^2')
plt.xlabel('x')
plt.ylabel('h(x)')
plt.grid(True)
plt.legend()
plt.show()
根据图可以知道在y轴上的y=2的地方是篮球起抛点(考试时不要写中文,下同,注意这个只是讲义!)
a i x = 0 ; h = 2 + 0 ? 0 Height = 2 m \begin{array}{|ll}\text{a}&\text{i}&x=0;h=2+0-0\\&&\text{Height}=2m\end{array} a?i?x=0;h=2+0?0Height=2m?
ii问的是在水平距离x=0.5的位置,球的高度是多少
ii
x
=
0.5
;
h
=
2
+
1.1
×
0.5
?
0.1
×
(
0.5
)
2
Height
=
2.525
m
\begin{array}{rl}\text{ii}&x=0.5;h=2+1.1\times0.5-0.1\times(0.5)^2\\\text{Height}&=2.525m\end{array}
iiHeight?x=0.5;h=2+1.1×0.5?0.1×(0.5)2=2.525m?
b Use the model to predict the height of the basketball when it is at a horizontal distance of 15 m from the player.
根据上面画的图可知,球都到地底下了了,所以模型不适用。
b
x
=
15
;
h
=
2
+
1.1
×
15
?
0.1
×
(
15
)
2
Height
=
?
4
m
\begin{array}{ll} {x=15;h=2+1.1\times15-0.1\times(15)^{2}}\\{\text{Height}=-4m}\end{array}
x=15;h=2+1.1×15?0.1×(15)2Height=?4m?
c?Comment?on?the?validity?of?this?prediction.
\text{c Comment on the validity of this prediction.}
c?Comment?on?the?validity?of?this?prediction.
c Height cannot be negative so the model is not valid when x=15m