Tóm tắt:

\(h_A=10\\ m=100g=0,1kg\\ v_A=5m/s\\ g=10m/s^2\)

a/ tìm h của vật tại điểm \(W_{đ_B}=\dfrac{1}{4}W_B\)(đặt là điểm B):

Cơ năng ban đầu của (đặt là điểm A) vật:

\(W_A=W_{đ_A}+W_{t_A}=\dfrac{1}{2}mv_A^2+mgh_A\\ W_A=\dfrac{1}{2}.0,1.5^2+0,1.10.10\\ W_A=11,25\left(J\right)\)

Áp dụng Định luật Bảo toàn cơ năng:
\(W_A=W_B\)

mà \(W_B=W_{đ_B}+W_{t_B}\)

 \(\Rightarrow W_{t_B}=W_B-W_{đ_B}\)

\(\Rightarrow W_{t_B}=W_B-\dfrac{1}{4}W_B\)

\(\Rightarrow mgh_B=\dfrac{3}{4}W_B\)

Thế số:

 \(0,1.10.h_B=\dfrac{3}{4}.11,25\\ h_B=8,4375m\)

b/ tính \(h_{max}\) và \(t_C\)? (đặt điểm cao nhất là C):

Áp dụng định luật bảo toàn Cơ năng:

\(W_A=mgh_{max}\) (\(W_{đ_C}=0\left(J\right)\))

\(\Rightarrow h_{max}=\dfrac{W_A}{mg}=\dfrac{11,25}{0,1.10}\\ \Rightarrow h_{max}=11,25m\)

thời gian để đạt độ cao cực đại:

\(v=v_0-gt_C\\ 0=5-10t_C\\ \Rightarrow t_C=0,5\left(s\right)\)
 

Exercise 1. Reorder the words to make correct sentences.

1. can / You / travel / sleep / when / you / by / driverless / car.

-> You can sleep when you travel by driveless car.

2. name / What / means / the / of / the / of / is / transport?

-> What is the name of the means of transport?

3. In / we / than / future, / will / travel / much / further / the / we / do / now.

In the future, we will travel much further than we do now.

4. Do / will / you / we / travel / for / think / to / other / planets / holidays? / the

-> Do you think we will travel to other planets for the  holidays?

5. I / runs / to / use / it / a / bamboo copter / because / on / electricity. / It / want / is / self-balancing.

-> I want to use a bamboo copter because it is self-balacing. It runs on electricity.

6. I / would / because / to / the / like / walker / it / use / runs / on / electricity.

-> I would  like to use walker because it runs on electricity. 

7. a / transport / trains / soon / means / become / will / popular / of / Bullet / in / Viet Nam.

-> Bullet trains  will soon become popular means of transport in VietNam.

8. have / in / will / we / 2030. / driverless / cars

-> We will have driveless cars in 2030.

9. cars / cities, / won’t / allow / in 2050.( chỗ này đề bị sai nhé!)

-> Cars won't be allowed in cities in 2050.
10. have / enough / parking / places / we / will / in / ten / years?

-> Will we have enough parking places in ten years?