问题 计算题

如图所示,水平地面上的物体在与水平方向成α=的拉力F作用下,由静止开始运动。物体质量为M=2.0kg,拉力F=12N,物体与地面间的动摩擦因数μ=0.25。

求:(1)拉力F作用时物体的加速度a1

(2)若2.0s后撤掉拉力F,物体减速滑行时的加速度a2

(3)物体由开始运动到停下来的总位移。

答案

(1)(2)(3)14.6m

题目分析:以物体为研究对象,物体的运动过程分为两段:前段物体在F作用下由静止开始做匀加速直线运动;后段撤去F,由于摩擦力的作用,物体作匀减速运动,直到停止。物体在这两段中的受力分析如图所示。

解:(1)根据牛顿第二定律物体在拉力F作用下的加速度:

其中           

代入上式       

(2)物体在前2.0s的位移

物体在撤去拉力F后的加速度大小为

(3)物体撤去F后的位移

点评:动力学的两类基本问题(1)已知受力情况求运动情况:已知物体的受力情况,由牛顿第二定律求出物体的加速度,再通过运动学公式就可以确定物体的运动情况.(2)已知运动情况求受力情况:已知物体的运动情况,根据运动学公式求出物体的加速度,再根据牛顿第二定律确定物体所受的力.在处理动力学的两类基本问题时,加速度是联系运动和力的纽带、桥梁

单项选择题
问答题

Task 5
Conversation: The following are two business letters. After reading them, you should give brief answers to the 5 questions (No. 56 through No. 60) that follow. The answers should be no more than 3 words. They should be written after the corresponding numbers on the Answer Sheet.

Letter One

Dear Sirs,
On 5th October I bought one of your expensive "Apollo" fountain pens from Julian’s, a big department store of this town. Unfortunately I have been unable to use the pen because it leaks and fails to write without making blots. I am very disappointed with my purchase.
On the advice of Julian’s manager I am returning the pen to you and enclose it with this letter for correction of the fault.
Please arrange for the pen to be fixed or replace it with a new one and send it to me as soon as possible.
Letter Two

Dear Sirs,
Thank you for your letter of 9th October enlosing the defective "Apollo" fountain pen, received today. We very much regret that the pen you bought has given you trouble.
Normally each of our pens is individually examined before being passed into store and it is hard to understand why this one escaped examination. We have passed your pen to our quality control department for inspection and report. Meanwhile, we are arranging to replace your pen with a new one.
We extend our apologies for the inconvenience this matter has caused you, but are confident that the replacement pen you will be receiving will prove satisfactory.

When was the Letter One writtenIt was written ______.