物理问题(-.-)

yohyoh

1)a car starts from rest with an initial acceleration of 6.0m/s^2. the magnitude of this acceleration decreases at a constant rate until it reaches zero after 10s, after which the car continues at a constant velocity. What is the time t required for the car to travel 400m from its starting point? (pay close attention to the fact that the acceleration is not constant.)

2)a projectile is launched from a tube mounted on a truck which is traveling with a constant speed of 30km/h along a flat road . the tube makes an angle of 40" to the vertical and points toward the front of the truck. projectile leaves the tube with an speed of 30 m/s relative to the truck. how far from the projection point will the projectile land on the ground? neglect wind resistance and the height of the tube above the ground.

3)while driving around the Wastch front, you turn a corner and are driving up a steep hill.suddenly, a small boy runs out on the street chasing a ball. you slam on the brakes and skid to a stop leaving a 50ft long skid mark on the street. the boy calmly walks away but a policeman watching from the sidewalk walks over and gives you a ticket for speeding. you are still shaking from the experience when he points out that the speed limit on this street is 25mph. after you recover your wits, you examine the situation more closely, you determine that the street makes an angle of 20 degrees with the horizontal and that the coefficient of static friction between your tires and the street is 0.80. you also find that the coefficient of kinetic friction between your tires and the street is 0.60. your car's information book says that the mass of your car is 1570kg. you yourself weigh 130lb. witnesses say that the boy had a weight of about 60lb and took 3.0 seconds to cross the 15ft street. will you fight the speeding ticket? explain your reasoning.

Lov瑜瑜4ever

字未免也太多了吧？
哪个参考书的问题吖？

yohyoh

回复 2# Lov瑜瑜4ever


    我学校老师给的疯狂假期作业。。。还有另外8题的说。。==

五官齐全

第一个问题 前十秒 average acceleration,a = 3ms^2   
first ten second distance = at^2 = 3(10^2) =300
final velocity = at = 3(10)= 30
then 400-300 = 100
100/30=....
剩下自己做

五官齐全

第一个问题 前十秒 average acceleration,a = 3ms^2   
first ten second distance = at^2 = 3(10^2) =300
final velocity = at = 3(10)= 30
then 400-300 = 100
100/30=....
剩下自己做

五官齐全

第3题 用kinetic friction

yohyoh

回复 5# 五官齐全


    first ten second distance = at^2 = 3(10^2) =300<<<<这个不是应该是1/2at^2=1/2 (3)(10)=150吗？

yohyoh

回复 6# 五官齐全


    kinetic friction是第几课的？

五官齐全

Paise 忘记 成1/2 忘记了 不过是lower six 的

yohyoh

回复 9# 五官齐全

哦哦。。那能大略跟我解释下第3题吗？我真的不太明白他在讲什么。。。

五官齐全

这个很麻烦 要换好Unit先 所以我懒惰做   你要注意 那个斜度（关系gravity), 然后你和车的重量（friction这个也有关系到斜度很tricky的) 来算deceleration..
算到后用50ft 就可以懂有没有超速
还有小孩子的重量和时间是玩你罢了
就这些重点

blazex

可以写下第三题的答案吗？我算到12.98m/s,为什么第一题是用average acceleration 而不用算出rate of change of acceleration?