题目

Physics
1) A rocket engine ejects 60kg of hot gas at a speed of 90m/s in 20s.
Find:a) The change in momentum of the hot gas and
b) The thrust(in Newtons)of the engine.
2) What is the radius of the circle in which a 240-kg motorcycle and rider are traveling if they are going 35m/s and the force exerted on the tires perpendicular to the velocity is 6,200N?
3) An 18,000kg van is moving with a speed of 15m/s.If Superman is to stop the van in 0.5s,what average force must he exert on the van?
4)A 10g object moving to the right with a speed of 20m/s makes an elastic head-on collision with a 15-g object moving in the opposite direction with a speed of 30cm/s.Find the velocity of each object after the collision.

提问时间：2020-12-09

答案

1、a、气体的动量=mv=60×90=5400
b、由题,由动量定理,Ft=动量变化=5400,所以F=5400/20=270(N)
2、由题,向心力F=mv^2/r,而向心力是由摩擦力提供的.
所以r=mv^2/F=240×35^2/6200=47.42m
3、由题,平均加速度a=v/t=15/0.5=30(m/s^2)
由牛二定律,F=ma=18000×30=540000(N)
4、确定一个是20m/s,另一个是30cm/s吗?