此文章來自奇摩知識+如有不便請留言告知
標題:
F.4 physics mechanics
發問:
F.4 physics: mechanics - vertical motion under gravity1. A ball is thrown vertically upwards with a speed of 15 ms-1 from the edge of a cliff. Another ball is thrown vertically downward with the same speed. Which ball hit the ground with a greater speed?F.4 physics: mechanics - Newton First Law2.... 顯示更多 F.4 physics: mechanics - vertical motion under gravity 1. A ball is thrown vertically upwards with a speed of 15 ms-1 from the edge of a cliff. Another ball is thrown vertically downward with the same speed. Which ball hit the ground with a greater speed? F.4 physics: mechanics - Newton First Law 2. Someone can pull away a tablecloth with the dishes above remaining on the table. Explain the theory behind. 更新: 2. in a straight line 姐係咩???? 同horizontal direction有冇關係??? 3. If you are standing in a bus, what happens to you when the bus? (a) starts moving? (b) stops moving? (c) turns a corner? Explain. 更新 2: 4. A car collides with a wall and is brought to stop in 0.1s. The initial speed of the car is 72 km/h, and the total mass of the car and the driver is 1000 kg. Find the average force acting on the car during collision. How may the force be reduced to protect the driver? Explain.
最佳解答:
1. Let h be the height of the cliff. Consider the 1st ball, use equation: v^2 = u^2 + 2a.s with u = 15 m/s, a = -g(=-10 m/s^2), s = -h, v=? hence, v^2 = 15^2 + 2.(-10).(-h) i.e. v^2 = 225 + 20h Consider the 2nd ball, use the same equation with u = -15 m/s, a = -g(=-10 m/s^2), s = -h, v =? hence, v^2 = (-15)^2 + 2.(-10).(-h) i.e. v^2 = 225 + 20h Therefore, both balls have the same speed just before striking the ground. 2. This is because of inertia. By Newton's First Law, objects try to remain their state of rest or unform motion in a straight line unless a force is applied to them. Sudden pulling of the tablecloth has not sufficient time to change the state of rest of the dishes placed on the tablecloth. They try to remain at their initial rest state. 2012-10-16 11:05:45 補充： 2. "In a straight line" means there is no change of direction of motion. 2012-10-16 11:08:25 補充： 3 You tends to (a) fall backward; (b) fall forward; (c) fall sideways away from the turn. This is because of inertia that you wants to maintain your original state of motion. 2012-10-16 11:11:54 補充： 4. 72 km/h = 20 m/s Retardation = 20/0.1 m/s^2 = 200 m/s^2 Force = 1000 x 200 N = 200,000 N Use "seat-belt" or "air bag".
其他解答:C8D74AB62542840B
留言列表
