Presentation on theme: "Free Fall Free fall: when an object is only affected by gravity"— Presentation transcript:
1 Free Fall Free fall: when an object is only affected by gravity Think about an apple falling from a tree.It starts at rest and gains speed as it falls, or accelerates.Gravity causes the apple to accelerate downward and is said to be in free fall.Free fall: when an object is only affected by gravitySI unit: m/s2 ( for acceleration due to gravity)g = 10 m/s2 on Earth.The letter g represents the acceleration due to gravity.
2 Equation: v = gtv = velocity or speed (m/s)g = acceleration due to gravity (10 m/s2 on Earth)t = elapsed time (s)**Hint - as soon as you see any of the following phrases in a word problem, write g = 10 m/s2 for a given: free fall, falling, dropped, thrown**
3 Remember, the object’s velocity increases by 10 m/s every second it falls Time fallingvelocity0s1 s2 s3 s4 s5 s6s7s8s9s
4 Check Your Understanding What would the speedometer reading on a falling rock be 4.5 seconds after it drops from rest?v = ?g = 10 m/s2t = 4.5sv = gtv = (10 m/s2) (4.5s)v = 45 m/s
5 How about 8 seconds? v = gt v = 80 m/s How about 15 seconds? g = 10 m/s2t = 8sv = gtv = (10 m/s2) (8s)v = 80 m/sHow about 15 seconds?t = 15sv = (10 m/s2) (15s)v = 150 m/s
6 Now consider an object thrown straight up Now consider an object thrown straight up. It will continue to move straight up, then it comes back down.At the highest point, the object changes its direction and the objects instantaneous speed is 0 m/s.Whether the object is moving up or down, the acceleration of the object is always 10 m/s2.It is just -10 m/s/s when the object is moving upBecause gravity is slowing the object down instead of speeding it upIt is decelerating!!
7 Because an object in free fall increases the rate of distance covered every second, we cannot use v =d/t to find distance.Equation: d = ½ gt2d = distance (m)g = acceleration due to gravity (10 m/s2 on Earth)t = elapsed time (s)
8 Use the distance formula to find how far the object will fall at each second Time fallingDistance travelled0 s1 s2 s3 s4 s5 s6 s7 s8 s9 s
9 Check Your Understanding What is the distance an object falls in one second?d = ?g = 10 m/s2t = 1 sd = ½ gt2d = ½ (10)(12)d = 5 m
10 Practice problem 1 Copy this down then solve: If it takes a rock that falls off a cliff 8 seconds to hit the ground, then what is its speed and how far has it fallen?
11 Practice problem 2 Write this down then solve If a rock falls 320 m, how long did it take for that rock to fall?
12 Challenge practice problem Think this one through – do you need to do any work to solve it?If you throw a ball straight up at 20 m/s, how long will it take for the ball to come to a complete stop before it starts to fall?
13 The problem with airWhen you drop a crumpled piece of paper and a flat sheet of paper they should fall and hit the ground at the same time, but do they?The reason the crumpled piece of paper hits the ground first is because it experiences less air resistanceA frictional force (means it goes against motion) caused by airThe greater the surface area of an object, the more air resistance it feels