Free Falling Objects Physic 11

Humour:

Freely Falling Objects  A freely falling object is any object moving freely under the influence of gravity alone.  It does not depend upon the initial motion of the object  Dropped – released from rest  Thrown downward  Thrown upward

Acceleration of Freely Falling Object:  All free-falling objects (on Earth) accelerate downwards at a rate of 9.81 m/s 2  The acceleration of an object in free fall is directed downward, regardless of the initial motion  The magnitude of free fall acceleration is g = 9.81 m/s 2

Acceleration of Free Fall cont….  We will neglect air resistance  Free fall motion is constantly accelerated motion in one dimension  Let upward be positive

Free Fall – an object dropped:  Initial velocity is zero  Let up be positive  Acceleration is  a = g = m/s 2 v i = 0 a = -g

Free Fall – an object thrown downward:  a = g = m/s 2  Initial velocity  0  With upward being positive, initial velocity will be negative v o ≠ 0 a = -g

Free Fall -- object thrown upward:  Initial velocity is upward, so positive  The instantaneous velocity at the maximum height is zero  a= g = m/s 2 everywhere in the motion v = 0 v i ≠ 0 a = -g

Thrown upward, cont…  If an object is projected upwards in a perfectly vertical direction, then it will slow down as it rises upward.  The instant at which it reaches the peak of its trajectory, its velocity is 0 m/s.  This value can be used as one of the motion parameters in the kinematic equations; for example, the final velocity (vf) after traveling to the peak would be assigned a value of 0 m/s.  If an object is projected upwards in a perfectly vertical direction, then the velocity at which it is projected is equal in magnitude and opposite in sign to the velocity that it has when it returns to the same height.  That is, a ball projected vertically with an upward velocity of +30 m/s will have a downward velocity of -30 m/s when it returns to the same height.

Ex #1: You throw a rock off a cliff, giving it a velocity of 8.3 m/s down. The instant you release the rock, your friend starts timing. You hear a splash in the river below at 6.9 sec. How high is the cliff? 1. V i =-8.3m/s, t=6.9s, a=-9.81m/s 2, d=? 2. Use: d = V i t + ½ at 2 3. d = V i t + ½ at 2 = (-8.3)(6.9) + ½ (-9.81)(6.9) 2 = ( ) = -290 m

Ex #2:Rex Things throws his mother's crystal vase vertically upwards with an initial velocity of 26.2 m/s. Determine the height to which the vase will rise above its initial height. 1. V i =+26.2m/s, a=-9.81m/s 2, V f =0, d=? 2. Use: V f 2 = V i 2 + 2ad 3. V f 2 = V i 2 + 2ad 0 = (26.2 )2 + 2(-9.81)d = (-19.62)d = d 34.9 m = d

Practise:  WS`s