Falling objects

Goal of the class To relate the motion of a freely falling object with constant acceleration To be able to calculate displacement, velocity, and time for objects in freefall. Question of the day: Is the acceleration due to gravity a constant?

Freefall Objects on Earth are attracted to the Earth due to the force of gravity This force pulls objects towards the centre of the planet (i.e. downwards) In a vacuum this value on the Earth’s surface is ag = -g = -9.81ms-2

Apollo 15 On 2nd August 1971 a demonstration showing the validity of Galileo's theory was shown by the crew of Apollo 15

SUVAT equations When dealing with the SUVAT equations the acceleration due to gravity will normally replace a with -g

Example problem Q) You throw a tennis ball directly upwards into the air with an initial velocity 15ms-1. If you throw it from an initial height of 1.5m above the floor, how long will it be in the air before it hits the floor?

Newton’s G The acceleration due to gravity is not a constant and is dependant on other factors G = Newton’s gravitational constant M = Mass of planet (object) Exercise: Find the units of G

Question Can you prove it using maths? Do you think for a larger planet made of the same material, will g be: Bigger? Smaller? The same? Can you prove it using maths?

Practice problems Estimate how long it would take your book to fall to the ground if you knocked it off the table. You drop a feather from rest off a building and it takes 8s to fall a height of 15m. Calculate the acceleration of the feather. Why is it not the same as g?

Oversimplification A spherical cow?

Homework We will have a short quiz after Chuseok now that we’ve finished the first 2 chapters (4-5 short questions to test understanding) Do as much practice as you see fit

Questions