Theories of motion Aristotle was a Greek philosopher who lived in the 4 th century BC Aristotle explained the behaviour of a falling body by saying that its speed depended on how much earth element it contained ID: 514387
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Slide1
Vertical Motion under GravitySlide2
Theories of motion
Aristotle was a Greek philosopher who lived in the 4
th
century BC.Aristotle explained the behaviour of a falling body by saying that its speed depended on how much earth element it contained. This suggested that a 2kg cat would fall twice as fast and half the time as a 1 kg cat dropped from the same height.Slide3
Theories of motion
Galileo used inclined planes (because freely falling bodies moved too fast to analyse) in his experiments
Through his experiments, he was able to show that Aristotle was incorrect and that in fact,
freely falling bodies actually fall with a uniform acceleration.
However, centuries later, Galileo
Galilei
noticed that in a hailstorm the small hailstones arrived at the same time as large hailstones, which caused him to doubt Aristotle’s theory.Slide4
Analysing vertical motion
Many people think that heavy objects fall faster than lighter ones.
The cause of this confusion is usually related to the effects of
air resistance.
Some falling objects are greatly affected by air resistance, (
e.g
feather or balloon).
However ignoring air resistance,
all free-falling bodies near the Earth’s surface will move with an equal downwards acceleration
.Slide5
Analysing vertical motion
https://
www.youtube.com
/watch?v=E43-CfukEgsSlide6
Analysing vertical motion
At the Earth’s surface, the acceleration due to gravity is
g
= 9.8 m s-2 down.Free fall simply implies that the motion of the body is affected only by gravity.Slide7
Analysing vertical motion
Since the acceleration of a free falling body is constant (uniform acceleration), it is appropriate to use the equations for uniform acceleration. (a = 9.8 m.s
-2
)It is often necessary to specify up and down as the positive or negative direction when doing such problems.Slide8
Example 1
A construction worker accidently knocks a brick from a building so that it falls vertically a distance of 50m to the ground. Using g = 9.8 m s
-2
calculate:a. The time the brick takes to fall the first 25mSlide9
Example 1
A construction worker accidently knocks a brick from a building so that it falls vertically a distance of 50m to the ground. Using g = 9.8 m s
-2
calculate:b. The time the brick takes to reach the groundSlide10
Example 1
A construction worker accidently knocks a brick from a building so that it falls vertically a distance of 50m to the ground. Using g = 9.8 m s
-2
calculate:c. The speed of the brick as it hits the ground.Slide11
Example 2
On winning a tennis match the victorious player, Nick, smashed the ball vertically into the air at
30 m s
-1. In this example, air resistance can be ignored and the acceleration due to gravity will be taken as 10 m s-2.a. Determine the maximum height reached by the ballSlide12
Example 2
On winning a tennis match the victorious player, Nick, smashed the ball vertically into the air at
30 m s
-1. In this example, air resistance can be ignored and the acceleration due to gravity will be taken as 10 m s-2.b. Calculate the time that the ball takes to return to its starting positionSlide13
Example 2
On winning a tennis match the victorious player, Nick, smashed the ball vertically into the air at
30 m s
-1. In this example, air resistance can be ignored and the acceleration due to gravity will be taken as 10 m s-2.c. Calculate the velocity of the ball 5.0 s after being hit by NickSlide14
Example 2
On winning a tennis match the victorious player, Nick, smashed the ball vertically into the air at
30 m s
-1. In this example, air resistance can be ignored and the acceleration due to gravity will be taken as 10 m s-2.d. Determine the acceleration of the ball at its maximum height.