s = ut + 1/2at2
u and a are positive
s
t
v = u + at
u is positive and a is negative
v
t
v2 = u2 + 2as
u is positive and a is negative
v
s
a-t graph for motion with uniform
velocity
a
t
Vertical motion under gravity
Motion of a free falling object
• Slope of v-t graph =
acceleration
• => g = (2.75-0.5)/(3*0.08)
• = 9.375 ms-2
Q. The experimental value is always smaller
than the actual value, why?
1. When the vibrator strikes the ticker tape, it
produces friction which slows down the
falling motion.
2. The falling motion is slowed down by air
resistance.
The “coin and feather”
experiment
Acceleration due to gravity (g)
coin
feather
fall in air
fall in vacuum
Why does the feather fall down
slower than the coin in air?
gravitational force
air resistance
fall in air
fall in vacuum
Air resistance depends on:
•
•
•
•
Size of the object
Shape of the object
Its velocity
=> parachute
• A ball is thrown vertically upwards
at an initial velocity of 100 m s-1.
• What is the acceleration of the ball
along the time of flight?
Acceleration always equals to 10 ms-2
pointing downwards