s = ut + 1/2at2

u and a are positive

s

t

v = u + atu is positive and a is negative

v

t

v2 = u2 + 2asu 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

fall in air fall in vacuum

coin

feather

Acceleration due to gravity (g)

Why does the feather fall down slower than the coin in air?

fall in air fall in vacuum

gravitational force

air resistance

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