Mechanics
Freefall under Gravity
Objective: Be able to model gravity
as an acceleration
M1
All objects that fall freely under gravity on Earth experience the same acceleration,
no matter how heavy they are. This acceleration is 9.8 ms-2 towards the centre of
the Earth.
2
g  9.8 ms 
Make sure that the acceleration is in the correct direction. If u = 3.5 ms-1 upwards,
then the acceleration is -9.8 ms-2 upwards. Each displacement, velocity and
acceleration must be in the same direction.
Eg.
A brick is thrown off the top of a 19.6m building. If the initial velocity is
1.5 ms-1 downwards, find:
a) the velocity of the brick when it hits the ground,
b) the time taken for the brick to hit the ground.
1. A ball is dropped from a height of 28m. How long after being released does the
ball hit the ground?
2.39 s
2. A brick is thrown downwards with an initial velocity of 10 ms-1. How far has it
travelled when its velocity is 108 ms-1?
590 m
3. A ball is thrown downwards off a building. If it takes 5 seconds to fall 30 m, what
was its initial velocity?
-18.5 ms-1
4. Another ball is thrown off a building. If it reaches a velocity of 49 ms-1, 12 m below
its starting point, what was its initial velocity? How long does it take to reach 49
ms-1?
46.5 ms-1 or -46.5 ms-1
2.01 s or 4.97 s
Eg.
A ball is thrown vertically upwards with velocity 14.7 ms-1 from a platform
19.6 m above the ground. Find:
a) the time taken for the ball to reach the ground,
b) the velocity of the ball when it reaches the ground.
Eg.
A particle is projected vertically upwards with a velocity of 34.3 ms-1. Find
how long after projection the particle is at a height of 49m above the point
of projection for:
a) the first time,
b) the second time.
1. A ball is dropped from a height of 35m with initial velocity 3 ms-1 upwards. How
long after being released does the ball hit the ground?
2. A brick is thrown upwards with an initial velocity of 10 ms-1. How far has it
travelled when its velocity is 10 ms-1 downwards?
3. A bullet is fired vertically upwards with an initial acceleration of 3000 ms-2, for 0.2
seconds, at which point it leaves the gun barrel. How long after leaving the gun
barrel does it return to strike the shooter?
A-Level Past Paper Question:
A ball is thrown vertically upwards from the ground with initial speed 11.2 ms-1.
a) Find the greatest height reached.
b) Calculate the length of time the ball is more than 2.8m above the ground.
c) State one assumption you have made in your solution of the problem.