Force due to Gravity
Gravity always tries to accelerate an object towards the centre of the earth at 9.81 m/s 2 . We could say: g = 9.81 m/s 2
If an object is dropped, the acceleration due to gravity creates a force on the object, known as the gravitational force, F g
.
F g
= mg
If an object is on the earth’s surface, the force is felt even if the object does not accelerate. We sense this force as our weight.
Weight = F g
Weight: the gravitational force acting on an object
Mass: the amount of matter that makes up an object
Ex. What is the weight of a 50 kg box?
F g
= mg
= 50 kg x 9.81 m/s 2
= 490.5 N
= 0.49 kN
Note that in the SI system, weight has units of N

On different planets an object’s mass is constant, but its weight will vary because g is different on different planets.
Ex.
On Mars g is about 3.74 m/s the 50 kg object on Mars?
2 . What is the weight of
F g
= mg
= 50 kg x 3.74 m/s 2
= 187 N
= 0.19 kN
If you diet, and want weight loss, you really want mass loss.
In the old Imperial units, the unit of force is the pound, and the unit of mass is the slug!
On earth, a weight of 2.2 lbs is equivalent to a mass of 1.0 kg.
Ex. What is your weight in N on earth?