Gravity is a driving force if you are going downhill and the wind is a driving
force if it is at your back - but a counter-force if you are going uphill!
Counter Forces or Resistive Forces
that push an object in the opposite direction to that it is already
travelling in make it decelerate - undergo negative acceleration or slow down.
These forces are called counter forces or resistive forces.
If driving forces do not equal counter
forces there is a net force acting on a system and the state of
rest or motion will be changed. As the counter force and driving force
do NOT cancel each other out there is either acceleration
(if the driving force wins!) or deceleration (if the counter force wins!).
Force Systems and Unbalanced Force Systems
forces will have no effect on the movement of an object: it will remain
stationary or, if it is already moving it will continue to move at the
same speed and in the same direction.
If the forces acting on an object do not cancel
each other out, an unbalanced force will act on the object.
unbalanced force will affect the movement of the object. How the movement
is affected depends on the direction and the size of the unbalanced force:
a stationary object will start to move in the direction of the unbalanced
an object moving in the direction of the force will speed up;
an object moving in the opposite direction
to the force will slow down;
the greater the size of the unbalanced force,
the faster the object will speed up or slow down.
When an unbalanced force acts on an object in a
particular direction its speed changes (it accelerates) in that direction.
The greater the force, the greater the acceleration. The bigger the mass
of an object, the greater the force needed to give the object a particular
is an equation that links force to the acceleration it causes.
It is called
the force (in newton)
the mass of the object the force is applied to (in kilogram)
the acceleration produced when the force acts on the mass
acceleration produced on all masses when they are
near the Earth's surface is called the acceleration due to gravity.
This is the same whatever the mass of the object!
It is about 10m/s2 on Earth.
you drop a box of feathers from the top of a building at the same time
as a friend drops a box (of same size and dimensions) of lead from the same height, they will both
reach the ground at the same time! They will both accelerate at 10m/s2. (We would have to make the containers the same size and shape to cancel out the effect of air resistance).
If we drop items from the same height on the moon, however, their shape and size makes no difference as there is no atmosphere and therefore no air resistance.
See this video:
force that acts on the mass of an object because of gravity is called
Therefore weight is a force and is measured in newtons.
Weight is the force of gravity acting on a
mass that is positioned in a gravitational field. It is measured in newtons. The weight of an object may be considered to act at a single point referred to as the object's 'centre of mass'