A force is a push or pull.
Force has both size and direction and is measured in newtons (N). Several forces can act on the same object at once. Adding them with their directions gives the resultant, or net, force.
A non-zero net force causes acceleration: a change in velocity. That can mean speeding up, slowing down or changing direction.
Mass and weight are different.
Mass describes inertia and is measured in kilograms (kg). Weight is the gravitational force acting on that mass. Near the Earth’s surface, weight is approximately mass × 9.81 N/kg.
A 100 kg mass therefore weighs about 981 N. Its mass does not change because a lift starts moving, although the support force acting on it can change.
Compare forces on one object.
- Upward supportThe rope or platform exerts a force.
- Downward weightGravity pulls the object down.
- Net forceThe difference determines acceleration.
With only weight and upward support acting, support must be 981 N for the forces to balance.
Balanced does not necessarily mean stopped.
When net force is zero, velocity remains constant. An object may stay at rest or continue moving steadily in a straight line. A steady upward journey therefore does not require a net upward force.
To accelerate upward, the upward force must exceed the downward force. To slow an upward-moving object, the net force acts downward.
A counterweight reduces an imbalance.
A traction lift’s counterweight offsets part of the weight on the car side. The balance changes as passengers enter and leave; it is not perfectly balanced at every load.
Even when gravitational forces balance, moving the system still involves accelerating mass and overcoming losses. The counterweight does not make the car weightless.
A closer look
Newton’s second law is F = ma for constant mass. In a simplified vertical model, support minus weight equals mass times upward acceleration. This explains why the support force felt by passengers changes during acceleration, rather than throughout every upward or downward journey.
Check your understanding
Can a lift moving steadily upward have balanced forces?
Reveal the explanation
Yes. Constant velocity means zero acceleration, so the net force is zero. Starting or stopping requires a change in that balance.