Weight is the force acting on an object due to gravity. The force of…

The idea

Weight is the force acting on an object because of gravity. Near the Earth it exists because the Earth has a gravitational field around it, a region in which any mass experiences a force. The weight of an object always acts towards the centre of the Earth, which on a flat classroom diagram looks like straight down but on a diagram of a satellite in orbit does not. The field is strongest at the surface and gets weaker with distance, so a satellite high above the Earth weighs less than the same satellite on the ground, though the field never vanishes. The gravitational force on an orbiting object continually changes the direction of its motion and so its velocity.

What earns the marks

• Weight being the force acting on an object due to gravity.
• A gravitational field being the region around a mass where another mass experiences a force.
• The weight acting towards the centre of the Earth.
• The field strength, and so the weight, decreasing as distance from the Earth increases.

Common mistakes and how to put them right

• Defining weight as how heavy something is, and quoting it in kilograms.

• Drawing weight straight down the page on a diagram of an object in orbit.

• Claiming there is no gravity in orbit because astronauts appear weightless.

• Confusing gravitational field strength with the weight itself.

• Saying weight is caused by the atmosphere pressing down.

Exam tips

• Use the field definition wording, 'the region where another mass experiences a force', which mirrors the accepted electric field answer.
• On any orbit diagram, draw the force arrow pointing at the centre of the Earth.
• Keep weight in newtons and mass in kilograms throughout, even in a written answer.