Weight, Resultant Forces & Vector Diagrams

Gravity and weight

• Weight is the force acting on an object due to gravity. Near the Earth it is due to the Earth's gravitational field
• Weight depends on the gravitational field strength where the object is (recall this equation):

$$W = m g$$

• $W$ weight in newtons, N; $m$ mass in kg; $g$ gravitational field strength in N/kg, always given (9.8 N/kg on Earth, 1.6 N/kg on the Moon)
• Worked example: a 70 kg person on Earth weighs $70 \times 9.8 = 686$ N; on the Moon, $70 \times 1.6 = 112$ N. The mass is the same in both places
• Weight and mass are directly proportional: $W \propto m$
• The weight of an object can be considered to act at a single point, its centre of mass
• Weight is measured with a calibrated spring balance (a newtonmeter); mass with a balance