Weight, Mass & Gravity (Edexcel GCSE Physics: Combined Science)

• Weight is defined as:

The force acting on an object due to gravitational attraction

• Planets have strong gravitational fields
  • Hence, they attract nearby masses with a strong gravitational force

• Because of weight:
  • Objects stay firmly on the ground
  • Objects will always fall to the ground
  • Satellites are kept in orbit

Some of the phenomena associated with gravitational attraction and the weight force

• Weight, mass and gravitational field strength are related using the equation:

• g is known as the acceleration due to gravity or the gravitational field strength
  • On Earth, this is equal to 9.81 m/s² (or N/kg)

• The weight that an object experiences depends on:
  • The object's mass
  • The mass of the planet attracting it

• Mass (measured in kilograms, kg) is related to the amount of matter in an object
• Weight (measured in newtons, N) is the force of gravity on a mass
  • The weight of an object and the mass of an object are directly proportional
  • The size of this force depends on the gravitational field strength (often called gravity, g, for short)

• Mass is commonly measured using a top pan balance
  • The weight can then be indirectly found through calculation using:

W = mg

• Where g on Earth is ≈ 10 N/kg
• Weight can be measured directly using a calibrated spring-balance, also known as a newton-meter
  • This device is a type of weighing scale which measures force in Newtons
  • It consists of a spring fixed at one end with a hook to attach an object at the other

Weight can be measured using a top-pan balance or a newton-meter

• An object’s mass always remains the same, however, its weight will differ depending on the strength of the gravitational field on different planets
• For example, the gravitational field strength on the Moon is 1.63 N/kg, meaning an object’s weight will be about 6 times less than on Earth

On the moon, your mass will stay the same but your weight will be much lower

• The value of g (gravitational field strength) varies from planet to planet depending on their mass and radius
• A few examples of varying gravitational field strength are shown below:

Gravitational field strength of the planets in our solar system

Step 1: List the known quantities

• • Weight on Mars, W_M = 190 N
  • Gravitational field strength on Mars, g_M = 3.8 N/kg
  • Gravitational field strength on Earth, g_E = 9.8 N/kg

Step 2: Write out the equation relating mass and weight and rearrange for mass

W = mg

• • Divide both sides by g:

Step 3: Calculate the student’s mass

• • The student’s mass is the same anywhere in the universe

Step 4: Calculate the student’s weight on Earth

W_E = m × g_E = 50 × 9.8 = 490 N