# 5.1.4 Weight, Mass & Gravity

### Defining Weight

• 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

• Mass (measured in kilograms, kg) is related to the amount of matter in an object
• The more mass an object has the larger the weight force it will experience
• Since weight is a force, it is measured in Newtons (N)
• The weight that an object experiences depends on:
• The object’s mass
• The mass of the planet attracting it

### Measuring Weight

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

#### Exam Tip

Since mass is measured in kilograms in Physics, if it is given in grams make sure to convert to kg by dividing the value by 1000!

### Centre of Mass

• The centre of mass of an object (sometimes called the centre of gravity) is defined as:

The point through which the weight of an object acts

• For a symmetrical object of uniform density, the centre of mass is located at the point of symmetry
• For example, the centre of mass of a sphere is at the centre

The centre of mass of a regular shape can be found by symmetry

• The centre of mass of an irregular object can be found by locating its balance point
• A broomstick has a centre of mass slightly closer to the head of the broom since there is more mass located there

The centre of mass of a broomstick which is also its balance point

#### Exam Tip

Since the centre of mass is a hypothetical point, it can lie inside or outside of a body. The centre of mass will constantly shift depending on the shape of a body. For example, a human body’s centre of mass is lower when learning forward than when stood upright

However, make sure that when you are drawing force diagrams to draw the forces as if they were acting on the centre of mass of the object!

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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