Weight, Mass & Gravity (Edexcel GCSE Physics)
Revision Note
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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
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)
Examiner Tips and Tricks
It is a common misconception that mass and weight are the same, but they are in fact very different
Since weight is a force - it is a vector quantity
Since mass is an amount - it is a scalar quantity
Measuring Weight
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
Examiner Tips and Tricks
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!
Weight & Gravity
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
Worked Example
A student estimates she would have a weight of 190 N on Mars. Calculate the weight of the student on Earth. The gravitational field strength on Earth is 9.8 N/kg The gravitational field strength on Mars is 3.8 N/kg.
Answer:
Step 1: List the known quantities
Weight on Mars, WM = 190 N
Gravitational field strength on Mars, gM = 3.8 N/kg
Gravitational field strength on Earth, gE = 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
WE = m × gE = 50 × 9.8 = 490 N
Examiner Tips and Tricks
You do not have to remember the gravitational field strength g on other planets, but just that it increases with the mass of the planet. The value of g will be given in your exam questions.
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