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Weight, Mass & Gravity (Edexcel GCSE Physics: Combined Science)
Revision Note
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 Tip
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 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!
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/kgThe gravitational field strength on Mars is 3.8 N/kg.
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 Tip
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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