Science
Gravity, force and weight
When an unbalanced force is applied to an object, the object will accelerate. The size of the acceleration depends upon the force applied and the mass of the object. There is a relationship between force, mass and acceleration (force = mass × acceleration).
Gravity is a force that attracts objects with mass towards each other. The weight of an object is the force on it due to gravity. There is a relationship between weight, mass and acceleration of free-fall (weight = mass × acceleration of free-fall). The gravitational field strength of the Earth is 10 N/kg and its acceleration of free-fall is 10 m/s2.
Mass and force
All objects have mass. It is a measure of how much material the object contains, and it is measured in kilograms (kg). When a force is applied to an object it will accelerate. The size of this acceleration depends on the size of the force and the mass of the object.
Size of the force
When an unbalanced force is applied to an object, the object will accelerate. The bigger the force, the greater the acceleration.
Doubling the size of the unbalanced force doubles the acceleration.
The mass
An object will accelerate in the direction of an unbalanced force. The bigger the mass of the object, the smaller the acceleration.
Doubling the mass halves the acceleration.
Calculating acceleration due to a force
You should know the equation that shows the relationship between force, mass and acceleration, and be able to use it.
The equation
force (N) = mass (kg) × acceleration (m/s2)
You should see that 1N is the force needed to give 1kg an acceleration of 1m/s2.
For example, the force needed to accelerate a 10kg mass by 5m/s2 is 10 × 5 = 50N.
The same force could accelerate a 1kg mass by 50m/s2 or a 100kg mass by 0.5m/s2.
- Question
A rocket has a mass of 2,000 kg.
The force created by its engine is 3,000 newtons.
Calculate the acceleration caused by this force.
- Answer
- Write down and rearrange the equation - force = mass × acceleration
- Rearrange the equation - acceleration = force ÷ mass
- Put in the values - acceleration = 3,000N ÷ 2,000kg
- Work out the answer and write it down - acceleration = 1.5m/s2
Try this actvity to practise using the equation.
Gravity
Gravity is a force that attracts objects towards each other. Any object with mass exerts a force of gravity. The greater the mass, the greater the force. The force of gravity between two objects decreases as the objects move further apart.
The force of gravity is not very noticeable unless one of the objects – such as the Earth or the Sun – has a huge mass.
Gravity attracts objects towards the centre of the Earth
Gravity:
- pulls objects on Earth towards the centre of the planet
- holds the Earth's atmosphere in place
- holds all the components of the solar system in orbit around the Sun
- holds all the components of the galaxy together
Weight
Weight is not the same as mass. The mass of a given object is the same everywhere, but its weight can change. The weight depends on the mass of the object and the strength of the gravitational field around it.
Gravitational field strength
The gravitational field strength of the Earth is 10 N/kg (ten newtons per kilogram). This means that an object with a mass of 1 kg would be attracted towards the centre of the Earth by a force of 10 N. We feel forces like this as weight.
You would weigh less on the Moon than you do on Earth because the gravitational field strength of the Moon is one-sixth that of the Earth. But note that your mass would stay the same.
Weight
If you drop an object on the Earth, it accelerates towards the centre of the Earth at 10 m/s2. This is called the acceleration of free-fall. You can calculate the weight of an object using this equation:
weight (N) = mass (kg) × acceleration of free-fall (m/s2)
- Question
A person has a mass of 60 kg. How much do they weigh on Earth, if the acceleration of free-fall is 10 m/s2?
- Answer
weight = mass × acceleration of free-fall
= 60 kg × 10 m/s2
= 600 N
- Question
How much would the same person weigh on the Moon, if the acceleration of free-fall there is 1.6m/s2?
- Answer
weight = mass × acceleration of free-fall
= 60 kg × 1.6 m/s2
= 96 N
Now try a Test Bite