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P5 C) Weight, Mass and Gravity
P5 C) Weight, Mass and Gravity
Gravity is the force of attraction between all masses. However, we only notice gravity from very large masses, such as planets or stars. Anything that is near a planet or a star is very strongly attracted towards the planet or star.
Gravity has two effects. The first effect is that gravity attracts everything to the ground (objects accelerate towards the ground). Secondly, gravity gives objects weight.
Weight and mass are not the same. Mass is the amount of stuff that is in an object; mass is not a force and it is measured in kilograms (kg). An object’s mass will be the same wherever it is situated; an object’s mass will be the same on the earth and the moon.
The weight of an object is the force of gravity pulling that object towards something. On earth, the weight of an object is the force that is pulling the object towards earth – the force comes about due to the gravitational field from the earth. Weight is a force and is measured in newtons (all forces are measured in newtons). The weight of an object is different depending on where the object is situated, and this is because different locations have different gravitational field strengths (GFS). On earth the gravitational field strength is around 9.8 Newtons per kg (N/kg). On the moon the gravitational field strength is much weaker at 1.6 Newtons per kg. Therefore, if we had the same object on the earth and the moon, the weight of the object on earth would be greater than the weight of the object on the moon. The mass of the object will be the same on both the earth and the moon (remember, the mass of an object is the amount of stuff that is in an object and the mass is the same everywhere).
We can work out the weight of an object by multiplying the mass by the gravitational field strength (GFS). The equation and formula triangle are shown below.
Gravity has two effects. The first effect is that gravity attracts everything to the ground (objects accelerate towards the ground). Secondly, gravity gives objects weight.
Weight and mass are not the same. Mass is the amount of stuff that is in an object; mass is not a force and it is measured in kilograms (kg). An object’s mass will be the same wherever it is situated; an object’s mass will be the same on the earth and the moon.
The weight of an object is the force of gravity pulling that object towards something. On earth, the weight of an object is the force that is pulling the object towards earth – the force comes about due to the gravitational field from the earth. Weight is a force and is measured in newtons (all forces are measured in newtons). The weight of an object is different depending on where the object is situated, and this is because different locations have different gravitational field strengths (GFS). On earth the gravitational field strength is around 9.8 Newtons per kg (N/kg). On the moon the gravitational field strength is much weaker at 1.6 Newtons per kg. Therefore, if we had the same object on the earth and the moon, the weight of the object on earth would be greater than the weight of the object on the moon. The mass of the object will be the same on both the earth and the moon (remember, the mass of an object is the amount of stuff that is in an object and the mass is the same everywhere).
We can work out the weight of an object by multiplying the mass by the gravitational field strength (GFS). The equation and formula triangle are shown below.
Weight is measured in Newtons (N), mass is measured in kilograms (kg) and gravitational field strength is measured in Newtons per kilogram (N/kg). Let’s now have a couple of examples using this formula triangle.
Example 1
I have a laptop that has a mass of 2.3 kg.
a) What is the weight of the laptop on earth? The GFS on earth is 9.8 N/kg.
b) What is the weight of the laptop on the moon? The GFS on the moon is 1.6 N/kg.
Part a – Earth
We work out weight by multiplying the mass by the gravitational field strength. The equation is shown below:
I have a laptop that has a mass of 2.3 kg.
a) What is the weight of the laptop on earth? The GFS on earth is 9.8 N/kg.
b) What is the weight of the laptop on the moon? The GFS on the moon is 1.6 N/kg.
Part a – Earth
We work out weight by multiplying the mass by the gravitational field strength. The equation is shown below:
We are told in the question that the mass of the laptop is 2.3 kg and the GFS on earth is 9.8 N/kg. Therefore, we find the weight by subbing these values into the formula.
Therefore, the weight of the laptop on earth is 22.54 N.
Part b – Moon
The second part of the question asks us to find the weight of the laptop on the moon. We work out the weight of the laptop on the moon by using the same equation.
Part b – Moon
The second part of the question asks us to find the weight of the laptop on the moon. We work out the weight of the laptop on the moon by using the same equation.
The mass of the laptop will be the same as before because the mass of an object is the amount of stuff in the object; this means that the mass of the laptop will still be 2.3 kg. The question tells us that the GFS on the moon is 1.6 N/kg. We find the weight by subbing in mass as 2.3 kg and GFS as 1.6 N/kg.
The weight of the laptop on the moon is 3.68 N.
Comparison
The weight of the laptop on earth (22.54 N) is greater than the weight of the laptop on the moon (3.68 N). This is because the gravitational field strength is greater on earth than the moon; the GFS on earth is 9.8 N/kg and the GFS on the moon is 1.6 N/kg. All objects will have a greater weight on earth compared to the moon.
Comparison
The weight of the laptop on earth (22.54 N) is greater than the weight of the laptop on the moon (3.68 N). This is because the gravitational field strength is greater on earth than the moon; the GFS on earth is 9.8 N/kg and the GFS on the moon is 1.6 N/kg. All objects will have a greater weight on earth compared to the moon.
Example 2
A rock on Neptune has a mass of 8 kg and a weight of 93.6 N. Find the gravitational field strength on Neptune.
The question is asking us to work out the gravitational field strength on Neptune. We find the calculation for GFS by covering g up in the formula triangle.
A rock on Neptune has a mass of 8 kg and a weight of 93.6 N. Find the gravitational field strength on Neptune.
The question is asking us to work out the gravitational field strength on Neptune. We find the calculation for GFS by covering g up in the formula triangle.
When we cover up g in the formula triangle, we see that we find GFS by dividing weight by mass. The question tells us that the weight of the rock is 93.6 N and the mass of the rock is 8 kg. We sub these values into the calculation.
Therefore, the gravitational field strength on Neptune is 11.7 N/kg.
Centre of Mass
Weight is a force that acts from a single point within an object. This single point is known as the centre of mass for an object, and we assume that all of the mass of an object is concentrated at this point. For a uniform object, the centre of mass will be in the centre of the object (a uniform object has a regular shape and the density of the object is the same throughout the whole object). The object below is a ball, and the centre of mass is at the centre of the ball.
Weight is a force that acts from a single point within an object. This single point is known as the centre of mass for an object, and we assume that all of the mass of an object is concentrated at this point. For a uniform object, the centre of mass will be in the centre of the object (a uniform object has a regular shape and the density of the object is the same throughout the whole object). The object below is a ball, and the centre of mass is at the centre of the ball.
Direct Proportion
The mass and weight of an object are directly proportional to each other; direct proportion means that the variables move in the same direction and by the same proportion. For example, if we were to halve the mass of an object, the weight of the object would also halve. If we were to triple the mass of an object, the weight of the object would also triple.
We measure the mass of an object using a mass balance (essentially a set of scales). We measure the weight of an object by using a spring balance (we measure the weight of our object by attaching it to the hook on the bottom of the spring balance. We then read the value on the scale; click here to see some pictures of what a spring balance looks like).
The mass and weight of an object are directly proportional to each other; direct proportion means that the variables move in the same direction and by the same proportion. For example, if we were to halve the mass of an object, the weight of the object would also halve. If we were to triple the mass of an object, the weight of the object would also triple.
We measure the mass of an object using a mass balance (essentially a set of scales). We measure the weight of an object by using a spring balance (we measure the weight of our object by attaching it to the hook on the bottom of the spring balance. We then read the value on the scale; click here to see some pictures of what a spring balance looks like).