On a bag of one kilogram, the force of gravity is 9.81 newtons, because the Earth pulls on every kilogram with 9.81 newtons. For any mass you calculate it with Fz is m times 9.81, with the mass in kilograms and the force in newtons. You draw that force as an arrow pointing down from the centre of gravity.
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A one-kilogram bag of sugar. How big is the force of gravity on that bag, in newtons? Make a guess first, before you watch on.
You see why the Earth pulls on every kilogram with 9.81 newtons, and how you use that to calculate the force of gravity for any mass: Fz is m times 9.81, with Fz in newtons and m in kilograms. Also the difference between mass and gravity, and why gravity on the Moon is about six times smaller while your mass stays the same. Then two sums, a rucksack and an apple, where you first convert the mass to kilograms. Finally how to draw a force: an arrow from the point of application, in the direction of the force, with a length according to the force scale.
For high school physics, in the chapter on forces. Halfway through there is a question and a pause: stop the video there and do the sum yourself first.
In this video
The full explanation, in writing
A bag of sugar
0:00A one-kilogram bag of sugar. How big is the force of gravity on that bag, in newtons? Just make a guess.
Nine point eight one per kilogram
0:13The Earth pulls on every kilogram with nine point eight one newtons. So on a one-kilogram bag of sugar the force of gravity is nine point eight one newtons, almost ten. For every mass: F z is m times nine point eight one, with F z in newtons and m in kilograms.
0:33Mind the difference between those two. Your mass is in kilograms, gravity is a force, in newtons. On the Moon your mass is exactly the same, but gravity is about six times smaller.
A rucksack and an apple
0:49Always convert the mass to kilograms first. A rucksack of six point five kilograms: six point five times nine point eight one is sixty-four newtons. An apple of one hundred and twenty grams: first to kilograms, that is zero point one two zero kilograms.
1:06Times nine point eight one gives one point one eight newtons. Your turn. A dog has a mass of twenty point zero kilograms. How big is the force of gravity on that dog?
A force as an arrow
1:22Twenty point zero times nine point eight one is one hundred and ninety-six newtons. You draw a force as an arrow. The arrow starts at the point of application, the point where the force acts on the object; for gravity, that is the centre of gravity.
1:38The arrow points in the direction of the force, and gravity always points straight down. The length of the arrow gives the magnitude, using a force scale: you agree how many newtons one centimetre stands for. If you choose one centimetre for fifty newtons, the arrow becomes one hundred and ninety-six divided by fifty, three point nine centimetres long.
Three things to remember
2:03Three things. You calculate gravity with F z is m times nine point eight one, with the mass in kilograms and the force in newtons. Mass and gravity are not the same: mass is in kilograms, gravity is a force in newtons. And you draw a force as an arrow from the point of application, in the direction of the force, with a length according to the force scale.
Two questions
2:29Two questions. One. A bike has a mass of fifteen point zero kilograms. How big is the force of gravity on the bike? Two. You draw a force of thirty newtons with the scale one centimetre for ten newtons. How long will the arrow be?
The answers
2:49The first: fifteen point zero times nine point eight one is one hundred and forty-seven newtons. The second: thirty divided by ten is three. The arrow will be three point zero centimetres long.
Next: the centre of gravity
3:02Want to know where the centre of gravity lies for an object without a regular shape? Then watch the video about the centre of gravity on my YouTube channel.
Frequently asked questions
How do you calculate the force of gravity?
With Fz is m times 9.81, with Fz in newtons and m in kilograms. The Earth pulls on every kilogram with 9.81 newtons. So a rucksack of 6.5 kilograms has a force of gravity of 6.5 times 9.81, which is 64 newtons.
What is the difference between mass and gravity?
You give mass in kilograms, and it is the same everywhere. Gravity is a force, in newtons, and depends on where you are. On the Moon your mass is exactly the same, but gravity is about six times smaller.
What do you do if the mass is given in grams?
Convert it to kilograms first. An apple of 120 grams is 0.120 kilograms, and 0.120 times 9.81 gives a force of gravity of 1.18 newtons.
How do you draw a force with a force scale?
As an arrow that starts at the point of application, for gravity the centre of gravity, and points in the direction of the force. The length follows from the force scale: with one centimetre for fifty newtons, a force of 196 newtons becomes an arrow of 196 divided by 50, so 3.9 centimetres.
Which way does gravity point?
Always straight down, towards the centre of the Earth. The arrow for gravity starts at the centre of gravity of the object.
Mass is in kilograms, gravity is a force in newtons.
Physics with Thomas makes physics explainers for secondary school. No words on screen, so what you see works in any language.
In this series
Types of forces3:06
Calculating gravity3:15
Centre of gravity2:48
Force meter: range and precision3:11
Elastic force and spring constant3:41
Directly proportional relationship3:37
About me
My name is Thomas Schuurmans, and I have a passion for physics and for understanding why things work the way they do. What I want is to get young people asking “why is that?” and “how does that work?” more often, and to hand them the tools of physics in a way that is simple and that you can see.
I graduated in applied physics at Delft University of Technology in 2003. After that I spent more than twenty years outside education: first at TNO, the Dutch applied research institute, then at a design agency, and eventually founding Proportion Global, through which I work on innovation questions in Africa, Latin America and South Asia. That work resembles physics more than you would expect: don't start from a solution, first understand what is going on, try something, be wrong, and look again.
Since 2026 I have been teaching physics to both lower and upper secondary classes, and I started a master's at the University of Amsterdam for my full teaching qualification. That is where I learned that a secondary school pupil's real attention for new material lasts about seven minutes. And my own weakness happens to be telling too many side stories.
That is where the idea came from: videos that explain one topic sharply and visually, inside those seven minutes. I make them for my own pupils. Then I publish them, because good explanation should be within reach of anyone who needs it, wherever you live and whatever language you think and speak in.