Forces and balance
A force is a push or a pull, measured in newtons. When all the forces on an object cancel each other out, it is in balance and its motion does not change.
You can start right here: nothing needed first.
Why
Why does the rope in a tug of war sometimes not move a centimetre, even though both teams are pulling with all their might?
At the fair or a school party, two teams pull on a rope. Everyone sweats and digs their heels in, and yet the red rag stays hanging in the middle. There is hard pulling going on, but nothing happens.
That is because the two forces are equally large and act in opposite directions. They cancel each other out. Only when one team pulls harder does the rope slide towards their side. This teaches you that it is not one force that counts, but all the forces together.
See it
Two teams pull on a crate. Use the sliders to set how hard each team pulls. Look at the arrows, the arrow for the resultant force and whether the crate moves.
The crate speeds up to the right
Description of the picture
A crate in the middle with a team pulling on it on the left and on the right. Each force is an arrow: the bigger the force, the longer the arrow. Below the crate the resultant force is shown as a single arrow, with its size in newtons. If both forces are equal, there is no resultant arrow and the crate stays put. Otherwise the crate slides towards the side of the bigger force.
Get it
A force is a push or a pull. When you push a trolley, pull a door open or kick a ball away, you exert a force. A force can set something moving, slow it down, change its direction or change its shape, like a ball you squash flat.
We measure forces in newtons, N for short. A small apple of 100 grams pulls on your hand with about 1 N. You measure forces with a force meter: a spring that stretches further the harder you pull.
A force has a size and a direction. That is why we draw forces as arrows. The length of the arrow shows how big the force is, the tip shows the direction. If several forces act on the same object, you combine them into one resultant force. If they point the same way, you add them. If they work against each other, you subtract, and the resultant force points towards the side of the bigger one.
If the resultant force is zero, the forces are balanced. An object that is standing still then stays still. An object that is moving keeps going straight on at the same speed. That is Newton's first law, also called the law of inertia. Only a resultant force changes the motion.
A ball you kick across the grass still comes to a stop because of friction. Friction is a force between two surfaces that acts against the motion. On a rough surface it is large, on ice or on rollers it is small. Without friction you could not even walk, because your shoes would slip away.
Forces in the same direction add up, opposite forces are subtracted. If the resultant force is 0 N, the forces are balanced and the motion does not change.
Worked example
Team A pulls a crate to the left with 350 N. Team B pulls to the right with 200 N and a helper with 100 N. How big is the resultant force and which way does the crate slide?
Add up the forces to the right.
200+
100= 300 The left side pulls with 350 N, the right with 300 N. They work against each other, so you subtract.
350−
300= 50 The bigger force points to the left, so the resultant force does too.
- Answer
The resultant force is 50 N to the left. The crate slides to the left.
Do it
Pick a level. You get a new exercise every time, checked straight away.
Misconceptions
Wrong: If something is moving, there must always be a force acting on it in the direction of motion.
An ice hockey puck glides on for a long time after it is hit, without anyone pushing it. It only slows down because of the small amount of friction.
Right: Without a resultant force, a moving object keeps going straight on at the same speed. A force is needed to change the motion, not to keep it going.
Wrong: If something is standing still, there are no forces acting on it.
A book on a table is pulled down by gravity and at the same time pushed up by the table.
Right: There are forces, but they are balanced: the resultant force is zero.
Wrong: Two forces of 40 N and 30 N always add up to 70 N.
That is only true if they point the same way. If they pull against each other, only 10 N is left.
Right: Always pay attention to the direction: add for the same direction, subtract for opposite directions.
Wrong: Friction is always a nuisance and useless.
Without friction your bike brakes would not work, tyres would skid on the road and you could not walk.
Right: Friction acts against motion. Sometimes you want it small, as with an ice skate, sometimes large, as when braking.
Try it at home
A rubber band as a force meter
What you need
- a strong rubber band
- a thick book
- a piece of string or ribbon
- a ruler
- four or five round pencils
How to do it
- Tie the string around the book and fasten the rubber band to the end of the string.
- Put the book on the table and pull very slowly on the rubber band until the book just starts to slide.
- Use the ruler to measure how long the rubber band is at that moment. A longer rubber band means a bigger force.
- Now lay the pencils side by side under the book, so that it can roll on them.
- Pull slowly again and measure how long the rubber band is now when the book starts to move.
Safely: Pull gently on the rubber band and keep it away from your face and eyes, because a rubber band can snap loose. Work on a table where the book cannot fall off.
What do you see, and why?
On the table the rubber band stretches much further than on the pencils. There is a lot of friction between the book and the table. On rolling pencils there is much less friction, so a smaller force is needed.
In real life
- On a slippery winter day the council spreads salt and sand on the pavement. The sand increases the friction, so you are less likely to slip.
- When a De Lijn bus brakes suddenly, you slide forwards. According to the law of inertia, your body simply wants to keep going straight on.
- A Christmas tree in a stand stays upright because the forces on it are balanced. A gust of wind can upset that balance.
- In the supermarket a full trolley is harder to get rolling than an empty one. You need a bigger force to change its motion.
History
In 1687 the English scholar Isaac Newton wrote his book on the laws of motion. The first law says that an object stays at rest or keeps moving in a straight line as long as no resultant force acts on it. The unit of force, the newton, is named in his honour.
At the table
Why do you have to push harder to get a full trolley moving than to keep it rolling afterwards?
A question to talk about together, for parents and children. There is no wrong answer.