Balanced and Unbalanced Forces

Forces: the pushes and pulls that shape motion

Every time something starts moving, stops, speeds up, slows down or changes direction, a force is responsible. A force is simply a push or a pull. We measure forces in newtons (N), named after Isaac Newton.

What makes forces interesting is that an object almost never has just one force acting on it. A book resting on a table has gravity pulling it down and the table pushing it up. A cyclist has the push of the pedals and the drag of air resistance and friction. To understand what an object will actually do, we have to look at all the forces together and work out whether they are balanced or unbalanced. That single idea explains an enormous amount of physics. (For the deeper rules behind it, see Newton's laws of motion.)

Forces have size and direction

A force is not just "how strong" — it also has a direction. A 10 N push to the right is very different from a 10 N push to the left. This is why we often draw forces as arrows: the length of the arrow shows the size of the force, and the way it points shows the direction.

A diagram that shows all the forces on a single object as arrows is called a free-body diagram. For a box sitting on the floor you would draw:

• An arrow pointing down for its weight (gravity).
• An arrow of equal length pointing up for the support force from the floor.

These two arrows are the same length but point opposite ways. That is your first example of balanced forces.

Balanced forces

Forces are balanced when they cancel each other out completely. The forces pulling one way are exactly matched by forces pulling the other way. When this happens, we say the resultant force (also called the net force) is zero.

Here is the crucial rule:

When forces are balanced, the object's motion does not change.

This does not mean the object must be still. Balanced forces lead to one of two situations:

1. A stationary object stays still (like a book on a table).
2. A moving object keeps moving at a constant speed in a straight line (like a car cruising at a steady 50 km/h on a flat road, where the engine's forward force exactly balances friction and air resistance).

This is Newton's first law: an object continues at rest or at constant velocity unless acted on by a resultant force. Motion does not need a force to keep going — it only needs a force to change.

Unbalanced forces

Forces are unbalanced when they do not cancel out. One direction "wins", leaving a resultant force that is not zero.

When forces are unbalanced, the object's motion changes.

An unbalanced force always causes one or more of these:

• The object speeds up (if the resultant force points the way it is moving).
• The object slows down (if the resultant force points the opposite way).
• The object changes direction.

Any change in speed or direction is called acceleration. So we can say it simply: an unbalanced force causes acceleration. The bigger the resultant force, the bigger the acceleration.

How to find the resultant force

The resultant force is the single force that has the same overall effect as all the forces combined. Finding it (for forces along one line) is easy:

• Forces in the same direction → add them.
• Forces in opposite directions → subtract them.
• The resultant points in the direction of the larger side.

Worked example 1 — adding forces. Two children push a stuck trolley in the same direction, one with 80 N and one with 120 N.

Resultant = 80 + 120 = 200 N in the pushing direction.

The forces are unbalanced, so the trolley accelerates forwards.

Worked example 2 — opposing forces. A car's engine provides a forward driving force of 3000 N. Friction and air resistance together push back with 1200 N.

Resultant = 3000 − 1200 = 1800 N forwards.

The forces are unbalanced and point forwards, so the car accelerates forwards.

Worked example 3 — a tug of war. The red team pulls left with 600 N; the blue team pulls right with 600 N.

Resultant = 600 − 600 = 0 N.

The forces are balanced. The rope does not move — until one team tires and the forces become unbalanced.

A real-world story: the skydiver

The skydiver is the perfect example of forces changing from unbalanced to balanced.

1. The moment they jump, gravity (their weight, perhaps 700 N) pulls them down. Air resistance is small. The forces are unbalanced with a big downward resultant — so they accelerate and speed up.
2. As they speed up, air resistance grows (faster movement means more air pushing back).
3. Eventually the upward air resistance grows until it exactly balances their 700 N weight. Now the resultant force is zero.
4. With balanced forces, they stop accelerating and fall at a steady, constant speed called terminal velocity.

When they open the parachute, air resistance suddenly becomes much larger than their weight. The forces are unbalanced upwards, so the resultant force points up — they decelerate to a new, slower terminal velocity, safe for landing. The whole skydive is a story of balanced and unbalanced forces.

Why this matters

This single idea — compare all the forces, find the resultant, and predict the motion — underlies how engineers design everything from bridges (which must have perfectly balanced forces so they don't move) to rockets (which need a huge unbalanced upward force to accelerate skyward). The forces you cannot see, like friction, are explored more in friction explained, and you can measure forces directly using the technique in measuring forces with springs.

Try it yourself! 🧪

Tug of war with elastic bands — see balance and imbalance.

You need a small object (a cork or eraser), two identical rubber bands, a ruler, and a friend.

1. Loop a rubber band onto each side of the cork.
2. You and your friend each pull one band, stretching them equally so the cork sits still in the middle. The forces are balanced — the cork does not move, even though both of you are pulling hard. The resultant force is zero.
3. Now have one person pull a little harder (stretch their band more). The cork jumps towards that person. The forces are now unbalanced, so there is a resultant force, and the cork accelerates in that direction.
4. Try pulling so that the cork moves at a steady, slow speed by keeping a constant difference — notice how hard it is, because any change in the resultant changes the motion.

You have just demonstrated Newton's first law: balanced forces leave the motion unchanged, while an unbalanced force always produces a change in motion.