Unbalanced Forces and Net Force: Discover How Forces Change Motion

A force is a push or pull that acts on an object. Forces can make objects start moving, stop moving, speed up, slow down, or change direction. You use forces every day when you kick a ball, open a door, or pick up your backpack.

You already learned about different types of forces in Force Strength and Effects of Different Forces. Now you will discover what happens when forces are not equal.

Balanced forces happen when two forces pushing in opposite directions are equal in strength. When forces are balanced, the net force is zero and the object does not change its motion it stays still or keeps moving the same way.

Unbalanced forces happen when the forces on an object are NOT equal. When forces are unbalanced, the net force is greater than zero and the object changes its motion. You can explore this idea more in Balanced Forces and Equal Opposing Forces.

Think about a tug-of-war game. If Team A pulls with 20 N and Team B pulls with 35 N, the forces are unbalanced. The rope moves toward Team B because Team B has the greater force.

Net force is the total combined force acting on an object after you add all the forces together, taking direction into account. It tells you whether the forces are balanced or unbalanced.

When two forces push in opposite directions, you subtract the smaller force from the larger force to find the net force. When two forces push in the same direction, you add them together.

Example: You push a ball to the right with 8 N and friction pushes back with 3 N. The net force is 8 3 = 5 N to the right. The ball moves right because the net force is unbalanced in that direction.

An unbalanced net force causes a change in motion. Here is what can happen:

When a larger force pushes in the same direction something is already moving, the object speeds up. When a larger force pushes against the motion, the object slows down. A sideways force can change direction.

Friction is a force that slows down objects that are moving. It acts in the opposite direction of motion when two surfaces rub together. A ball rolling on carpet slows down because of friction.

Gravity is a force that always pulls objects straight downward toward Earth. When you drop a leaf or a rock, gravity pulls both of them down. You learned more about this in Gravitational Forces and Effects on Objects.

Forces like gravity and friction are always acting on objects around you. Even a book sitting still on a table has balanced forces gravity pulls it down and the table pushes it up with equal force.

Force: A force is a push or pull that acts on an object. Forces can change the motion, direction, or shape of objects. For example, kicking a ball is a push force.

Net Force: Net force is the combined result of all forces acting on an object. You find it by adding forces in the same direction or subtracting forces in opposite directions. It tells you if forces are balanced or unbalanced.

Unbalanced Forces: Unbalanced forces are unequal forces that cause motion to change. For example, if one team pulls harder in tug-of-war, the rope moves their way because the forces are unbalanced.

Balanced Forces: Balanced forces cancel each other out so nothing changes. For example, two equally strong people pushing on opposite sides of a door keep it still because the forces are balanced.

Friction: Friction is the rubbing force between surfaces that slows things down. For example, a ball rolling on grass slows down and stops because of friction between the ball and the grass.

Motion: Motion means an object is changing its position from one place to another. When an unbalanced force acts on an object, it causes a change in motion the object can start, stop, speed up, slow down, or turn.

Gravity: Gravity is a force that pulls all objects downward toward Earth. It is why things fall when you drop them. Gravity acts on every object that has mass.

Newton (N): A Newton is the unit scientists use to measure the strength of a force. When you see "10 N," it means a force of 10 Newtons is being applied.

Try these activities to strengthen your understanding of unbalanced forces and net force:

• Push a toy car with a small force, then a large force. Notice how the car moves differently each time this shows how force strength affects motion.
• Play tug-of-war with a friend. Try pulling with equal force, then have one person pull harder. Watch how the rope moves when forces become unbalanced.
• Roll a ball on a smooth floor and then on a carpet. You will see how Motion in Materials and Movement Through Air and Water is affected by friction on different surfaces.

Before mastering unbalanced forces and net force, it helps to understand these foundational topics:

• Force Strength and Effects of Different Forces You learned how the size of a force affects what happens to an object. A bigger force causes a bigger change in motion.
• Gravitational Forces and Effects on Objects You discovered that gravity is a pulling force that acts on all objects and pulls them toward Earth.
• Magnetic Forces, Attraction and Repulsion You explored how magnets can push or pull objects without touching them, which is another example of a force causing motion.
• Motion in Materials and Movement Through Air and Water You learned how objects move through different materials and how resistance forces like friction slow them down.

Understanding unbalanced forces and net force connects to many other important science ideas. Here is how they all fit together:

• Balanced Forces and Equal Opposing Forces This topic is closely related because balanced forces are the opposite of unbalanced forces. When you understand both, you can predict whether an object will move or stay still.
• Magnetic Forces, Fields and Interaction Magnetic forces are a great example of forces that can be balanced or unbalanced. You will see how magnetic attraction and repulsion create net forces on objects.
• Machine Types, Levers, Pulleys, and Inclined Planes Simple machines use forces to make work easier. Understanding net force helps you see how machines change the direction or size of forces.
• Work and Force, Mechanical Advantage When an unbalanced force moves an object, work is done. This topic builds directly on your understanding of net force and motion.
• Energy Types, Potential and Kinetic Energy When an unbalanced force causes an object to move, it gains kinetic energy. You will explore this connection in your next science lessons.
• Energy Conversion, Transformations Between Forms Forces cause energy to change from one form to another. Your knowledge of net force will help you understand how energy transforms.
• Physical Properties, Mass, Volume, and Density The mass of an object affects how much force is needed to change its motion. Heavier objects need a greater net force to start moving.