Net Force

Net Force

Middle School

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Learning Objectives

• Define net force and explain how it affects an object's motion.

• Explain the difference between balanced and unbalanced forces.

• Calculate the net force when forces act in the same or opposite directions.

• Predict the direction of motion based on the calculated net force.

Key Vocabulary

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Force

A push or a pull that has the ability to change the motion of an object.

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Net Force

The total combination of all forces that are acting on a single object at the same time.

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Balanced Forces

Equal forces acting on one object in opposite directions, resulting in a net force of zero.

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Unbalanced Forces

Unequal forces that cause a change in an object's motion, speed, or its direction.

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Friction

A force that works against motion when two surfaces are rubbing or in contact with each other.

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Gravity

The fundamental force that pulls all objects on Earth downward toward the center of the planet.

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What is Net Force?

• A force is a push or pull on an object, measured in Newtons (N).

• The net force is the overall force found by combining all forces on an object.

• When the net force is not zero, it causes the object to accelerate.

• The object's acceleration is always in the same direction as the net force.

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Multiple Choice

What is the term for the total amount of force acting on an object?

• Gravity

• Net Force

• Friction

• Acceleration

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Balanced vs. Unbalanced Forces

Balanced Forces

• Forces are equal and in opposite directions, making the net force zero.

• They do not cause a change in the object's state of motion.

• A book resting on a table is an example, with its net force being 0 N.

Unbalanced Forces

• Forces are unequal, making the net force greater than zero.

• They cause a change in the object's motion, known as acceleration.

• In tug-of-war, the rope moves in the direction of the larger force.

Multiple Choice

What is the result of balanced forces acting on an object?

• The object accelerates.

• The object's motion does not change.

• The object changes direction.

• The object stops moving immediately.

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Calculating Net Force

Same Direction

• ​When multiple forces act on an object in the same direction, you add the forces together.

• ​​The result of adding these forces together is a stronger overall force called the net force.

• ​For example, 25 N + 20 N in the same direction gives a total net force of 45 N.

Opposite Directions

• ​If forces are in opposite directions, you find the net force by subtracting the smaller force from the larger one.

• ​​If unbalanced, the object moves in the direction of the greater force; if balanced, the net force is 0 N.

• ​For example, 12 N to the right and 10 N to the left results in a net force of 2 N right.

Multiple Choice

Two people push a box in the same direction. One pushes with a force of 10 N and the other with a force of 15 N. What is the net force on the box?

• 5 N

• 10 N

• 15 N

• 25 N

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Common Misconceptions

Misconception	Correction
If an object is not moving, there are no forces acting on it.	An object can have multiple balanced forces, resulting in zero net force.
A net force of zero always means the object is stationary.	A net force of zero means no acceleration, not just being stationary.
Unbalanced forces only make things start moving.	Unbalanced forces change motion: speeding up, slowing down, or changing direction.

Multiple Choice

A box is being pushed to the right with 20 N of force and pulled to the left with 30 N of force. How will the box's motion be affected?

• It will move right with a force of 10 N.

• It will move left with a force of 10 N.

• It will not move.

• It will move left with a force of 50 N.

Multiple Choice

If you see an object moving at a constant speed in a straight line, what can you conclude about the forces acting on it?

• The forces are unbalanced.

• There are no forces acting on it.

• The forces are balanced.

• The force of gravity is strongest.

Multiple Choice

Two people are pushing a heavy crate to the right. The net force on the crate is 150 N to the right. If Person A is pushing with a force of 70 N, what force is Person B applying?

• 80 N to the right

• 80 N to the left

• 220 N to the right

• 70 N to the right

Multiple Choice

An object has a downward gravitational force of 15 N. A table pushes upward on it with a force of 15 N. If a person then pushes the object to the right with a force of 10 N, what is the net force and resulting motion?

• The net force is 0 N, and the object does not move.

• The net force is 40 N, and the object moves diagonally.

• The net force is 10 N to the right, and the object accelerates to the right.

• The net force is 15 N downward, and the object falls.

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Summary

• The net force is the sum of all forces acting on an object.

• Balanced forces have a net force of zero and cause no change in motion.

• Unbalanced forces have a net force greater than zero, causing the object to accelerate.

• Add forces in the same direction and subtract them in opposite directions.

Poll

On a scale of 1-4, how confident are you about calculating and describing net force?

• 1

• 2

• 3

• 4