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Forces and Motion - Balanced and Unbalanced Forces

Grade 7IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A force is a push or pull acting upon an object as a result of its interaction with another object, measured in Newtons (NN).

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Forces are vector quantities, meaning they have both a magnitude (size) and a specific direction.

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The Net Force (FnetF_{net}) is the vector sum of all individual forces acting on an object. It determines the object's state of motion.

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Balanced Forces occur when the net force acting on an object is zero (Fnet=0NF_{net} = 0 N). This results in no change in the object's motion: if it is at rest, it remains at rest; if it is moving, it continues at a constant velocity (vv).

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Unbalanced Forces occur when the net force acting on an object is not zero (Fnet≠0NF_{net} \neq 0 N). This causes the object to accelerate (aa), which means a change in speed, direction, or both.

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Newton’s First Law (Law of Inertia) states that an object will maintain its state of motion unless acted upon by an unbalanced external force.

📐Formulae

Fnet=∑F=F1+F2+...+FnF_{net} = \sum F = F_1 + F_2 + ... + F_n

F=m×aF = m \times a

Fnet=Fapplied−FfrictionF_{net} = F_{applied} - F_{friction}

Fnet=0  ⟹  a=0F_{net} = 0 \implies a = 0

💡Examples

Problem 1:

A wooden crate is pushed to the right with a force of 25N25 N. At the same time, friction acts against the movement with a force of 10N10 N to the left. Calculate the net force acting on the crate.

Solution:

Fnet=25N−10N=15NF_{net} = 25 N - 10 N = 15 N to the right.

Explanation:

Since the forces are acting in opposite directions, we subtract the smaller force from the larger one. Because Fnet≠0F_{net} \neq 0, the forces are unbalanced, and the crate will accelerate to the right.

Problem 2:

A lamp sits on a desk. Gravity pulls the lamp down with a force of 12N12 N, and the desk pushes up (normal force) with a force of 12N12 N. What is the net force and the state of motion?

Solution:

Fnet=12N−12N=0NF_{net} = 12 N - 12 N = 0 N

Explanation:

The forces are equal in magnitude and opposite in direction, resulting in a net force of zero. These are balanced forces, so the lamp remains at rest.

Problem 3:

A car is driving at a constant velocity of 20m/s20 m/s. If the engine provides a forward thrust of 2000N2000 N, what is the magnitude of the air resistance and friction acting against it?

Solution:

Ffriction=2000NF_{friction} = 2000 N

Explanation:

According to Newton's First Law, if an object moves at a constant velocity (vv), the acceleration is zero (a=0a = 0), which means the net force must be Fnet=0NF_{net} = 0 N. Therefore, the backward resistive forces must exactly balance the forward thrust.

Balanced and Unbalanced Forces - Revision Notes & Key Formulas | IB Grade 7 Science