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Forces and Energy - Forces (Gravity, Friction, Magnetism)

Grade 5IB

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

🔑Concepts

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

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Gravity is a non-contact force that pulls objects toward each other. On Earth, gravity pulls everything toward the center of the planet. Weight is the measure of the force of gravity on an object's mass (mm).

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Friction is a contact force that opposes motion when two surfaces slide or attempt to slide across each other. It generates heat and depends on the texture of the surfaces.

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Magnetism is a non-contact force produced by the motion of electric charges. Magnets have two poles: North (NN) and South (SS). Like poles repel (N−NN-N or S−SS-S), and opposite poles attract (N−SN-S).

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Air resistance is a type of friction (drag) that acts on objects moving through the air, acting in the opposite direction to the object's motion.

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Mass is the amount of 'stuff' or matter in an object, measured in kilograms (kgkg), and it does not change regardless of location. Weight changes depending on the local gravitational field strength (gg).

📐Formulae

W=m×gW = m \times g

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

gEarth≈9.8 m/s2≈10 N/kgg_{Earth} \approx 9.8 \text{ m/s}^2 \approx 10 \text{ N/kg}

💡Examples

Problem 1:

An astronaut has a mass of 70 kg70\text{ kg}. If the gravitational field strength on the Moon is gmoon≈1.6 N/kgg_{moon} \approx 1.6\text{ N/kg}, what is the astronaut's weight on the Moon?

Solution:

W=70 kg×1.6 N/kg=112 NW = 70\text{ kg} \times 1.6\text{ N/kg} = 112\text{ N}

Explanation:

To find the weight, we multiply the mass of the astronaut by the gravity of the Moon. Note that the mass remains 70 kg70\text{ kg} even though the weight is much less than on Earth.

Problem 2:

A wooden block is pushed across a floor with a force of 20 N20\text{ N} to the right. The friction between the block and the floor is 5 N5\text{ N}. What is the net force (FnetF_{net}) acting on the block?

Solution:

Fnet=20 N−5 N=15 N to the rightF_{net} = 20\text{ N} - 5\text{ N} = 15\text{ N} \text{ to the right}

Explanation:

Friction always acts in the opposite direction of motion. To find the net force, we subtract the frictional force from the applied pushing force.

Problem 3:

Two magnets are placed near each other. Magnet A has its North pole facing Magnet B's North pole. What force will occur?

Solution:

A force of repulsion (FrepelF_{repel}).

Explanation:

According to the laws of magnetism, like poles (North and North) push away from each other, creating a magnetic repulsion force.

Forces (Gravity, Friction, Magnetism) - Revision Notes & Key Formulas | IB Grade 5 Science