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Forces and Motion - Gravity, Friction, and Magnetism

Grade 6IB

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

🔑Concepts

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Force is a push or pull exerted on an object, measured in Newtons (NN). It can change an object's speed, direction, or shape.

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Gravity is a non-contact force of attraction between all objects with mass. The weight of an object is the gravitational force acting on it, calculated as W=m×gW = m \times g.

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Mass (mm) is the amount of matter in an object and is measured in kilograms (kgkg). Unlike weight, mass does not change based on location.

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Friction is a contact force that opposes motion between two surfaces. It depends on the texture of the surfaces and the normal force pressing them together.

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Magnetism is a non-contact force. Magnets have two poles: North (NN) and South (SS). Like poles repel (N↔NN \leftrightarrow N), and opposite poles attract (N→←SN \rightarrow \leftarrow S).

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

📐Formulae

F=m×aF = m \times a

W=m×gW = m \times g

gEarth≈9.8 m/s2g_{Earth} \approx 9.8 \, m/s^2

Fnet=F1+F2+...+FnF_{net} = F_1 + F_2 + ... + F_n

💡Examples

Problem 1:

A student has a mass of 45 kg45 \, kg. Calculate their weight on Earth where g=9.8 m/s2g = 9.8 \, m/s^2.

Solution:

W=45 kg×9.8 m/s2=441 NW = 45 \, kg \times 9.8 \, m/s^2 = 441 \, N

Explanation:

Weight is the force of gravity on an object. By using the formula W=mgW = mg, we multiply the mass by Earth's gravitational acceleration.

Problem 2:

A box is pushed with a force of 20 N20 \, N to the right, while friction exerts a force of 5 N5 \, N to the left. What is the net force (FnetF_{net})?

Solution:

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

Explanation:

Forces acting in opposite directions are subtracted to find the resultant or net force.

Problem 3:

An astronaut's mass is 80 kg80 \, kg. On the Moon, the gravitational acceleration is gmoon≈1.6 m/s2g_{moon} \approx 1.6 \, m/s^2. What is the astronaut's mass and weight on the Moon?

Solution:

Mass=80 kgMass = 80 \, kg; Weight=80 kg×1.6 m/s2=128 NWeight = 80 \, kg \times 1.6 \, m/s^2 = 128 \, N

Explanation:

The mass remains constant at 80 kg80 \, kg because the amount of matter doesn't change. The weight decreases because the Moon's gravity is weaker than Earth's.