describe and use the concept of weight as the effect of a gravitational field on a mass and recall that the weight of an object is equal to the product of its mass and the acceleration of free fall

Momentum and Newton’s Laws of Motion

Newton’s First Law (Law of Inertia)

⚡ A body remains at rest or moves in a straight line at constant speed unless acted upon by an external force. Think of a soccer ball (⚽) on a quiet field – it won’t move unless someone kicks it. The ball’s inertia keeps it still, and the kick provides the force that changes its motion.

Newton’s Second Law (Law of Acceleration)

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Mathematically:

\$\mathbf{F}_{\text{net}} = m\,\mathbf{a}\$

🚀 If you push a light skateboard harder than a heavy one, the light skateboard will accelerate more. The heavier the mass, the more force you need to achieve the same acceleration.

Newton’s Third Law (Action–Reaction)

For every action, there is an equal and opposite reaction. When you jump off a diving board, your legs exert a downward force on the board, and the board pushes you upward with an equal force. The forces act on different bodies but are equal in magnitude and opposite in direction.

Weight and Gravitational Force

What is Weight?

Weight is the force exerted on a mass by Earth's gravitational field. It depends on both the mass of the object and the acceleration due to gravity (\$g\$). The formula is:

\$W = m\,g\$

⚖️ Think of weight as the “pull” you feel when you hold an apple. The heavier the apple (larger \$m\$) or the stronger the pull of gravity (larger \$g\$), the greater the weight.

Gravitational Acceleration (\$g\$)

On Earth, \$g\$ is approximately \$9.81\,\text{m/s}^2\$. This value can vary slightly depending on where you are on the planet, but for most school calculations we use \$9.81\,\text{m/s}^2\$.

Weight Table Example

Key Takeaways

• Weight is a force, not a mass. It is calculated as \$W = m\,g\$.
• Mass (\$m\$) is a measure of how much matter an object contains and is constant everywhere.
• Gravity (\$g\$) provides the acceleration that turns mass into weight.
• Newton’s laws describe how forces affect motion, and weight is one of those forces acting on a mass.

Quick Practice Questions

1. Calculate the weight of a 3 kg object on Earth.
2. Explain why a feather and a hammer have the same weight on Earth.
3. Describe how Newton’s Second Law applies when you push a shopping cart.