physiworld
LESSON
CONCEPTS

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This is a demo preview of one section from the Dynamics module.

The full Physiworld platform contains over 100 interactive pages of physics simulations, guided explanations, quizzes, progress tracking, and XP-based learning — entirely free to use.

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Why Do Things Move?

A football flies through the air. A car brakes at a red light. A swimmer pushes off a wall and glides forward. None of these happen by accident — every motion has a force behind it, and every force follows rules.

In this lesson, you will discover those rules by playing with three simulators, solving challenges, and seeing Newton's laws in action. Start by dropping a ball.

What happens when gravity pulls and the ground pushes back?

Let's solve this together ↓

1
0.8
Adjust sliders, then bounce the ball

If you push a wall, why don’t you move — but you still feel a force?

This is explained by Newton's Third Law ↓

Action & Reaction

Whenever two objects interact, they push on each other. These forces always come in pairs. You cannot have a single force acting alone — every action has an equal and opposite reaction.

$$F_{\text{action}} = -F_{\text{reaction}}$$
\( F_{\text{action}} \) = force you apply \( F_{\text{reaction}} \) = equal & opposite

Let's put numbers to it ↓

Problem: An astronaut pushes a satellite with \(40\,\text{N}\) in space.

Step 1: Identify action force: \( F_{\text{action}} = +40\,\text{N} \)

Step 2: Apply Newton’s Third Law

\[ F_{\text{reaction}} = -40\,\text{N} \]

Answer: The satellite pushes back with 40 N in the opposite direction.

Click the example to continue

Now you try ↓

Action–Reaction Challenge

1 A swimmer pushes backward on the water with 150 N. What is the magnitude of the force the water exerts on the swimmer?
N
Solve the math question

What decides how fast something accelerates?

Let’s explore this together ↓

50 N
10 kg
Adjust force and mass, then press the object
Force and Mass

More force means more acceleration. More mass means less acceleration for the same force. This trade-off is Newton's Second Law — and it governs everything from rockets to roller coasters.

The formula behind it ↓

$$F = m \cdot a$$
\( F \) = force (N) \( m \) = mass (kg) \( a \) = acceleration (m/s²)
1 N
1 kg × 1 m/s²
2×
force → 2× acceleration
2×
mass → ½ acceleration

Let's put numbers to it ↓

Problem: A car of mass \(1000\,\text{kg}\) accelerates at \(3\,\text{m/s}^2\).

Step 1: Use the formula: \( F = m \cdot a \)

Step 2: Substitute values: \( F = 1000 \times 3 \)

\[ F = 3000\,\text{N} \]

Answer: The net force is 3000 N.

Now you try ↓

Force & Motion Challenge

2 A box of mass 200 kg accelerates at \(4\,\text{m/s}^2\). What is the net force acting on the box?
N
Solve the math question

What happens when force stops but motion continues?

Try to solve this ↓

45°
50 m/s
Adjust angle and speed, then press the ball
Why the path curves

Horizontal speed stays constant (no horizontal force after launch). Vertical speed changes constantly (gravity pulls down). The combination creates a parabola — the same curve every thrown ball, bullet, and water fountain follows.

$$F_{\text{net}} = F_{\text{applied}} - F_{\text{friction}}$$
\(F_{\text{net}}\) = net force \(F_{\text{applied}}\) = driving force \(F_{\text{friction}}\) = resistance

See a worked example ↓

Problem: A sled is pulled with \(900\,\text{N}\). Friction is \(200\,\text{N}\) and air resistance is \(150\,\text{N}\).

Step 1: Total resistance = \(350\,\text{N}\)

Step 2: \(F_{\text{net}} = 900 - 350\)

\[ F_{\text{net}} = 550\,\text{N} \]

Answer: \(550\,\text{N}\)

Now you try ↓

Net Force Challenge

3 A boat engine produces 700 N thrust. Water resistance is 200 N. What is the net force?
N
Solve the math question
Summary

You’ve learned how forces create motion: forces come in pairs, acceleration depends on mass and force, and net force determines what actually happens. Next, you’ll explore how multiple forces combine — including forces at angles and equilibrium.

Next: Forces in Action