Forces and Newton's Laws of Motion
Grade 10 · Science · Study Guide
Master Newton's three laws of motion and understand how forces cause objects to accelerate, stay still, or interact.
Isaac Newton's three laws of motion, published in 1687, form the foundation of classical mechanics. The First Law (Law of Inertia) states that an object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force. This is why passengers lurch forward when a car brakes suddenly — their bodies tend to keep moving.
The Second Law quantifies how force affects motion: F = ma (net force equals mass times acceleration). If you double the force on an object, you double its acceleration. If you double the mass, you halve the acceleration for the same force. This law also explains why it's harder to push a loaded shopping cart than an empty one. The Third Law states that for every action, there is an equal and opposite reaction. When you push against a wall, the wall pushes back on you with equal force — and this reaction force acts on a different object.
Force is measured in Newtons (N): 1 N = 1 kg·m/s². Common forces include gravity, friction, tension, and normal force. When all forces on an object are balanced (net force = 0), the object is in equilibrium and doesn't accelerate. Free body diagrams are essential tools for visualizing and analyzing all forces acting on an object.
| Law | Statement | Real-World Example |
|---|---|---|
| 1st Law (Inertia) | Objects resist changes in motion | Passengers lurch forward when brakes are applied |
| 2nd Law (F=ma) | Force = mass × acceleration | Heavier car needs more force to accelerate |
| 3rd Law (Action-Reaction) | Equal and opposite reaction | Rocket propulsion: exhaust pushes down, rocket goes up |
Key terms
- Newton's First Law
- An object remains at rest or in uniform motion unless acted upon by a net external force (Law of Inertia).
- Newton's Second Law
- The net force on an object equals its mass times its acceleration: F = ma.
- Newton's Third Law
- For every action force, there is an equal and opposite reaction force acting on a different object.
- Inertia
- An object's resistance to changes in its state of motion; proportional to mass.
- Net force
- The vector sum of all forces acting on an object; determines acceleration.
- Equilibrium
- State in which the net force on an object is zero, resulting in no acceleration.
Flashcards
- Force
- A push or pull on an object with a specific magnitude and direction; measured in Newtons (N).
- Mass
- The amount of matter in an object; determines its inertia; measured in kilograms (kg).
- Acceleration
- The rate of change of velocity; caused by a net force; measured in m/s².
- Normal force
- The perpendicular support force exerted by a surface on an object resting on it.
- Friction
- A force opposing relative motion between surfaces in contact; depends on surface type and normal force.
- Free body diagram
- A diagram showing all forces acting on an object as arrows indicating direction and magnitude.
Practice questions
A 10 kg object experiences a net force of 30 N. What is its acceleration?
- 3 m/s²
- 30 m/s²
- 300 m/s²
- 0.3 m/s²
Show answer
Answer: 3 m/s²
Using F = ma: a = F/m = 30 N ÷ 10 kg = 3 m/s².
An object moving at constant velocity has a net force of zero acting on it.
Show answer
Answer: True
Newton's First Law: constant velocity means no acceleration, which requires net force = 0. All forces are balanced.
Newton's ___ law states that for every action there is an equal and opposite reaction.
Show answer
Answer: third
Newton's Third Law: action-reaction pairs are equal in magnitude, opposite in direction, and act on different objects.
A ball resting on a table has its weight balanced by the table's normal force. This is an example of:
- Newton's First Law
- Newton's Second Law
- Newton's Third Law
- Friction
Show answer
Answer: Newton's First Law
The ball is in equilibrium (net force = 0, no acceleration) because the normal force from the table exactly balances gravity — consistent with the First Law (inertia/equilibrium).
An object with greater mass has greater inertia.
Show answer
Answer: True
Inertia is resistance to changes in motion, and it is directly proportional to mass. A more massive truck is harder to start moving or stop than a lighter bicycle.
Fun facts
- In space, where there is no air resistance or friction, a thrown object would travel in a straight line forever — Newton's First Law in action.
- Newton published his three laws of motion in 'Principia Mathematica' at age 44, but reportedly developed the core ideas during a plague-year quarantine.
- The thrust of a rocket engine works because of Newton's Third Law — hot gas is expelled downward at high speed, and the rocket is pushed upward with equal force.
Get a personalized course on Forces and Newton's Laws of Motion built around your child.