Forces and Motion
Grade 6 · Science · Study Guide
Learn how forces cause objects to move, speed up, slow down, and change direction.
A force is a push or a pull. Forces are everywhere — gravity pulls you down, friction slows your skateboard, and tension keeps a rope bridge from collapsing. Forces are measured in units called Newtons (N), named after the physicist Isaac Newton.
Newton's First Law of Motion says that an object at rest stays at rest, and an object in motion stays in motion — unless an outside force acts on it. This tendency to resist changes in motion is called inertia. A heavy truck has more inertia than a bicycle, which is why it takes longer to stop.
Newton's Second Law explains how force, mass, and acceleration are related: Force = Mass × Acceleration (F = ma). If you apply the same force to a heavy object and a light object, the lighter one will accelerate more. This is why a small car and a semi-truck react very differently when you push them.
Newton's Third Law states that for every action there is an equal and opposite reaction. When you jump off a diving board, you push down on the board (action) and the board pushes you up (reaction). Rockets use this principle — they shoot exhaust gas downward and shoot upward as a result.
Friction is a force that opposes motion between two surfaces. It can be useful (it lets you walk without slipping) or unhelpful (it slows cars down and wears out engine parts). Gravity is a force of attraction between objects with mass. On Earth, gravity accelerates falling objects at 9.8 m/s².
| Law | Statement | Real-World Example |
|---|---|---|
| 1st Law (Inertia) | Objects resist changes in motion | A ball keeps rolling until friction stops it |
| 2nd Law (F=ma) | Force = Mass × Acceleration | Kicking a soccer ball vs. a bowling ball |
| 3rd Law (Action-Reaction) | Equal and opposite reaction | A rocket launches by expelling exhaust downward |
Key terms
- Force
- A push or pull that can change an object's motion. Measured in Newtons.
- Inertia
- The tendency of an object to resist changes in its state of motion.
- Newton's First Law
- An object at rest stays at rest and an object in motion stays in motion unless acted on by an external force.
- Newton's Second Law
- Force equals mass times acceleration (F = ma).
- Newton's Third Law
- For every action force there is an equal and opposite reaction force.
- Friction
- A force that opposes the relative motion of two surfaces in contact.
- Gravity
- A force of attraction between objects with mass; pulls objects toward Earth at 9.8 m/s².
- Net force
- The total force acting on an object after all forces are combined.
Flashcards
- Force
- A push or pull; measured in Newtons (N).
- Inertia
- Resistance to changes in motion; greater mass = greater inertia.
- F = ma
- Newton's Second Law: Force equals mass times acceleration.
- Newton's Third Law
- Every action has an equal and opposite reaction force.
- Friction
- A force that resists motion between surfaces in contact.
- Gravity
- Attraction between masses; pulls objects toward Earth at 9.8 m/s².
Practice questions
Which of Newton's laws states that F = ma?
- First Law
- Second Law
- Third Law
- Law of Gravity
Show answer
Answer: Second Law
Newton's Second Law (F = ma) describes the relationship between force, mass, and acceleration.
An object with more mass has more inertia.
Show answer
Answer: True
Inertia is directly proportional to mass — a semi-truck is much harder to stop than a bicycle because it has more mass.
Forces are measured in units called ___.
Show answer
Answer: Newtons
The Newton (N) is the SI unit of force, named after Isaac Newton. One Newton is the force needed to accelerate 1 kg at 1 m/s².
A rocket launches by pushing exhaust gas downward. This is an example of which law?
- Newton's First Law
- Newton's Second Law
- Newton's Third Law
- The Law of Gravity
Show answer
Answer: Newton's Third Law
The rocket pushes exhaust down (action), and the exhaust pushes the rocket up (reaction) — equal and opposite forces.
Friction always makes objects move faster.
Show answer
Answer: False
Friction opposes motion and slows objects down. It converts kinetic energy into heat.
Fun facts
- In space, with no friction or air resistance, a spacecraft launched by Apollo can still be cruising through space 50+ years later — Newton's First Law in action.
- The fastest rocket ever launched (Parker Solar Probe) reached 430,000 mph — about 0.064% the speed of light.
- Friction generates heat — that's why you rub your hands together to warm them up.
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