Forces and Newton's Laws
Understand how forces affect motion through Newton's three laws
Forces in Action
Push, pull, and motion
A force is a push or a pull on an object. Forces are vector quantities—they have both magnitude (size) and direction. The SI unit for force is the Newton (N).
Common types of forces include:
- Weight: The force of gravity pulling down (W = mg)
- Normal reaction: The perpendicular force from a surface
- Friction: Opposes motion between surfaces
- Tension: Pulling force in ropes or cables
- Air resistance: Friction from moving through air
Newton's First Law (Inertia)
An object at rest stays at rest, and an object in motion stays in motion with constant velocity, unless acted upon by an unbalanced force.
Newton's Second Law (F = ma)
The acceleration of an object is directly proportional to the net force and inversely proportional to its mass: F = ma
Newton's Third Law (Action-Reaction)
For every action, there is an equal and opposite reaction. Forces always occur in pairs.
Key Exam Point
Action-reaction pairs act on different objects. When you push a wall, you push the wall (action) and the wall pushes you back (reaction). These forces don't cancel out because they act on different things.
Force diagrams (free-body diagrams) show all forces acting on an object using arrows. The length of each arrow represents the magnitude, and the direction shows which way the force acts.
The resultant force (net force) is the single force that has the same effect as all forces combined. If forces are balanced (resultant = 0), there is no acceleration.
Adjust Values
Newton's Laws
- 1st Law: Objects stay at rest or constant velocity unless acted on by unbalanced force
- 2nd Law: F = ma (force equals mass times acceleration)
- 3rd Law: Every action has an equal and opposite reaction
Force
Question:
A 1200 kg car experiences a driving force of 4000 N and friction of 1000 N. Calculate the acceleration.
Answer:
Step 1: Find the resultant force
Resultant = Driving force - Friction
Resultant = 4000 - 1000 = 3000 N
Step 2: Apply F = ma
a = F / m
a = 3000 / 1200 = 2.5 m/s²
A 5 kg object experiences a net force of 20 N. What is its acceleration?