Why does a canoe move forward when the canoeist pushes water backwards with their paddle — and why does it move faster when they push harder?
Key idea
The Concept of Force
A force can set an object moving, change its speed or direction, or change its shape.
Direction always matters
Force is measured in newtons (N) and always needs a direction along with its magnitude — like friction opposing motion, or gravity pulling downward.
Key idea
Balanced and Unbalanced Forces
1
Balanced forces
Equal in magnitude, opposite in direction — they cancel out, so the object doesn't move (or its motion doesn't change).
→
2
Unbalanced forces
A non-zero net force results, and the object moves (or accelerates) in the direction of the larger or combined force.
Key idea
Finding the Net Force
Two forces on a block | Net force
10 N and 6 N, same direction
10 + 6 = 16 N, in that direction
10 N right and 6 N left
10 − 6 = 4 N, towards the right
6 N right and 10 N left
10 − 6 = 4 N, towards the left
Key idea
The Force of Friction
Push a box and it may not movefriction between the box and the floor opposes your applied force.
Motion needs a net forceThe box starts moving only once your push exceeds friction. Once moving, if you stop pushing, friction alone brings it to rest — this is why a rolling ball or a coasting bicycle eventually stops.
Key idea
Newton's First Law of Motion
The law
An object at rest remains at rest, and an object in motion continues to move with constant velocity, unless a net force acts upon it.
Named for a tendency to resist change
Newton called this tendency inertia — first argued by Galileo, who reasoned a moving object needs no force to keep moving forever, if nothing opposes it.
Key idea
Newton's Second Law of Motion
1
The law
A net force accelerates an object in its own direction. Acceleration is proportional to the net force, and inversely proportional to the object's mass.
→
2
The equation
a = F ÷ m, or F = ma 1 newton is the force that gives a 1 kg object an acceleration of 1 m/s².
Key idea
Weight: The Force of Gravity
Falling objects accelerate under gravity at g ≈ 9.8 m/s² (often rounded to 10 m/s² for quick estimates).
F = mg
Example 6.4: A 30 kg barbell needs an upward force of F = mg = 30 kg × 9.8 m/s² = 294 N from the weightlifter, just to hold it steady.
Key idea
Slowing Down Gently Reduces Force
A longer stop, a smaller forceA fielder pulls their hands back while catching a fast ball. Stretching out the time it takes the ball's velocity to reach zero reduces the acceleration — and so the force needed.
Airbagsand coconuts — A car airbag cushions a passenger, increasing stopping time to lower the force of impact. A coconut cracked on hard ground stops almost instantly — needing a very large force, which breaks its shell.
Key idea
Newton's Third Law of Motion
The law
Whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first.
Two forces, two different objects
These paired forces act on different objects — so unlike balanced forces on one object, they never cancel each other out.
Key idea
Action and Reaction, Side by Side
Paddle pushes water backwards
Water pushes paddle (and canoe) forward
Key idea
Forces on a System of Objects
Two boxes connected by a string, pulled by force F: treat internal tension as invisible, and the whole system as one object.
a = F ÷ (m₁ + m₂)
The system accelerates just like one object of combined mass.
Box 2 → Box 1
Chapter · Key terms to remember
Key Terms
Force
A push or pull, measured in newtons (N).
Balanced / Unbalanced forces
Cancel out / leave a non-zero net force.
Friction
A force opposing relative motion between surfaces.
Chapter · Key terms to remember
More Key Terms
Newton's First Law
No net force means no change in motion (inertia).
Newton's Second Law
F = ma — force, mass, and acceleration linked.
Weight
The gravitational force on an object, F = mg.
Chapter · Key terms to remember
More Key Terms
Newton's Third Law
Every force has an equal, opposite reaction force.
Tension
The internal pulling force transmitted through a string.
Questions for your notebook
Write these down, then discuss
1
Using a horizontal force F, a table is moved across the floor at a constant velocity. How much is the frictional force exerted by the floor on the table?
2
100 oarsmen row a snake boat, each applying a horizontal force of 200 N. 95 row backwards (to propel the boat forward) but 5 mistakenly row in the opposite direction. What is the net force on the boat?
3
A sailor jumps out from a small boat forward, onto the shore. Will the boat move? If yes, in which direction, and why?
Project as-is — students copy the questions, then the class discusses answers together.