How Forces Affect Motion 🚀

Kicking • Striking • Squeezing — force in action!

A force is simply a push or a pull on an object. It's what causes motion to start, stop, or change.

• A force can make a resting object move.
• It can change the speed of a moving object.
• It can change the direction of a moving object (e.g. a bat hitting a ball).
• It can even change the shape of an object (e.g. squeezing a lemon).
• Force is a vector quantity — it always needs a magnitude (strength) AND a direction to be described.
• SI unit of force: newton, symbol N. (Full word starts small — "newton", but symbol is capital "N".)

A spring balance

• A spring balance is used to measure the magnitude of a force — not just weight, but any force.
• Weight of an object = the gravitational force with which the Earth pulls it.
• Pulling the free end of the spring balance stretches the spring inside — the reading shows the pulling force.

Tug of war = balanced forces if no one moves

In real life, more than one force usually acts on an object at once. What matters for motion is the net (resultant) force.

• Balanced forces: equal in magnitude, opposite in direction → object does not move; net force = 0.
• Unbalanced forces: net force ≠ 0 → object's motion changes (starts moving / speeds up / changes direction).

Same direction → forces add up

Two simple rules for net force:

Same direction → F₁ + F₂

Opposite direction → F₁ − F₂ (bigger force wins)

Net force acts in the direction of the bigger push

Friction opposes the push

• Friction is a force that acts opposite to the direction of motion (or attempted motion) between two surfaces in contact.
• An object stops on its own after being pushed because friction gradually reduces its velocity to zero.
• To keep an object moving at constant velocity, the applied force must exactly balance friction (net force = 0).
• Friction depends on the nature of the surfaces in contact — smoother surfaces → less friction → object travels farther.

4 forces on a pushed box: Applied, Friction, Weight, Normal force

• Besides applied force & friction, weight (down) and the normal force (up, from the surface) also act — these two stay balanced.

Activity: rubber band launches coins across surfaces

Spring balance measures friction force directly

Cart & pulley experiment

Falling cup pulls cart with constant force

• For the same mass: bigger force → bigger acceleration.
• For the same force: bigger mass → smaller acceleration.

• 1 newton = the force that gives a 1 kg mass an acceleration of 1 m/s².
• Acceleration due to gravity, g ≈ 9.8 m/s² (take 10 m/s² for quick estimates). It does NOT depend on the object's mass.
• (Advanced) The fuller form of the law: rate of change of momentum (mass × velocity) is proportional to the net force — this works even when mass itself is changing.

Big idea: for the same change in velocity, spreading the change over a longer time reduces the force needed (and vice-versa) — since force also depends on how quickly velocity changes.

Airbags increase stopping time → less force → fewer injuries

Coconut stops in a flash → huge force → shell breaks!

• Catching a cricket ball: the fielder pulls hands back → increases the time to stop the ball → reduces the force felt.
• Airbags: inflate to give the passenger more time to stop → smaller force on the body → reduces injury.
• Cracking a coconut: it hits the ground and stops almost instantly → very large force → shell breaks.

• Forces always occur in pairs — but the two forces act on two different objects, so they do NOT cancel each other out.
• Applies to every kind of force — contact forces (pushing, friction) AND non-contact forces (magnetic, electrostatic, gravitational).

Push the table → table pushes you back!

Everyday examples:

• Walking/running: feet push the ground backward → ground pushes feet forward (friction helps you move here!).
• Climbing a tree: legs push the trunk down → friction pushes the climber up.

Climbing uses friction as the reaction force

Paddle pushes water back → water pushes canoe forward

Two spring balances always show equal readings

Earth pulls the fruit — the fruit pulls Earth back equally!

• Action-reaction forces are always equal in magnitude, but the acceleration they cause can be very different.
• Why? Because a = F/m — and the two objects usually have very different masses.
• Example: Earth and a falling fruit pull each other with equal force, but Earth's mass is enormous → its acceleration is too tiny to notice, while the fruit clearly accelerates towards Earth.

Magnetic force pair

Electrostatic force pair

A rocket = engine + body, treated together

• When two or more connected objects (e.g. joined by a string) are pulled by a force, we can treat them as one single system.
• Internal forces (like the tension between the objects) act inside the system and cancel out — no need to consider them.
• Only external forces decide how the whole system accelerates:

• This trick makes analysing complex, connected systems much simpler — and gives the same answer as studying each object separately.

Balloon activity: air rushes out → balloon shoots forward

Each extra turn of rope = much more grip!

Wrapping a rope once around a post and pulling the other end needs a lot of force to hold a heavy load. Add one more turn — and suddenly it's much easier to hold! Friction between the rope and the post doesn't grow in a simple straight-line way — a small change in contact can create a huge change in grip. This is exactly how large ships are held safely at a pier using ropes around bollards.