Forces: Gravity, Resistance and Mechanisms

🪂 Imagine you drop a feather and a football from the same height at the same time. Which hits the ground first? Most people say the football — but why? The answer involves invisible forces acting on every object, every second of every day. In this lesson you will uncover the science behind gravity, discover why parachutes work, find out why fish are shaped the way they are, and learn how a single person can lift a car using nothing but a simple machine. Forces are everywhere — let's investigate!

🌍 Gravity is a pulling force that acts between any two objects that have mass. Mass is the amount of matter in an object, measured in kilograms (kg). The Earth has an enormous mass, so it pulls every object near it downward toward its centre. This pull is what we call weight. Weight is measured in Newtons (N) using a force meter (also called a Newton meter). On Earth, every 1 kg of mass experiences about 10 N of gravitational pull. So a 5 kg bag of potatoes weighs roughly 50 N. Gravity also acts between Earth and the Moon, keeping the Moon in orbit, and between Earth and the Sun.

⬇️ When an object is unsupported — nothing holding it up — gravity accelerates it downward. In a vacuum (a space with no air at all) every object falls at exactly the same rate, no matter its mass. This was famously demonstrated on the Moon in 1971 when astronaut David Scott dropped a hammer and a feather simultaneously: they hit the surface together. Back on Earth, air gets in the way. Air resistance is a friction force between a moving object and the air particles surrounding it. The larger the surface area facing the direction of travel, the greater the air resistance, and the more it slows the object down.

🐟 Water resistance works in the same way as air resistance but in liquids. Water particles are much more densely packed than air particles, so water resistance is far greater. This is why streamlining matters so much underwater. A streamlined shape — narrow at the front and smoothly tapered — cuts through water with minimal resistance. Fish, dolphins and submarines all share this torpedo-like shape for exactly this reason. Friction is another resistance force. It acts between two solid surfaces that are touching and moving against each other. Friction always opposes motion — it acts in the opposite direction to movement — and converts kinetic energy (movement energy) into heat.

🔧 Sometimes we need to move a very heavy object using only a small force. Simple machines let us do this by changing the size or direction of a force. A lever is a rigid bar that pivots on a fixed point called a fulcrum. When you push down on the long side of a lever, the short side pushes up with a much greater force. The further your effort force is from the fulcrum, the greater the lifting effect — called mechanical advantage. A see-saw, a wheelbarrow and a pair of scissors are all levers. A pulley is a wheel with a rope running over it; using multiple pulleys in a system means each pull of the rope moves a smaller weight over the same distance with less force.

⚙️ Gears are toothed wheels that interlock and transfer motion from one shaft to another. When a small gear meshes with a large gear, the large gear turns more slowly but with greater turning force (called torque). When a large gear drives a small gear, the small gear spins faster but with less force. Cyclists use this principle constantly: a low gear (small chainring to large sprocket) makes pedalling easier on a steep hill because each pedal turn produces more force at the wheel, even though the bike moves more slowly. A high gear does the opposite — less force but much faster wheel rotation on flat ground.