Force is what changes an object's motion. The SI unit is the newton, named after Isaac Newton: one newton is the force that accelerates one kilogram by one metre per second per second. A medium apple resting in your hand presses down with roughly one newton. Two older units remain common. The kilogram-force is the weight of one kilogram under standard gravity, exactly 9.80665 newtons. The pound-force is the weight of one pound under the same gravity, about 4.448 newtons.
Where you meet it
In daily life, people usually express force as weight in kilograms. Bathroom scales, luggage scales and the "load" figures on shelves and ladders all do this, and that's fine as long as everything stays on Earth. Force units become important when you need to compare with specifications written in newtons. Car and caravan towing is a good example. A towbar has a maximum nose weight, the downward force the trailer's hitch puts on it, usually quoted in kilograms. Engineering data for the same towbar may give vertical loads in newtons. A 75 kg nose weight corresponds to about 735 newtons.
Mass and weight are not the same
The root of most confusion is that a kilogram is a unit of mass, the amount of matter in an object, while a newton is a unit of force. On the Moon, a 10 kg bag still has a mass of 10 kg, but it weighs only about a sixth as much as on Earth. Even on Earth, gravity varies by around half a percent between the poles and the equator, which is why precision scales are calibrated for where they're used. For everyday conversions, using standard gravity is accurate enough.
Worked conversions
A gym cable machine stack marked 50 kg exerts about 490 newtons on the cable when lifted slowly. A spring balance reading 20 pounds is measuring about 89 newtons. A small car with a mass of 1,200 kg presses on the ground with a total weight of about 11,770 newtons, spread across four tyres, so each tyre carries roughly 2,940 newtons, or about 300 kilograms-force, if the weight is evenly distributed. In practice the front tyres usually carry more, because the engine is at the front.
Force in sport and fitness
Force-measuring plates and smart gym equipment increasingly report results in newtons. A jump test might show a peak force of 2,000 newtons, which means about 204 kilograms-force: more than double the jumper's weight, because the force needed to accelerate the body upwards adds to the force needed to support it. Climbing ropes and harnesses are rated in kilonewtons. A climbing rope's impact force of around 8 to 9 kN is equivalent to the weight of roughly 800 to 900 kilograms, and carabiners are commonly rated at 20 kN or more.
Checking your answer
A quick way to check is to remember that one kilogram weighs about ten newtons, and one pound weighs about four and a half. An adult weighing 70 kg exerts about 690 newtons standing still. If a force for a person or household object comes out in tens of thousands of newtons, check whether a value in grams was entered as kilograms, or whether kilonewtons were taken as newtons.
One more worked example
A car tyre at 2.3 bar gauge pressure pushes on the road through a contact patch roughly the size of a postcard, around 150 square centimetres. Pressure times area gives the force: 230,000 pascals multiplied by 0.015 square metres is about 3,450 newtons, or around 350 kilograms-force. That's close to the share of the car's weight each tyre carries, which is no coincidence: the contact patch grows or shrinks until the tyre pressure supports the load.