Forcemeter
What does something do if it is not moving? It stays still. What are most objects around you doing at this moment? They are staying still. These two observations led scientists to believe that the natural state for any object is to be at rest. In the seventeenth century a scientist called Isaac Newton made a great many investigations. One of his investigations was on how the objects in the Solar System moved. He found that when he studied objects in space their natural state was not to be at rest but to be moving. From his investigations Newton concluded that everything stays at rest or moves steadily in a straight line until it is acted upon by a force.

Think about what happens when you want to move an object. For example, you may like to push the mouse across the mouse mat. When you try this, the mouse does not immediately shoot away. It stays where it is until you have given it enough of a push to start it moving. We could say that this reluctance to move is a kind of laziness. Scientists have a name for it. They call it inertia.

Everything possesses inertia. If you sit on a trolley and ask your friend to push you, they will find they have to push quite hard to get you moving. This is due to your inertia. If you are standing up in a bus and the driver brakes suddenly, you are flung forwards and have to move your feet to keep your balance. This is also due to your inertia. When your friend pushes you on a trolley your inertia tends to keep you where you are. When you are on the rapidly slowing bus your inertia tends to keep you moving fast.

The laziness or inertia of an object depends on the amount of matter in it. Matter is made from particles called atoms and molecules which are packed together to make the object. The amount of matter in an object is called its mass. Scientists have found a way of measuring the mass of an object by giving it a push and then measuring how fast its speed increased. This increase in speed is called acceleration. The mass of an object can then be found by timing how long it takes to accelerate.

One way of finding the mass of an object is to put it on a spring, stretch the spring, then let the object go. The spring then pulls on the object and makes it accelerate. The speed at which the object accelerates or bounces on the spring can be used to measure its mass. The object will accelerate or bounce on the spring in the same way, no matter where it is in the universe, because its mass does not change. It always contains the same number of atoms or molecules.

A spring can also be used to find the weight of an object. The object is simply placed on a spring and the force of gravity pulls the object down and stretches the spring. The weight of the object depends on the mass of the object and also the force of gravity acting on it. This means that the weight of an object is not the same in every part of the universe because the force of gravity varies from place to place. For example, the force of gravity on the Earth is six times greater than the force of gravity on the Moon but is less than half the force of gravity on Jupiter - the largest planet in the Solar System. The weight of an object is also measured in different units from its mass. The unit of force is the newton and weight is a kind of force, so the weight is measured in newtons.

Over the years some confusion has developed over the ideas of mass and weight. For example, you may see the weight of a food item measured in grams while many people are unsure what the mass of an object is or have never heard of a newton. When you use a forcemeter, you may find that the scale is measured in either newtons or grams. When you make your readings always remember that mass is measured in grams and weight is measured in newtons.

Do planets move in straight lines in the Solar system?
No, they don't. They move in curved paths called orbits. The reason for this is that the Sun's gravity pulls on them and this makes them move in a curved path called an orbit. If the Sun's gravity did not exist the planets would move away in a straight line. A spacecraft called Voyager has left the Solar System and is no longer being pulled by the Sun's gravity. It will travel in a straight line across space until it reaches another star. The star's gravity may then make the spacecraft move in orbit around it.

What is pushing the spacecraft along in space?
Nothing. A forward force was provided by the spacecraft rocket engines a long time ago and there is no other force to oppose it so the spacecraft just keeps on going.

Could an object keep on going in a straight line in the air for a long time?
No. The object is pulled by the Earth's gravity and is pushed on by the air. Gravity makes the object move in a curve and land back on the Earth. The push of the air slows the object down.

Could an object go in a straight line for a long time on the Earth's surface?
No, the force of friction between the object and the Earth's surface immediately begins to slow it down and eventually makes it stop.

Astronauts are weightless in space but are they also massless?
No. If they were massless you would not be able to see them because they would not be there. There is a special chair with springs, which an astronaut can use to show that he or she is not massless. The astronaut sits in the chair and rocks once. When a person rocks they go from rest to movement. This is acceleration. The rate at which an astronaut accelerates in the chair is the same in space as it is on the ground.

Why can a spring be used in a forcemeter?
Because it extends in a regular way as it is pulled. When a small mass is put on a spring it stretches a small amount. When double the mass is put on the spring, it stretches double the original amount. When three times the mass is added the spring stretches three times as much and so on. When the masses are removed the spring returns to its original length.

If the spring did not extend in a regular way and return to normal when the masses were removed it could not be used in a forcemeter.

Can the word weight be used instead of mass?
Many people use the word weight when they really mean the mass of an object. They also use the word weight to mean an object with a weight such as a balance has a set of weights. It would be more correct to call these half-kilogram and kilogram 'weights' masses.