Forces v Energy (Part II)…

The next thrilling instalment in the ‘Forces v Energy‘ series – who will win? Well, neither because they are totally different things so they cannot have any sort of competition or conflict. It would be like saying ‘Custard v Empathy’ – which will win?

Forces

• Forces are measured in Newton.
• Forces cause objects to speed up, slow down, change direction or change shape.

A force is a push or a pull and you may be asked about a number of different ones. Don’t for now worry too much about “The Four Fundamental Forces” that Prof Cox describes in the Symphony of Science video – we will concern ourselves with forces you experience on a larger scale.

Friction is a force that tends to oppose motion – it slows things down or enables things to grip surfaces. Life would be very difficult in a frictionless Universe – you wouldn’t be able to stop walking or change direction! In air we call friction air resistance or drag, and in water it tends to be called water resistance or drag.

Gravity is the very weak force that pulls all particles of matter towards each other. We happen to be standing on a big object (planet Earth) so the large gravitational pull that it generates (because of its size) overwhelms any other gravitational tugs we may experience. On Earth, gravity pulls on you with a force of about 10 Newton for every kilogram of your mass. If you were in deep space, where gravity would pull at you equally from all sides, you would seem weightless. On the Moon, which is about 1/6 the mass of Earth, your weight would be 1/6 of your weight on Earth (1.6 N/kg).

A tension force is the force applied to a string, chain, wire or other solid material that is supporting a load. Imagine a conker on a string. Gravity is causing the conker to pull down on the string (there is no obvious stretching) and so tension is the force that opposes the conker. Elastic materials can exert a force when they are stretched or compressed because something that is elastic will try to return to its original length (size). In the picture of Green Arrow, the string is under tension and the bow is balancing this with an elastic force. A solid material like a table, when it has an object like a book resting on it, returns the force of gravity with what is called a normal force. If an object is floating in water, we call the force that opposes gravity upthrust.

The electromagnetic force is what makes your hair stand on air when you touch a Van De Graaff generator, or allows a balloon that has been rubbed on a sweater to stick to the wall. When two objects have an opposite charge they are attracted and pull on one another. If an object has an excess of electrons then it becomes negatively charged. When two negatively charged objects are brought together they repel one another. This is a push force.  This is the force that pushes the electrons around a circuit. The voltage is a measure of the electrical force applied to the circuit – the higher the force, the faster the electrons move and hence the higher the current. Some materials can become magnetised, where one end becomes a north seeking pole and the other end is the south seeking pole (it is not possible to have one without the other). Bringing two south seeking poles together causes them to repel one another, but opposite magnetic poles attract one another.

A vehicle with a motor, or a human being with muscles, can also apply forces to things. These forces are simply called applied forces. A car will accelerate from rest until the applied force from the engine is equalled by the frictional forces opposing it (drag) – it will then travel at a constant speed. When the brakes are pressed, the drag forces are increased so the car slows down. You can read more about this process in the post Force and Linear Motion.

Two short videos – the first poses a question and the second one answers it!

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 Questions…

1. What two forces are acting on a duck as it floats, motionless, on a pond?
2. An astronaut has a mass of  90 kg. What would his weight be in deep space?
3. The astronaut puts on his 150 kg space suit so that he can walk on the moon. How much will he weigh in total on the surface of the moon?
4. [Harder] How would you expect the mass of a space craft to change as it journeyed from Earth to the Moon and back again?

ps – It would be custard, of course.