Forces and how to teach them
A great Newton’s third law experiment to try below – more ideas on my Forces Pinterest page here
One of the many problems faced by physics teachers is that the terminology we use in a very precise way is often used very arbitrarily in the ‘real world’
Forces is a bad one;
- “She forced me to do it”
- “It was an act of force”
- A force of nature
- The armed forces
- Possibly the worst of the lot when it comes to confusing the concepts –“May the Force be with you “ !
The Force? something we can carry around with us?
Before we start teaching Forces we should look at the common misconceptions
Probably the biggest single misconception is that when you push or throw an object that there continues to be a force in the direction of motion. It’s natural and common sense to assume that a driving force is needed to keep an object moving at a steady speed because that is our everyday experience.
When you stop pedalling on a bike you come to stop. Therefore you must need a force to maintain a constant speed – dont you ? . We live in a world with friction, but as we cant see it often it isnt accounted for.
Carol Davenport @DrDav has written a blog for making the invisible visible, its for Ks2 but it gives a good overview of what our students should know ( but probably dont !) here
Teach momentum first?
Another issue is that we usually haven’t taught momentum yet. Should we do that before we teach forces?
I dont do an in depth quantitative analysis of momentum. I usually just ask the question ‘Which would hurt you most if it hit you a beetle or a train?
Students usually respond- a train, then you point out what would happen if the train was moving really slowly and the beetle really fast? (I once had a Rhinocerous Beetle bounce off my forehead when cycling down a hill – it hurt !!)
So there are 2 properties we need to consider , the mass and how fast it is moving – its velocity. Multiply these two things together and we have momentum. Momentum is useful for scientists when studying collisions or how difficult it is to stop a moving object.
It is an interesting point to consider as to whether momentum is a more useful property than simply velocity when setting speed limits. Although the debate I had with a police officer who pulled me over on my motorbike for travelling the same velocity as the 4×4 in front of me that had 10 times my mass, was short lived and fruitless.
Videos – Lots of collision videos are around. Im wary of using many of the car crash ones as they may involve fatalities. Get your students to find them
Bubble soccer – The ultimate momentum experiments?
Stop a train?
Another problem with teaching physics is that students come into our classroom with a set of belief systems. They think they understand forces and they have models in their heads which sadly are usually wrong. This research indicates merely telling people the right things may not be effective at all
In this blog we look at the role of confusion here
So what actually is a Force?
Its a question few people can accurately answer
Identifying, measuring and representing forces – Misconceptions we may need to deal with
Pupils often think that:
- only animate objects can exert a force – thus, if an object is at rest on a table, there are no forces acting on it
- a rigid solid cannot be compressed or stretched
- all forces need objects to be in contact to have an effect
- friction only occurs between solid objects
- things fall naturally – no forces are involved; barriers stop things falling
- gravity stops acting when the object hits the ground
- there is ‘more gravity the higher up you go’ because things dropped from higher up suffer greater damage when they hit the floor
- mass and weight mean the same thing and they are equal at all times
- mass and volume mean the same thing
- gravity only affects heavy things
- gravity only works one way – the Earth attracts the Moon but the Moon does not attract the Earth
- Earth’s magnetism and/or spin create gravity
- astronauts are weightless in an orbiting spacecraft because there is no gravity
- magnetic field lines are really there – a magnetic field really is a pattern of lines
- all metals are attracted to a magnet
- all magnets are made of iron
- magnetic poles are always at the end of the magnet
- larger magnets are stronger than smaller magnets
- all materials that can be magnetised are magnets
- objects float in water because they’re ‘lighter’ than water or sink because they are heavier; wood always floats and metal always sinks
- all floating objects float because they have air in them
- all objects containing air float
- there is no/less gravity in water
- power, work, energy and force all mean the same thing.
- if an object is moving there must be a force acting on it
- if an object is stationary there are no forces acting on it
- if a force acts on an object it will inevitably move
- force is a property of an object; an object has force and, when the force ‘runs out’, it stops moving
- acceleration can only occur in the same direction as an object is moving
- when dropped in a vacuum, heavier objects will reach the ground first
- falling objects stay at the same speed as they fall
- opening a parachute during freefall makes the skydiver go upwards
- rocket propulsion is due to exhaust gases pushing on something behind the rocket.
- Pupils often confuse speed, acceleration and velocity; distance–time and speed–time graphs.
What they already know
One of my favourite activities is to use a plastic crossbow – you can buy them here
put a 1N apple on my head wear safety goggles and ask for a volunteer to shoot the apple off my head on the count of three.
On the count of 2 I stop and ask the class Should they aim straight at the apple, above it or below it? Virtually every student says above Why? get them to think for a bit What about if I move further away? They all know you have to aim even higher. So what do you know? That forces, in this case gravity, change the motion of a body and the longer the force acts for, the greater the change in motion
We can do an analysis on the crossbow bolt to really assess our students understanding of Forces
The reason for using an apple that weighs 100g is that we can reinforce that it weighs 1 Newton. Then we can ‘show’ a Joule – Throw the apple 1 metre in the air and you have done 1 Joule of work (Work (J) = Force (N) x Distance (m) travelled in the direction of the force) So if we consider a chemical store of energy – such as a packet of biscuits . How many Joules would there be stored in a packet of biscuits ? Get your students to guess – biscuits given to the 3 closest ? They massively underestimate as the answer is often over 2000,000 Joules. So I eat a packet of biscuits and I can throw an apple in the air two million times ! or fire one two thousand kilometres high ! Link to this to power stations and why coal and oil are used as fuels. In reality we cant do this , why not? Relate answers to the ideas that we are massively inefficient at transferring energy – You can burn poo as a fuel source ! We can go cross- curricular with biology and look at that we are homiotherms and hence transfer around 75 Watts of thermal energy . Poikilotherms such as snakes and crocodiles dont need anything like as much food as they dont need to regulate their internal temperature. Cue some gross videos of massive snakes and crocodiles eating things
I find many students struggle to analyse graphs and to see the point. I want them to see that graphs tell stories – See below for one of the ideas I use
It is counter intuitive that a feather and a bowling ball would fall at the same rate if there was no drag . Here is Brian Cox
or the Hammer and Feather on the moon – Or discuss conspiracy theories here
A man and a cat fall out of a plane at the same time. Sketch on the same Velocity – Time graph their motion . Get students to think about what information do they need – This may help
To be updated
The ubiquitous skydiver
- only equal masses can be balanced on a seesaw
the pivot has to be between the two forces that are acting
- forces only cause changes in speed, not direction
- when a force makes an object travel on a circular path the object must get faster, i.e. the force must cause a change in speed, not just direction
- cornering objects are forced outwards by a force called centrifugal force.
Forces and pressure – misconceptions and ways of teaching
Pupils often think that:
- pressure and force are synonymous
- pressure in liquids and gases can be stronger in one direction than another
- pressure arises from moving liquids or gases; still gas or liquid cannot exert a pressure
- moving fluids cause higher pressures
- liquids rise in a drinking straw because of ‘suction’
- fluid pressure only acts downward.