Forces_Concept_2
Pushing and pushing back
Pushing and pushing back
Consider the process of stretching an elastic band. When you were holding the stretched band your hands were not moving. But you could still feel a force acting on them. Your pull and that of the elastic band were cancelling each other out. such situations, the forces are said to be balanced.
Forces may also be balanced' when something significant is happening for example, when a skydiver is in 'free fail', The skydiver is falling at a constant speed: there is no change in the Speed or direction of the fall, or in the shape of the skydiver, The forces of gravity and friction (air resistance) acting on the falling skydiver are balanced (see below). When the skydiver reaches the ground, there wi\i be a change in the speed of fall as the forces become unbalanced (and perhaps a change in the shape of the skydiver) before the forces are balanced again.
Consider the book on the table, use a bigger book for the following: Place the book on the table: can you see the balanced forces? Gravity is making the book push down against the table the table is pushing back stopping the book from falling to the floor. Place your hand between the book and the table: can you feel the book pressing down and the table pushing up? If the book is heavy enough, your hand may be changing shape. These forces are normally acting between the book and the table, but you may not be aware of them unless you intervene in this way.
Now take the book off the table, The book is no longer pushing down on the table, so what is the table doing? Put your arm out straight and hold the book on your upturned hand. Keep the forces balanced: push up as much as the book pushes down. Don't stop pushing up, but take the book away: what happens? Your arm shoots up. Why doesn't the table do this? How does it 'know* when to stop pushing? The explanation is that the table exerts a purely reactive force: one that only exists in reaction to another force. This is similar to the way that friction only works to slow something down, When an object stops moving, friction does not throw it back the way it came. Your arm keeps a book in the air by providing an active force through the muscles.
All around you, there are examples of balanced forces such as the book placed on the table. Most structures have to resist moving or changing shape when a force is applied to them. If the force is taken away, they will still not move or change shape. Of course, the strength of any structure has limits: if the applied force is too great, it will collapse or move. If an elephant had been placed on the table, the outcome might have been different!
According to Newton's Third Law of Motion, for every action, .there is an equal and opposite reaction. So any force will be experienced by two things at once. Think of an arm-wrestling match. Who, apart from the two antagonists knows whether they are both truly straining or both faking effort? Only those directly involved in the force are able to feel it, in opposite directions.
For example, when you impart a force through a slap on the back both the hand and the back fee! the force If you immerse yourself in the front row of a rugby scrum, you win fee! the force pushing against you regardless of whether it is your team or the opposition who are doing the pushing.
Now imagine yourself as a skydiver jumping from an aeroplane (see concept 1), There will come a point your fall to the ground when you cannot get any faster. Why is that? Surely gravity will have its effect all the way to the ground. As you fall you will experience an updraft from the air caused by your rapid downwards motion through it. As you move through the air it pushes back against you. There will come a point when it is pushing back against you as hard as gravity is pulling you. At this point, you will not be able to fall any faster: you will have reached terminal velocity.
As this upwards push of the air, or air resistance, increases in proportion to the surface area you are presenting to the air beneath you, the only way to fail faster is to reduce your downwards surface area by making yourself more 'streamlined'. If you have been falling in the i beiiy-down, limbs-out' position you are clearly not trying to go very fast Your fastest possible speed in that position will be 55ms (metres per second) as determined by experiments. A head-first. or feet-first dive will allow a much more streamlined (low air resistance) fall, giving a speed up to 80ms.
However; I suspect that when you jump out of an aeroplane, your intention is to make as slow a descent as possible. Gravity is still pulling you down with the same force: you cannot change that. The best strategy is to make yourself as un-aerodynamic (or as poorly streamlined) as possible by increasing your downward facing surface area. A handy way of doing that is to use a parachute, This makes the upthrust from the air much greater, effectively causing air resistance to balance the downward force of gravity at a much lower terminal velocity. You are still falling but you will ht the ground at a much lower speed than you would without the parachute,
Children will constantly encounter opposed forces balancing each other in everyday life. A detailed examination of such situations will allow greater understanding,
Balanced forces — where the forces acting on an object are equal and opposite, causing no change in the object's speed direction or shape.
Unbalanced forces - where the forces acting on an object are greater in one direction than another, causing a change in its speed, direction or shape.
If a body is not moving, then there is no force acting upon it.
The child needs to consider situations where forces are balanced and there is no movement. Suggest that he or she pushes against -the classroom walk there will (hopefully) be no movement, but child should be able to feel a force. Take the child through the activity of placing a hand between a heavy book and a table to feel the forces .
If an object is moving, there must be a force acting in that direction.
If you push a toy car and then let go, it immediately starts to slow down. The only horizontal force acting on it is friction. The child needs to distinguish between movement and acceleration (a change in the speed or direction of movement). If there is acceleration then there is a force in the direction of the acceleration. Whenever something is slowing down, the dominant force acting on it is opposite to the direction of movement.
Pushing and pushing back (identification)
Children identify (verbally in a class discussion, or by making diagrams individually or small groups) situations in which they push and get pushed back (for example: leaning against a wall, sitting on a bench), Help them to see that the push back does not follow the original push (except in a fight).