FORCES 1. What is a force? contact forces and non-contact forces
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Suitable for GCSE/IGCSE Physics/Science courses or their equivalent
This page will answer questions such as ...
What are scalar and vector quantities?
What is a force? What does it do?
What is a contact force?
What is a non-contact force?
What is a force?
The first thing to say is you can't see a force!
BUT, you can observe its effect and often quantify it with equations.
The unit of force is the newton (N).
A force is a push or pull effect, acting on an object when it interacts with something.
The result of the interaction depends on the nature and magnitude of the forces involved.
The value of force can be very small or very large, from zero to an 'immeasurable' value at the centre of a black hole!
You are familiar with the results of electrical, magnetic or gravitational forces and mechanical forces such those acting in the spring of a clock.
You will learn formula to do calculations on gravity or acceleration.
What are scalar and vector quantities?
A scalar quantity only has magnitude (a numerical quantity), but no specific direction.
A vector quantity has both magnitude and specific direction.
Contact forces and non-contact forces - a comparison with examples
Interactions between objects
A 'force' interaction is a pair of equal and opposite forces acting on two different objects e.g.
If you push down on the floor, the floor pushes back up on you.
The forces of you and the floor are equal and both objects experience a force.
This is an example of Newton's 3rd Law which can be stated in various ways:
When the moon is pulled towards the Earth by the Earth's gravitational field force, there is an equal and opposite force operating as the moon's gravitational field pulls the Earth towards. If the forces were not equal, either the moon would drift away into space or collide with the Earth! Fortunately, its a good example of a non-contact force operating!
All objects standing motionless on the ground are examples of opposite contact forces operating. The weight of the object acting as a downward due to gravity is balanced by an upward push from the ground as it is minutely compressed. If the forces were not balanced, either the ground would sink or the object would be raised up!
BUT, take care with such descriptions, analysis of the above situation reveals some complications!
Consider the flask of liquid standing motionless on a laboratory bench. There are two sets of forces operating shown by the arrows of opposing direction, but the same length - same magnitude of force. Both sets of forces are examples of Newton's 3rd Law, but don't mix the two up!
The normal contact force due to the weight of the object acting (pushing) down on the surface of the bench (F1) is balanced by the bench under minute compression pushing back up to an equal and opposite extent onto the flask (F2).
At the same time both the flask and the Earth (including the bench) are mutually attracting each other (F3 and F4) to an equal and opposite extent due to the non-contact force of gravity (it makes no difference whether the objects are in contact or not here, gravity acts throughout everything!).
In the cases described so far there is no resultant force, everything is balanced. If the forces were not balanced and there was some net resultant force, the object would move or be reshaped - something would change!
For stationary objects, if the resultant force acting on the object is zero the object is said to be in equilibrium (effectively means a state of balance).
More complex force situations
(a) A 'free body force diagram' of a cyclist showing all the forces acting on the body (not to a force scale)
A 'free body force diagram' should show every forces acting on an isolated object (body) or system but shows none of the forces it exerts on the surroundings. The size of the arrows can indicate the relative magnitude (size) of the force.
There are four forces acting on the body (= bike + cyclist):
(b) A 'free body force diagram' of a swimmer showing all the forces acting on the body (not to a force scale)
(c) A 'free body force diagram' of a parachutist showing all the forces acting on the body (not to a force scale)
(d) Skiing involves the forces of F1 weight (gravity force acting on skier), F2 friction (between snow and ski) and F3 air resistance (friction between skier's clothing and the surrounding atmosphere brushing over the surface).
In diagrams to resolve numerical problems, the length of the arrow should equal the magnitude of the force OR a numerical force value indicated on the arrow.
At the moment for amusement only!
all non-equilibrium situations!
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