FORCES 1. What is a force? contact forces and non-contact forces

Doc Brown's Physics Revision Notes

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.

Examples:

speed (m/s), distance, mass, time, temperature (K or oC), potential difference (V)

A vector quantity has both magnitude and specific direction.

Examples:

velocity (m/s), rate of change of position in a specific direction (compare with speed above)

(you can think of velocity as 'speed in a particular direction', but take care in how you use the words speed and velocity!)

acceleration (m/s2), rate of increase in velocity in a specific direction

momentum (mass x velocity)

forces are vector quantities e.g. electrostatic, gravity, magnetic, pushing, pulling, tension

On diagrams vector quantities are usually depicted with an arrow, the length of the arrow can show the magnitude and the direction (angle) of the arrow shows the direction along which the quantity acts.

In the diagram above, you have two cyclist travelling at the same speed of 2 m/s, but in opposite directions. Therefore, although they have the same speed (same length of arrow), they have different velocities because they are travelling in different directions. Note that the velocity of the left cyclist is formally given a negative sign to indicate the opposite direction of motion (it doesn't mean going slower or slowing down!).

 


 

Contact forces and non-contact forces - a comparison with examples

Contact forces

If two objects have to be touching for the force to act, the force would be described as a contact force. The two objects will be pushing or pulling  on each other e.g.

friction between two surfaces rubbing against each other, the force moving an object forwards is partially countered acted by a force of friction acting in the opposite direction e.g. tyre of a car in contact with the road, pressing the brake pad onto the disc of a car's braking system,

air resistance as an object moves through the atmosphere, friction between the object and air,

the tension in the wires attached to the hook of a crane, if stationary, the tension in the cable is balanced by the weight of the object the crane is lifting

an object resting on a surface involves weight and compression (see 'interactions' section further down the page).

Non-contact forces

If the objects are subjected to a force, but do not need to be in contact with each other, the force would be described as a non-contact force e.g. three classic non-contact force fields acting between objects that are not touching ...

gravity - the gravitational attraction force between any two objects

note: a falling object in air experiences the non-contact force of gravity and the contact force of friction between the object and air.

magnetism - the magnetic force of attraction of iron towards a magnet or two like poles of magnets repelling each other - magnetic field effects

electrostatic force - the attraction (+ -) or repulsion (- - or + +) interaction of two objects carrying an electrical charge - electrical field effects

note: gravity only involves attraction (as far as we know?), but magnetic and electrical forces involve both the forces of attraction and repulsion.

 


 

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:

to every action there is an equal and opposite reaction,

whenever a force acts on one body, an equal and opposite force acts on some other body,

when two objects interact, the forces they exert on each other are equal in magnitude, but opposite in direction.

The two forces are called an interaction pair of forces and they must be of the same type and the same size but acting in opposite directions on the different objects.

On diagrams both forces will be shown by arrows indicating both the direction and magnitude of the vector quantities.

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!

The same argument applies to explain the Earth orbiting the Sun, both bodies experience the same numerically equal force, but acting in opposite directions. The diagram below illustrates these two gravitational force situations.

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):

F1 is the air resistance due to friction between the surface of the bike + cyclist combination and the air.

F2 is the weight of the bike + cyclist combination acting on the road

F3 is the thrust or push of the bike from the power generated by the cyclist

F4 is the normal contact force on the road surface, this is equal to F2 unless the cyclist rises or falls!

 

(b) A 'free body force diagram' of a swimmer showing all the forces acting on the body (not to a force scale)

F1 is the water resistance due to friction between the swimmer and the water

F2 is the weight of the swimmer acting on the water

F3 is the thrust or push of the swimmer from the power generated by the swimming action

F4 is the upthrust of the water on the swimmer (buoyancy effect)

 

(c) A 'free body force diagram' of a parachutist showing all the forces acting on the body (not to a force scale)

F1 is the air resistance (drag effect) due to friction between the parachutist and the air.

F2 is the weight of the parachutist due to gravity, pulling the parachutist downwards.

If F1 = F2 the parachutist will fall at a constant speed, a constant velocity if no side wind.

F3 is a push on the parachutist by a side wind. If it is zero the parachutist will fall vertically.

Note that the parachutist can pull on the cords of the chute and alter the direction of the drag effect to manoeuvre into a safe and intended landing location!

 

(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!



INDEX


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