# Forces

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• Created by: Jmsmcn
• Created on: 24-09-17 11:12

## Contact and Non-Contact Forces

A force is a vector quantity and vectors have magnitude and direction. Other vector quantities include: force, velocity, displacement, acceleration, momentum.

Some physical quantities only have a magnitude these are scalar quantities. Other scalar quantities include: speed, distance, mass, temperature, time.

A force is a push or a pull on an object that is coaused by it interacting with something.

When two objects have to be touching for something to act it is a contact force. These include: friction, air resistance, tensison in ropes, normal contact force

If two object do not need to be touching for the force to act the force is non-contact. These include: magneetic force, gravitational force, electrostatic force.

When two objects interact there is a force on both objects. An interaction pair is a pair of forces that are equal and opposite on two interacting object (the chair pushes on the ground and the ground pushes on the chair equally). The sun's attraction to the Earth and the Earth's attraction to the sun is the same

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## Weight, Mass and Gravity

Gravity is a force of attraction between masses. It makes things fall to the ground and gives things a weight

Mass is the amount of "stuff" in an object and does not change. Weight is a force acting on an object due to gravity. Gravity varies with location (it is stronger with larger masses and the closer you are to the mass). Because wiegth depends on the gravitational field strentgh it varies with location.

Weigh is a force so it is measured in Newtons (N) with a spring balance. You can think of the force acting on a single point this is the centre of mass. Mass is not a force and it is measured in kilograms with a mass balance

Mass and weight are directly proportional because of the equation:

Weight = Mass (KG) X Gravitational Field Strength (N/KG)

The gravitation field strength on Earth is 9.8 N/KG

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## Resultant Forces and Work Done

Free body diagrams show the forces acting on an object. The size of the arrows shows the magnitude of the object and the direction of the force

A resultant force is the overall force on a point or object. If you have lots of forces working on a single point the conbination of those forces is the resultant force. You add forces that act in the same direction and subtract forces that act in the opposite direction

When a force moves an object through a distance energy is transferred and work is done on the object. To make something move or keep it movng a force must be applied. The thing applying the force needs energy the force moves the energy from one store to another

Energy can be transfered usefully or it can be wasted. When you push an object you transfer energy usefully to kinetic energy and waste it in the form of heat because of friction

Work done (J) = Force (N) X Distance (m)

1 joule of work is done when a 1 netwon force  causes an object to move 1 metre.

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## Calculating Forces

You can use scale drawing to find a resultant force. Draw all the forces tip to tail and measure the distance between the start and the end.

An object is at equilibrium if the forces are balanced. If all the forces combine on an object to make 0 it is at equilibrium it will not speed up or slow down. The tip to tail diagram will force a full shape.

If a force acts at an angle you can split it into a vertical and horizontal force. Using a scale diagram you make it a right angle triangle. When they act together they have the same effect as a single force

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## Forces and Elasticity

Stretching, compressing and bending requires at least two forces as with one force the object would just move. This transfers energy

An object which has been elastically deformed can go back to its shape when the force is removed. An object has been inelastically deformed if it doesn't return to its shape when the force is removed. When s a spring is compressed energy is transfered to elastic potential energy.

The extention of a spring is directly proportional to the force:

Force (N) = Spring Constant (N/m - measure of stiffness) X Extention (M)

The spring constant is differnt for different springs - a stiffer spring has a higher constant. The equaltion can be used for compression as well

There is a point at which the force and extention is not proportional. On a graph this is when starts to curve. This point is the limit of proportionality.

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## More on Springs

You can work out the energy stroed in a spring as long as it has not passed its limit of proportionality.

Elastic Potential Energy (J)  = 0.5 X Spring Constant (N/m) X Extention (m)

For elastic deofrmation this calulates the energy stored and the amount of energy transfered to the spring. This is also the amount of energy that is relased when the spring returns to its normal shape.

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## Moments

A moment is the turning effect of a force. A force can cause and object to rotate the size of the moment depends on the force applied and the distance it is applied at. The pivot is the point the force turns around. To get the maximum force you need to push at a right angle otherwise the perpedicular distance is less

If the clockwise force equals the anti-clockwise force the object does not turn

Moment (Nm / J) = Force (N) X Distance (m)

Levers make it easy to do work because they increase the distance from the pivot. This makes it easier to do work.

Gears transmit rotational forces. A force transmitted to a larger gear will cause a bigger moment as the distance is larger. The larger gears will turn slower than the smaller gears.

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## Fluid Pressure

Pressure is the force per unit area. Fluids are subsnatces that flwo. As they can move they collide with things this exerts a force. Pressure is exerted normal normal to any surface in contact with the the fluid

Pressure (Pa) = Force (N) / Area (m^2)

Pressure in liquids depends on depth and density. Liquids do not vary in density but gasses do . Density is a measure of compactness. The more dense an object the more particles that surfaces can collide with. As depth increases the number of particles above increases the weight of these particlles adds pressure.

Pressure at a certain depth (Pa) = height of the column (m) X Gravitaional Filed Strength (N/kg) X Density (kg/m^3)

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## Upthrust and Atmospheric Pressure

Objects in a fluid experience upthrust. When an object is submerged the liquid applies a pressure from every direction and because pressure increases with depth the pressure is greatest on the bottom of the object (pushing up). This causes an upward resultant force.

If the upthrust is equal to the weight of the of the object it floats because the forces balance (this happens when the object is less dense than water). If some of an object is above water it is because the weight of the whole object is equal to the weigth of the water displaced.

If an object weighs more than the force of upthrust (it is more dense than water it will sink) becaue it will never displace enough water to equal. Submarines use this - to sink they fill up with wtaer so they are more dense and to come up they pump the water out so they are less dense.

The atmosphere is a layer of air the surrounds the Earth. it creates pressure because air molecules collide with the surface. As you go higher up the air pressure decreases. This is because the atmosphere gets less dense so there are less collisions. Also there is less atmosphere above you so there is less weight.

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## Distance, Displacement, Speed, and Velocity

Distance is scalar and displacement  is vector. So if you walk 5m north and them 5m south the distance is 10m but the displacement is 0m

Speed is scalar and it is just how fast you are going in any direction. But velocity is how fast you are going in a specific direction. As a result an object can be going at a constant speed but with a changing velocity as it is changing direction.

Distance travelled (m) = speed (m/s) X time (s)

Walking = 1.5 m/s

Running = 3 m/s

Cycling = 6 m/s

Car = 25 m/s

Train = 55 m/s

Plane = 250 m/s

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## Acceleration

Acceleration is how quickly you are speeding up. Decceleration is negative acceleration

Acceleration (m/s^2) = change in velocity (m/s) / time (s)

You may to estimate acceleration

Uniform acceleration means a constant acceleration - accelereation due to gravity is unifrom acceleration

Final velocity^2 - initial velocity^2 = 2 X Acceleration (m/s^2) X Distance (m)

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## Distance-Time, and Velocity-Time Graphs

Distance time graphs show journeys. Gradient = Speed, flat sections are stationary, straight uphill section mean it is tarvelling at a steady speed, curves mean it is speeding up or slowing down.

Velocity time graphs also show journeys. Gradient = acceleration, Flat sections are when it travels at a steady speed, uphill is acceleration, downhill is deceleration, curves are changing acceleration.

The area under the velocity time graph is equal to the distance travelles in that time interval

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## Terminal Velocity

Friction is always there to slow things down this is because if it is moving it will have things rubbing against it. To travel at a steady speed you need to equal the friction force.

Drag (resistance in a fluid) is resistance in a fluid. As speed increases drag increases. To reduce drag the object needs to be streamlined

Parachutes work in the opposite way they increase drag to slow you down.

Objects falling rach a terminal velocit where they do not speed up any more. This is because the friction from the air equals the downward weight force. This menas the resultant force is zero so it moves at a steady speed

Terminal velocity depends on the objects drag in comparison to its weight. So a person with a parachute has a lower terminal velocity as there is more drag

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## Newton's First and Second Laws

First Law:

If the resultant force is zero the object will move at a steady speed (including if that speed is 0). If the resultant force is not zero the object is: starting, stopping, speeding up, slowing down, or changing direction.

Second Law:

Acceleration is proportional to force. The larger the resultant force the more it accelerates (for the same mass), they are directly proportional. Acceleration is inversely proportional to mass, so an object with a larger mass will have less acceleration for the same force. This is all summed up in this equation:

Force (N) = Mass (kg) X Acceleration (m/s^2)

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## Inertia and Newton's Third Law

Inertia is the tendency for motion to remain unchanged. An object will stay at resut until acted upon by an unbalanced force and an obejct will remain in motion until the same happens.

An object inertial mass is a measure of how difficult it is for an object to chaneg velocity. It is found with the equation: mass = force/acceleration

Newton's Third Law:

This says that when two object interact the forces they exert on eachother are equal and opposite. When you push a wall it pushes back with the same force in the oppsoite direction until you stop.

If two ice scaters push against eachother the ligher one will have more acceleration because they have the same force

For it to be Newton's third law the forces must be the same

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## Stopping Distances

Stopping distances are important because the longer it takes to stop the more danger the thing you are stopping for (person in the road) is in)

The stopping distance is determined by the thinking distnace and the braking distance

The thinking distance is how far the car travels while the driver reacts. This is determined by your speed (travel faster in the time while the driver reacts) and your reaction time.

The braking distnace is the distance taken to stop when the brakes are applied. It is determined by your speed, the weather (the amount of friction on the road), the condition of the tyres (amount of friction on the tyres), how good the breaks are (how much force they can apply)

The less friction the more time the car will skid for.

Braking relies on friction, when the brake is pushed the brake pad is pushed on to the wheel, this causes friction while means kinetic energy is transfered to heat and sound. The fatser the car is going the more energy the car has so the longer it take. If there is a very large deceleration it can be dangerous as the car can skid.

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## Reaction Times

Reaction times vary person to person. But it can be affected by tiredness, drugs, or alcohol. Distractions can also affect the time taken

You can do the ruler drop test to work out recation time using the v^2-u^2 = 2as reaction and the   a = v/t equation

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## More on Stopping Distance

Drivers should leave enough space to stop so if they need to they can do it safely, Speed limits are very important because they affect the stopping distance

Speed affects braking distance more the thinking distance because of the amount of energy you need to tranfer - when the speed is double the energy is 4 times as much ( 2 squared)

30 mph : 9m + 14m

50 mph: 15m + 38m

70 mph: 21m + 75m

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## Momentum

Momentum is about how much "oomph" an object has. The greater the mass of the object and the greater the velocity the more momentum the object has:

Momentum (kg m/s) = mass (kg) X velocity (m/s)

Momentum before = momentum after: in a closed system this always applies:

If you crash a moving car into a parked car they lock together and move together at a lower speed but the momentum is the same because at the start there was less mass (an increase in mass is a decrease in velocity)

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## Changes in Momentum

Forces cause changes in momentum. If a force causes an object to speed up the momentum increases:

Force (N) = change in momentum (kg m/s) / change in time (s)

This menas if soemthing stops very fast the force is very high this is why cars are designed to slow people down over a longer period of time in a crash, because the longer the time the less force there is.

Cars have many safety features: crumple zones (increase time), seat belts stretch (increase time), airbags (lower you gradually on to the dashboard).

Bike helmets are designed to be curshed - this increases time, crash mats are designed to cushion you so the time is increased

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