P5-Forces

Vector quantity

Magnitude + direction

Vector Quantities:

* Force
* Velocity
* Displacement
* Acceleration
* Momentum

Scalar quantity

Only have magnitude

scalar quantities:

* speed
* distance
* mass
* temperature
* time

Contact and Non- contact forces

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

* Contact

\-friction

\-air resistance

\-tension in ropes

etc

* Non contact

\-Magnetic force,

\-gravitational force

\-electrostatic force

etc

Interaction pair

When two objects interact there is a force produced on both objects. An interaction pair is a pair of forces that are equal and opposite and act on two interacting objects.

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The sun and earth are attracted by the gravitational force which is a non contact force

An equal but opposite force of attraction is felt by both sun and earth

A chair exerts a force on the ground, the ground pushes back at the chair with the same force. Equal but opposite

Gravitational force

The force between masses

\-gravity attracts all masses

\-the bigger the stronger forces of attraction which is only really noticeable in planets

Gravity makes all things fall to the ground and gives everything a weight

Gravitational field strength

\-Varies with location

\-the closer it is to the mass causing the field the stronger the gravitational field strength is

Centre of mass

Forces of gravity work on a single point the centre of mass where the whole mass is concentrated

Newton meter

Measures weight (N) also a spring balance can measure weight

Equation for weight

Weight (N)=Mass (kg) x Gravitational Field Strength (N/kg)

Resultant force

Left - Right and up-down

Equation for work done

Work done (J)= F (N) x s (distance moved (m))

Finding resultant force

Draw all forces acting on the object from tip to tail and use Pythagoras by joining the lines up

Equilibrium in force

An object is in equilibrium if the forces are balanced

Resolving awkward forces

split into horizontal and vertical components

Stretching, compressing and bending

2 or more transfers of energy into an object at once causing one of these effects to occur

An object which has been elastically deformed can go back to its original shape and length after the force has been removed.

An object which has been inelastically deformed doesn’t return to its original shape

Extension

F(N)=k (N/m) x e (m) (K= stiffness of the material stretching)

Change in proportionality of extension

Known as the limit of proportionality after the amount of force reaches a certain threshold extension is no longer proportional to force.

Extension practical

Calculate weight of masses used through W=mg

Test weights by using identical spring and see if the increase of extension after each weight is added stays the same up to at least 5 weights if it is you need to uses lighter weights or you won’t get enough measurements for your graph


1. Measure the natural length of the spring before any weights are added with a millimetre ruler clamped to the stand
2. Take reading at eye level and add a marker at the bottom of the spring to make the reading more accurate
3. Add a mass to the spring and allow it to come to rest.
4. Record the mass and measure the new length of the spring (the extension is the change in length)
5. Repeat this process until you have at least 6 measurements
6. Plot a force extension graph it will only start to curve if you exceed the limit of proportionality but doesn’t matter if you don’t

If the line of best fit is a straight line it means there is a linear relationship between force and extension (they’re directly proportional) F=ke

K= gradient of the line

When the line begins to bend the relationship is now non linear

Work done for stretching can be found by doing #

Ee (J)= 1/2 k (N/m) x e² (m)

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The energy in the elastic potential store is equal to the area under the force-extension graph

Displacement

Distance moved + direction

Distance travelled equation

s=vt or distance travelled (m)= speed (m/s) x time (use mean speed if changing speed)

Speeds of different things

A person:

* walking- 1.5 m/s
* running- 3 m/s
* cycling - 6 m/s

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Vehicles

* A car- 25 m/s
* A train- 30 m/s
* A plane- 250 m/s

Acceleration

Change in velocity/time (s) or a=Δv/ t

Constant (Uniform) Acceleration

v² - u² = 2as or Final velocity (m/s) ² - Initial velocity (m/s) = 2 x Acceleration (m/s²) x Distance (m)

Distance-Time

If straight line stopped

gradient indicates speed

Curves indicate acceleration or deceleration

Velocity time graphs

Gradient = acceleration

Flat - Steady speed no acceleration

Curved-increasing or decreasing acceleration

Uphill- acceleration

Downhill-Deceleration

Friction

Slows down things if an object has no force propelling it along

Friction always acts in the opposite direction to movement

You get friction when two surfaces are in contact or when an object passes though a fluid (drag) .

You can reduce friction between surfaces with lubricants.

Drag

\-Increases as speed increases

\-Drag is the resistance you get in fluid (a liquid or gas)

\-Air resistance is a type of drag

\-Streamlining the object is the most important factor in reducing drag

\-Streamlined-where an object is designed to allow fluid to flow easily across it , reducing drag. (parachutes work in the opposite way)

Frictional forces from fluids increase with speed. A car much more friction to work against when travelling at 70mph compared to 30. So at 70 mph the engine has to work harder

terminal velocity

When a falling object first sets off the force of gravity is much greater than frictional force slowing it down

As speed increases friction builds up

This gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force (so the resultant force is zero)

It will have reached its maximum speed or terminal velocity and will fall at a steady speed

Streamlining

\-depending on shape and area

\-the less streamlined an object is, the lower its terminal velocity

\-Objects with large surface areas tend to have lower terminal velocities.

\-e.g. if you dropped a marble off a roof it would have a higher terminal velocity than a beach ball as there is more air resistance acting on the beach ball. #

\-so the ball spends lest time accelerating before the air resistance is large enough to equal the accelerating force

Newton’s First Law

A resultant force is needed to make something start moving, speed up or slow down:

If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object zero, it’ll just carry on moving at the same velocity (same speed and direction)

Velocity will only change if there is a non zero resultant force which means it must be accelerating or decelerating.

Newtons second law

Acceleration is proportional to resultant force

The larger the resultant force acting on an object acting on an object the more the object accelerates - they are directly proportional

Acceleration is also inversely proportional to the mass of the object

Force (N) = Mass (kg) x Acceleration (m/s²)

Required practical (extension)

Force and extension of a spring

Clamp stand

heavy weight on the stand

two clamps and bosses

attach metre rule and spring

Top of the spring must be at 0

bottom of spring has a pointer

add 1N weight to the spring and measure the extension

Plot extension against the weight

extension is directly proportional to the weight

A rubber band is non linear

the spring is elastic as the weight returns back to its original shape

If we add to much weight you can overstretch the spring and even after we have taken away the weights the spring would now still show an extension

this is called inelastic deformation

F=k x e

spring constant should be the same at all points before the limit of proportionality

Inertia

An object will stay still or keep the same motion unless you apply a resultant force.

Inertial mass

Measure of how difficult it is to change the velocity of an object. The ratio of force needed to accelerate an object over the acceleration produced

An object with a large inertial mass will require a larger force to produce a given acceleration than an object with a smaller inertial mass

Newtons third law of motion

Whenever two objects interact, the forces they exert on each other are equal and opposite .

Required practical acceleration

trolley holding up a piece of card with a gap in the middle that will interrupt the signal of the light gate twice. Then measure both lengths of card so the computer can work out its acceleration.

Connect the trolley to a piece of string that goes over a pulley and is then connected on the other side to a hook( of known mass) which can have masses added to it

The weight of the hook and other masses attached provide the accelerating force

Make a starting line

Use the formula F=ma

Place the trolley on the starting line so the string is taut and release it

Record the acceleration measured by the light gate as acceleration of the whole system

Investigate the effect of mass by adding mass to the trolley

Investigate the effect of force by adding mass to the the hook, keeping the mass of the whole system the same

Stopping distance

Total distance travelled from when the driver sees the obstruction to them coming to a stop

Thinking Distance + Braking distance

Thinking distance is the distance travelled by the car during the driver’s reaction time

Breaking distance how far car travels when breaks are applied

Stopping distance increases with speed

14m at 30 mph , 55m at 60 mph , 75m at 70 mph

Reaction time

Ruler drop test

Acceleration due to gravity is 9.8 (m/s²) roughly

affected by tiredness, drugs or alcohol

Forces and braking

Breaks convert kinetic energy into thermal energy due to the friction between the breaks and the wheel

This could cause the breaks to overheat and the driver may lose control

Force = mass x acceleration

30 m/s to 0 in 10 seconds

Losing 3 m/s² per second

Momentum

momentum is in all moving objects

non moving objects have no momentum

momentum= mass x velocity

(kg m/s) (kg) (m/s)

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In a closed system the total momentum before an event is equal to the total momentum after the event

Momentum is conserved in a closed system

Recoil in a canon shooting a canon ball