Forces

Force definition

• A push or pull that acts on an object due to its interaction with another object (N) e.g. a hand pushing a box

• Forces have direction and magnitude (vector)

Contact forces

• Contact- objects physically touching e.g. friction, air resistance, tension, normal, contact forces (equal and opposite forces in each direction)

Non contact forces

• Non-contact- don’t require objects to be touching e.g. gravitational, magnetic and electrostatic (‘field of influence’) magnetic and electrostatic can be attractive or repulsive

• Strength will decrease as objects get further apart

Scalar quantities

• Only have magnitude (size) and no direction

• E.g. speed, distance, mass, temperature, time, power

Vector quantities

• Have both magnitude and direction

• E.g. velocity, force, acceleration, momentum, displacement, weight

How to express vectors?

Arrows- length is magnitude and way pointing is direction

Free body diagrams

• Force arrows- represent all forces acting on an object

• All have to have both magnitude and direction

• Length of arrows show magnitude- some cancel each other out and what we have left is resultant ‘the overall force’

• Look at horizontal and vertical separately

• If there is no resultant force, the object is in equilibrium

Scale diagrams

• On scaled paper, allow the tip of the verticals vector to touch the tail of the horizontal vector

• Draw a line from starting point to end across (making a triangle) and measure with ruler

• Convert into newtons using scale

• Direction- measure angle of point

• If arrows join up perfectly- there is 0 resultant force and the forces are balanced in equilibrium

Resolve vectors

• Represent force with same angle as direction and draw to a scale

• Draw horizontal and vertical lines

• Convert and find forces

Elasticity

• A force can cause an object to compress, stretch or bend e.g. in a spring or ball (less elastic so harder to notice)

• Must have more than 1 force applied e.g. contact force

Types of deformation

• Elastic- returns back to original shape

• Inelastic- stays deformed ‘plastic’

Extension

• Increase in length of a spring when it’s stretched

• Measure how springs length changes as we add a downwards force

• Springs own mass will be exerting weight, which takes away from the natural length

• A mass on the bottom of a spring will increase the length- which we can measure as the extension

• An equal but opposite force will be exerted upwards- perfectly balanced

Extension increasing in proportion

• Increasing force (adding more mass), the extension increases proportionally

• F ∝ E

• The extent of the extension depends on the spring constant F = ke

What does the spring constant tell us?

• How many newtons it would take to stretch the object by 1m

• Higher spring constant = stiffer material

  • Requires more force

Hookes law

• Force and extension are directly proportional

• Elastic deformation- object can return to original state

• However, it has an elastic limit where Hookes law no longer applies and object cannot return to original state (inelastically deformed)

Lower and higher spring constant

• A lower spring constant- more elastic

• Higher- less elastic

• A measure of the energy required to stretch an object

What is elastic potential energy?

The energy transferred to an object as it is stretched

Force x extension graphs

• On only the straight part of the line, the gradient will be the spring constant

• Area under curve- energy transferred to spring as elastic potential energy

• Before it reaches ‘elastic limit’ or ‘limit of proportionality’

Speed

• Scalar- only has magnitude e.g. plane moving 250m/s

• Distance (scalar) / time taken

• S = d/t

• Speed = distance / time

Velocity

• Vector- has both magnitude and direction e.g. person cycling 6m/s east

• Displacement (vector) / time taken

• V = s/t

• Velocity = displacement / time

Distance

Scalar- only gives magnitude e.g. 10 meters

Displacement

Vector- has direction and magnitude e.g. person running 40 meters east

What happens to the velocity when objects don’t move at a constant speed?

We have to divide the total distance / displacement and divide this by the total time

This gives the average velocity

What is acceleration?

• The rate of change in velocity

• (How quickly something speeds up or slows down)

• Acceleration has direction as well as magnitude- can be negative if the object slows down (decelerates)

• Acceleration is the average- it may have accelerated more in the first few seconds OTHERWISE it is uniform

If missing values from an acceleration equation…

• Assume the initial velocity is 0- it started stationary

• Assume the acceleration on a dropped item is 9.8 due to the force of gravity

Distance/time graphs

• Allow us the visualise how far something has travelled in a certain period of time

• The gradient of the line at any point tells you the speed at this point

• Straight line- constant speed

• Flat line- stationary, both gradient and speed 0

• Steeper- gradient/speed increasing (acceleration) )

• Decreasing gradient ( deceleration

• On an accelerating curve, have to draw tangent to find gradient at point

Velocity/time graphs

• Gradient of steep line shows acceleration or deceleration (negative)

• Flat sections show velocity is constant- only use y value

• When curve gets steeper, rate of acceleration is increasing

• Distance is the area under the line- can be split into triangles and rectangles

• OR if curve, distance can be found by counting squares

Terminal velocity definition

• Where velocity remains constant- so no longer accelerating or decelerating

Stages of terminal velocity

• When an object first falls, its weight downwards is larger than air resistance upwards, resultant force downwards, accelerates

• However as velocity and acceleration increases, air resistance increases until it equals the weight

• There is no resultant force: so we say the object has reached terminal velocity- it will stay at this velocity until a sudden change

Newtons 1st law

• A resultant force is required to change the motion of an object

• Motion wont change if resultant force is 0- stays stationary or constant

Newtons 2nd law

• If a non-zero resultant force acts on an object, it will cause the object to accelerate

Rules for Newtons 2nd law

• Stationary- start moving

• If forces act in the opposite direction, the object would slow down or even stop

• Change in direction

• If act in same direction, speeds up

Circular motion definition

• Direction is always changing slightly- this also changes velocity so it is acceleration

• E.g. orbit of moon, speed remains constant but still accelerating because direction always changing because the earths mass exerts a gravitational pull on the moon, so the moon has a constant changing velocity but constant speed

Newtons 2nd law part 2

• The size of the resultant force is directly proportional to the acceleration it causes

• F=mxa

Inertia definition

• A tendency for the motion of an object to remain unchanged

• Unless acted on by resultant force, objects at rest will stay at rest and those in motion will stay in motion

Inertial mass definition

How difficult it is to change an objects velocity (how big the force required is)

Force/acceleration

Newtons 3rd law

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

Equal (magnitude), opposite (direction)

Stopping distance definition

the minimum distance required to stop a vehicle in an emergency

Stopping distance = thinking distance + braking distance

Thinking distance definition

How far the car travels during the drivers reaction time (Time between the driver seeing the hazard and applying brakes)

Things that affect thinking distance

1. Speed (faster further you’ll travel)

2. Reaction time (vary between people, tiredness, drunk, drugs, distracted)

Braking distance definition

Distance taken to stop under the braking force

Things that affect braking distance

• Speed and mass- both increase kinetic energy which needs to be reduced to stop

• Quality of brakes- worn or faulty cannot apply as much pressure, can’t slow car down quickly

• Condition of tyres

• As speed increases, total stopping distance increases

Momentum

• Vector quantity- magnitude and direction

• In a closed system, the total momentum before an event is the same as the total momentum after

• Find total momentum before

• Positive- to the right

• Stationary objects will always have 0 momentum