forces

what is a force

a push or pull acting on an object

what can a force do

change shape (stretch or squash), change its motion (accelerate or decelerate) and change its state of rest (begin moving).

types of forces

contact forces (friction, air resistance, tension) and non-contact forces (gravitational, magnetic, electrostatic)

newtons first law

an object will remain at rest or in uniform motion unless acted upon by a resultant force

newtons second law

the acceleration of an object is directly proportional to its mass (F=ma)

Newtons third law

for every action, there is an equal and opposite reaction

weight

the force of gravity acting on an objects mass

terminal velocity

the constant speed that a freely falling object eventually reaches when the resistance of the medium through which it is falling prevents further acceleration

resultant force

the overall force acting on an object after all forces have been taken into account

free body diagrams

diagrams showing all the forces acting on an object

equilibrium

when the resultant force on an object is zero

distance

a scalar quantity that refers to “how much ground an object has covered during its motion

displacement

a vector quantity that refers to “how far out of place an object is” and the objects overall change in position

representing vectors

an arrow, direction of arrow = direction of vector quantity, length represents the magnitude and a number can be added to indicate force magnitude

force calculation

mass (kg) X acceleration (m/s²) or m x a

inertia

the tendency to stay stationary or continue with uniform motion

inertial mass

measure of how difficult it is to change the motion of an object

mass

the amount of matter in a substance/ object, measured in kg

acceleration

an objects change in velocity (faster/ slower) measured in m/s²

weight equation

weight (N) = mass (kg) X gravitational field strength 9.8 (m/s²) or w = m x g

work

to move an object with an applied force

work equation

work (J) = force (N) x distance (m)

friction

when 2 surfaces slide past each other, creating a friction force

ways to reduce friction

• lubricant

• smooth surfaces

• smaller surface area

elastic objects

an object that regains its shape when the forces deforming it (squashing or stretching) are removed

compression

a force is exerted on an object, compressing it - e.g. spring

tension

when a force pulls on an object

hookes law

force applied (N) = spring constant (N/m) X extension (m) or F = k x e

spring constant

how stiff a spring is, steeper on graph stiffer the spring is, unit is K and is measured in N/m

distance travelled equation

distance travelled = speed x time or s = v x t

acceleration equation

acceleration = velocity / time or a = v/t

final velocity equation

final velocity = initial velocity + 2 x acceleration x distance or v² = u² + 2as

thinking distance

distance travelled in between the driver realising he needs to break and actually breaking

braking distance

distance taken to stop once brakes are applied

factors affecting thinking distance

• car speed

• intoxication

• concentration/ tiredness

• visibility

factors affecting braking distance

• car speed

• vehicle mass

• brake condition

• tread

• weatherst

stopping distance

thinking distance + braking distance

momentum

can be thought as “unstoppability” and is a vector

momentum equation

momentum (kgm/s) = mass (kg) x velocity (m/s) or p=m x v

momentum examples

a bullet is hard to stop as it is fast, and a rhino is hard to stop as it has a large mass

conservation of momentum

in a collision between 2 objects, momentum is conserved (kept the same), total momentum before collision = total after

crumple zones

areas of a vehicle designed to crush in a controlled way during a collision (bonnet and rear)

air bags

increases the time taken or the heads momentum to reach 0 - reducing the force and acts as a soft cushion that prevents cuts

seat belts

stop you travelling forwards or around if you collide, stretches in a collision

impact force equation

force (N) = (mass (kg) x change in velocity (m/s)) / time taken (s) or F= (m x △v) / t

moment

turning effect of a force

moment equation

moment (Nm) = force (N) x distance (m) or M = f x d

levers

act as a force multiplier (reduce force needed to perform tasks)

balancing moments

when an object is not turning around a pivot, the total clockwise moment must be balanced by the anti-clockwise moment

gears & pulleys

simple machines used to transmit rotational forces whilst also increasing/ decreasing this rotational force (turning effect)

pressure in fluids equation

pressure (pascals or N/m²) = force (N) / area (m²) or P = f/a

fluid

liquid or a gas

pressure equation

pressure (N/m²) = density (Kg/m³) x gravity (N/kg or m/s²) x height (m) or p = ρ x g x h

water dams

have a thicker base than top to manage the pressure

upthrust

the upward force that a fluid exerts on a body/object floating in it

archimedes principal

an object totally/partially immersed in a fluid is subject to an upward force equal in magnitude (size) to the weight of fluid it displaces

floating

an object floats when its weight is equal to the upthrust, it sinks when its weight is greater than the upthrust

atmospheric pressure

air pressure is caused by the force of air gas particles bumping into things, it’s equal in all directions

collapsing can practical

air pressure in can = pressure outside, heating the can with water replaces air with water vapour and the atmospheric pressure crushes the can

finding centre of mass on symmetrical objects

where axes of symmetry meet

finding centre of mass on irregular objects

• using a small hole in the object, hang it (freely swinging)

• hang a plumb line (string with weight on the end) from the same suspension point

• mark the vertical line directly below the suspension point

• the centre of mass is where they meet

centre of mass

an imaginary point where the whole weight of the body seems to act, if a body is hanging freely at rest, its centre of gravity is always vertically below the pivot

stability

measure of the body’s ability to maintain its original position