Chapter 3

Distance per unit time

Length between 2 distinct points

Displacement per unit time

Distance travelled in a particular direction from a reference point

Change in velocity per unit time

Quantity that has both magnitude and a given direction

Quantity that has only magnitude

• Acceleration

• Velocity

• Displacement

• Momentum

• Energy

• Speed

• Temperature

• Distance

g = 9.81

Speed = distance / time

Velocity = displacement / time

Velocity

Displacement

• v = u + at

• v² = u² + 2as

• s = ut + ½at²

• s = ½(u+v)t

Horizontal and vertical components of motion

Stopping distance = thinking distance + braking distance

Distance the car travels between the driver seeing the hazard and applying the brakes

Distance the car travels between the driver applying the brakes and the car coming to rest

thinking distance = speed of car * reaction time of driver

d = (1/2mv^2) / F

• tiredness

• alcohol + drugs

• distractions

• age of driver

• higher speed

• poor road conditions

• poor condition of tyres

• mass of car

f = ma

single force which has the same effect as the sum of all the forces acting on a body

the force that casues a mass of 1 kg to have an acceleration of 1 ms^-2

w = mg

the force in a string or cable when stretched

the force that acts perpendicular to the point of contact of the body and the surface

upwards force that a fluid exerts on a body floating in it due to the fluid being displaced

the force between 2 surfaces in contact with each other

the frictional force experienced by an object travelling through a fluid

• cross-sectional area of object

• density of fluid

• object initially accelerates as the downwards gravitational force is greater than the resistive forces

• drag on the object increases as it accelerates

• eventually drag = weight of object

• resultant force = 0

• falls at a constant velocity (terminal velocity)

resultant force acting on an object = 0

the turning effect of a force around a fixed point

the product of a force and the perpendicular distance of its line of action from the point

when 2 equal, antiparallel forces act to produce a rotation

turning effect of a couple

states that for an object in rotational equilibrium, the sum of anticlockwise moments equals the sum of clockwise moments

the single point at which all of the mass of the object can be assumed to be situated

(for a symmetrical body of constant density, the centre of mass will be at the centre of the object)

the single point through which the entire weight of the object can be thought to act

• make 3 holes in random places on the edge of the shape

• hang shape from first hole

• let shape swing and come to rest

• use plumb line to draw a vertical line down from the point of suspension to the bottom of the shape

• repeat steps from a different hole

• there will now be 2 lines on the shape, where they cross is the centre of mass / gravity of the shape

mass per unit volume

density = mass / volume

states that the upthrust exerted on an object is equal to the weight of the fluid displaced by the object

force per unit area

pressure = force / area

p = hpg

pressure increases with depth in water because of the force exerted by the increased weight of the water above

upthrust = weight downwards

upthrust = Ahpg

energy transferred when a force of 1 newton causes an object to move a distance of 1 metre in the direction of the force

w = fd

.

w = Fcosθ

energy cannot be created or destroyed, it can only be transferred from one form to another

energy an object has due to its position in a gravitational field

E = mgh

energy an object has due to its speed

E = 1/2mv^2

• mass

• velocity

loss in GPE = gain in kinetic energy

rate of energy transfer

joules transferred per second

P = Fv

P = wd / t

the ratio of useful energy output to total energy input

efficiency = useful energy output / total energy input

the object will return to its original shape when the deforming force is removed

the object won’t return to its original shape when the deforming force is removed

• 2 equal and opposite forces acting on a wire in order to stretch it

• extension

• 2 or more forces that have the effect of reducing the volume of the object on which they are acting, or reducing the length of a spring

• compression

the extension of an object is proportional to the force that causes it, provided that the elastic limit is not exceeded

F = kx

work done

• E = 1/2Fx

• E = 1/2kx^2

force per unit cross-sectional area

extension per unit length

the maximum stress a material can withstand while being pulled or stretched, before it fails or breaks

the ratio between stress and strain

young modulus = stress / strain

a body will remain at rest or continue to move at a constant velocity unless acted on by an external force

the resultant force on an object is proportional to the rate of change of momentum of the object, and the momentum change takes place in the direction of the force

f = ma

if object A exerts a force on object B, then object B will exert an equal and opposite force on object A

F = ma

p = mv

• momentum is a vector quantity - it has both magnitude and direction

• momentum is directly proportional to both mass and velocity - doubling either means that the momentum will also double

• momentum is conserved in any collision or interaction

the total momentum before a collision equals the total momentum after the collision, provided that no external forces are involved

the product of a force and the time for which the force acts

impulse = force * change in time

rate of change of momentum

F = change in momentum / change in time

impulse

collision in which the momentum and the kinetic energy are conserved

collision in which the momentum is conserved but some of the kinetic energy is transferred to other forms in the collision