Physics - Motion, Forces and Energy

Mass

Measures how much matter is in an object. Unit: kg

Density

The ratio of mass to volume of a substance

Weight

The downward force of gravity acting on an object due to its mass. Unit: N(Newton)

Density formula

Density = Mass/Volume Unit: kg/m3, g/cm3

Density of water

1000 kg/m3, 1.0 kg/dm3, 1.0 g/cm3

Density in liquids

When a liquid is poured into a container the liquids with the highest density will be towards the bottom and liquids with lower density will be at the top.

Speed formula

Speed = Distance/Time Unit: m/s

What is used to measure small distances that cannot be measured by a ruler

Micrometer screw gauge

Scalar quantity

Has only magnitude no direction. ex. Distance, Speed, Energy, Temperature

Vector quantity

Has both magnitude and direction. ex. Displacement, Velocity, Momentum, Force

Distance

Length a body has travelled between two points

Displacement

The shortest length between the initial and final positions of an object, including direction.

How are resultant vectors found

The magnitude between two vectors at right angles.

Velocity

Speed in a given direction

Velocity formula

Velocity = Displacement/Time, Unit: m/s

Acceleration

The rate of change of velocity

Acceleration formula

a= v-u/t unit:m/s2

In a displacement-time graph how do you find speed/velocity

The gradient of the line, (y2-y1/x2-x1), If gradient is calculated on a slope it is the average speed.

Deceleration in a displacement-time graph

If line is curved and gradient is negative than the object is decelerating. A negative gradient shows that an object is returning to its starting point.

Constant acceleration in a velocity-time graph

Line is straight

Changing acceleration in a velocity-time graph

Line is curved

How to find acceleration in a velocity time graph

Acceleration is determined by the gradient of the line in a velocity-time graph. A steeper gradient indicates higher acceleration.

How to find distance travelled in a speed-time graph

Area under the graph is the distance travelled.

Terminal velocity

• When an object is dropped it accelerates and the only thing acting on the object is its weight(downward force)

• When drag force(upward force) acts upon the object acceleration decreases

• When the upward force becomes equal to the downward force and resultant force is equal to 0 terminal velocity is reached.

Formula for Weight

Weight = Mass x G(9.8) Unit for G: N/kg

Force

Can change the size and shape of an object

Force applied on elastic solids

Elastic solids can stretch when force is applied and will return to its original shape when force is removed.

Spring constant

The force required per unit of extension.

Force formula in relation to spring constant

F = k/x Unit: N/m

How to find spring constant on a force-extension graph

The gradient of the linear section of the graph is equal to spring constant k.

Limit of proportionality

The point at which the graph stops being linear and curves and no longer obeys hookes law so extension is irreversible.

Resultant force

The resultant force is a single force that describes the actions of all the combined forces acting on an object.

Resulatnt force is equal to 0

If resultant force is equal to 0 than object remains at rest or moves with constant velocity in a straight line.

Force formula(with acceleration)

Force = Mass x Acceleration

Friction

A force acting between two surfaces that can impede motion and result in heating. Can act on object moving through a liquid or gas.

Moment

Moment of a force is the measure of its turning effect.

Moment formula

Moment = Force x Perpendicular distance

In terms of moment when is equillibrium reached

When the anticlockwise moment is equal to the colckwise moment there is no resultant force and equillibrium is reached.

Centre of gravity

The point on an object at which all of an objects weight can be considered to act on.

Plumb bob

A mass hanging from a string to define a vertical line and is used to find centre of gravity of an object.

How does position of gravity affect the stability of simple objects?

• If the centre of gravity is below the suspension point it will be in a stable equillibrium

• If the centre of gravity is above the suspension point it will be in an unstable equillibrium

• If object is an a new position everytime it is displaced it will be in a neutral equillibrium.

• Objects with lower centre of gravity and wider base are more stable.

Momentum formula

Momentum= Mass x Velocity

Impulse formula

Impulse = Force x Time for which force acts

Impulse = mv-mu(Change in momentum)

Principle of conservation of momentum

The total momentum before is equal to the total momentum after in a closed system where no external forces act on the object.

Conservation of momentum formula

m1v1 = m2v2 (Initial momentum = Final momentum)

Kinetic energy formula

KE = 1/2mv2

Gravitational Potential energy formula

GPE= mgh where m is mass, g is acceleration due to gravity, and h is height.

Principle of conservation of energy

Energy cannot be created or destroyed only transformed from one form to another. Thus total amount of energy is constant.

Work formula(In relation to distance)

Work = Force x Distance Unit: J(joules)

Power formula

Power = Work done/ Time taken Unit:Watts

Power = Change in energy/ Time taken

Coneversion of watt to joule

1 W = 1 J/s

Kilwatts and megawatts to watts

1kW = 10³ W

1MW = 10⁶ W

Efficiency Formula

Efficiency = Useful energy output/Total energy input x 100

Efficiency = Useful power output/Total energy input x 100

Pressure

Pressure is a force per unit area

Pressure formula

Pressure = Force/Area Unit:Pa(pascals)

Pacals to newton per m2

1Pa = 1 N/m2

Volume of a liquid column

V = Change in height x Cross-sectional area

Mass of a liquid column

Mass = Density x Volume

Weight of a liquid column

mg = Density x Volume(Change in height x Cross-sectional area) x g

Force = Weight

Pressure in liquids

Pressure = Density x Height x g(9.8) Unit:Pa, kg/m³

Pressure of a gas

P = Atmospheric pressure x Height xDensity x g