Physics revisionDensity

Density = ________ SI units ___________ other units ____________.

Density is a property of the material, it does not change if shape changes.

Density of water is 1g/cm3 or 1000kg/m3.

Objects whose density is lower than water’s float, and objects whose density is larger than

water’s sink.

Two experiments to find density:

1. Finding the density of a liquid 2. Finding the density of an irregular solid.

There are three steps needed to find the mass of a liquid. They are :

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There are three steps to find the volume of an irregular solid. They are :

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Precaution: Read measuring cylinder at eye level.

Vectors and scalers

Scalers are quantities that have size (magnitude) only.

Example : mass, time, distance, speed, volume, temperature.

Vectors are quantities that have size (magnitude) and direction.

Example : velocity, acceleration, force, weight,displacement.

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Distance – time and speed – time graphs

The speed of an object is how fast it goes. It depends on the distance travelled and the

time taken.

Speed = ________ units ___________

The velocity of an object is very similar to speed but being a vector , it tells you in whic

direction the object is moving as well.

The acceleration of an object tells you by how much the velocity increases. It depends on

the initial speed, the final speed and the time taken.

The deceleration ( or negative acceleration ) of an object tells you by how much the

velocity decreases. It depends on the initial speed, the final speed and the time taken.

Distance - time graph

The speed of an object can be found by finding the gradient.

Example find the speed of the red line for the first part of the journey.

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Question: Does the blue line have a higher or lower speed? Give a reason for your answer.

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Velocity – time graphs

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Equations of motion

When an object is in motion, we can measure certain quantiies

Distance s, time t, initial speed u, final speed v and acceleration a.

The equations are :

s = __________________ s = __________________ v = _______________

v2 = _________________

words to remember :

at rest and stationary mean that the speed is 0m/s

deceleration means negative acceleration. This means that the acceleration should have

a minus.

Free fall means that the acceleration is 10m/s2 and the initial speed is 0m/s.

In free fall the mass DOES NOT affect the speed or time of falling.

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When something is thrown vertically upwards, the acceleration is -10m/s2 ( since it is slowing

down) and the final speed is 0m/s.

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Newton’s laws of motion

Newton’s 1st law states that :

If the forces are balanced, _____________________________________________________

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Newton’s 2nd law states that :

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Newton’s 3rd law states that :

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The resultant force is the ‘answer’ to combined forces.

If the forces are acting in the same direction, you add them up.

The resultant force is 3 + 4 = 7N

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If the forces are acting in the opposite direction, you subract them

Some forces have specific names such as weight, reaction force, drag ( or air

resistance), friction and thrust ( forward force also called engine or driving force)

In the above image:

The weight of the car is equal to the reaction forces of theground on the tyres. In this

case there is no upward or downward resultant force.

The driving force is larger than the air resistance. This means that there is a resultant

force forward and so there is an acceleration. The acceleration can then be found using

the equation F = ma.

If the driving force and the air resistance are equal to each other then there is no

resultant force and the car moves with constant speed.

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In a parachute falling, if the weight and the drag are the same,

the resultat force is 0N and so the parachute falls with a

constant speed known as terminal velocity.

Momentum

Momentum = _____________________________ units _____________

Momentum increases as mass increases and as velocity increases.

When an object hits something else some of the momentum is transfered to the other object.

The principle of conservation of momentum states that :

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Mu1 + mu2 = Mv1 + mv2.

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The same thing goes for a seperation ( or an expolsion). This could be the case in :

Guns and cannons

Rockets and exploding fuel

Trolleys held together by a spring

People pushing each other away and others

Important : In a seperation, the total intitial momentum is 0kgm/s since te two initial speeds

are 0m/s and the total final momentum is also 0kgm/s since momentum is conserved. This

means that the objects move in opposite directions ( one positive and one negative)

P.S. the object with the smaller mass moves with the larger velocity.

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Centre of gravity

Centre of gravity __________________________________________________________

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In a uniform object the centre of gravity is in the middle. If the centre of gravity is above the

point of contact, the object will balance ( in equilibrium.) . If not the object will lose its

balance.

In the following diagrams mark which of these is in equailibrium

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Equilibrium can be on three types

Stable : difficult to make it topple

Unstable : easy to make it topple

Neutral : impossible to make it topple.

To make an object more stable, the centre of gravity must be lowered. This can be dine in

two ways:

Widen the base

Make the base heavier than the top

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Moments : is the ________________________________________________________

equation : _____________________________________ units ________________

Moment is directly proportional to the force and to the perpendicular distance from the

pivot.

This means that a long level will be easier to turn because it produces a large moment

even if the force is small.

Principle of moments states that ________________________________________

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Forces which act at the pivot do not produce any moment because the distance is 0m

These are usually the weight of the rod and the reaction force at the pivot.

To find these forces we usually use another condition for equilibrium which is

Total upward forces = total downward forces

Hooke’s law

This states that _________________________________________________________

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If the elastic limit is exceeded, the spring becomes permanently deformed and it does

not obey Hooke’s law any more.

Three steps to measure the extension of a spring.

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When a graph of force against extension is plotted, the graph is a straight line through the

origin.

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If a spring is stiffer then the line would be steeper ( more inclined towards the force axis)

If the spring is softer then the line would be less steep ( more inclined towards the

extension axis)

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Pressure

Pressure due to solids depends on two factors , Force and area

Equation : Pressure = ___________________ units ________________

The larger the force the larger the pressure.

The larger the area the smaller the pressure.

Pressure due to liquids depends on three factors, density, gravity and depth

The larger the depth the larger the pressure

The larger the density the larger the pressure

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Pressure does not depend on the shape of the container.

Hydraulic machines :

Hydraulic machines are also known as force multipliers because a small force is exerted

on a small piston and it produces a much larger force on a larger piston.

There are two properties of liquids which make this possible.

Liquids are incompressible

Pressure is transmitted equally throughout the liquid.

Examples: hydraulic jacks, hydraulic brakes.

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