Forces and their interactions

What are the 2 types all quantities are either?

• Scalar

• Vector

What is the difference between scalar and vector quantities?

• Scalar forces - only magnitude

• Vector forces - magnitude and direction

Name 3 scalar quantities and their corresponding vector quantity

• Distance and Displacement

• Speed and Velocity

• Mass and Weight

Name 5 other scalar quantities

• energy

• volume

• density

• temperature

• power

Name 3 other vector quantities

• force

• acceleration

• momentum

In vector representations, what represents the direction and magnitude of the vector?

• Direction of arrow: direction

• Length of arrow: magnitude

What is a force defined as?

A push or pull that acts on an object due to the interaction with another object

Name 3 effects a force can have on a body:

• change its speed

• change its direction

• change its shape

Give example of change in speed, change in direction, change in shape by forces

• An engine can increase the speed of a vehicle due to a force called thrust

• A comet’s direction can be affected by gravitational attraction

• A spring can have its shape changed by the force from a heavy load

Name 8 important forces to be familiar with:

• Weight

• Electrostatic force

• Thrust

• Air resistance(drag)

• Upthrust

• Compression

• Tension

• Reaction force

What 2 types can all forces be categorised into?

• contact

• non-contact

What can a contact force be defined as?

A force which acts between objects that are physically touching

Give 4 example of contact forces:

• friction

• air resistance

• tension

• reaction force

What is friction and when does it occur?

• a force that opposes motion

• occurs when objects rub against each other

What is air resistance and when does it occur?

• a type of friction

• occurs when an object moves through air

What is tension and when does it occur?

• A force that pulls 2 objects connected by a length, such as a string or rope

• occurs when a force is applied to the length

What is a reaction force and when does it occur?

• a force that pushes touching objects apart

• occurs when objects are supported by a surface

What is a non-contact force?

A force which acts at a distance, without any contact between bodies, due to the action of a field

Name 3 examples of non-contact forces

• Gravitational force

• Electrostatic force

• magnetic force

What is gravitational attraction and give an example of it

• the attractive force experienced by 2 objects with mass

• e.g. attraction between a proton and an electron

What is an electrostatic force and give an example of it

• A force experienced by charged objects which can be attractive or repulsive

• e.g. the attraction between a proton and an electron

What is a magnetic force and give an example of it

• A force experienced between magnetic poles that can be attractive or repulsive

• e.g. the attraction between the North and South poles of magnets

If Ben drags a sledge behind him as he climbs up a hill, describe the contact and non-contact forces involved:

Contact forces:

• Ben pulls on a rope attached to the sledge producing tension in the rope

• The ground supports Ben and the sledge with a reaction force

• There is friction between the sledge and the ground which opposes the motion

• There is also friction between Ben’s shoes and the ground allowing Ben to climb the hill without slipping down

• A very small force due to air resistance slows Ben’s motion up the hill

Non-contact forces:

• The gravitational attraction between the Earth and Ben pulls Ben and the sledge downwards

What is the magnitude of a force measured in?

Newtons

How can the direction be described as?

Left, right up down or even using angles.

How can the direction of a force be imagined with?

a number line

When describing a force with an angle, what do you need to mention?

The angle with respect to either the vertical or horizontal

What is a force pair?

When there is an interaction between two objects and a force is exerted on each object by another

Explain for example the force pair when a laptop rests on a desk

• The laptop exerts a downward force on the desk

• The desk exerts an upward force on the laptop

How can force pairs be represented in vector diagrams?

By arrows

What is weight defined as?

the force acting on an object due to gravitational attraction

Name 3 things that happen because of weight

• objects stay firmly on the ground

• objects will always fall to the ground

• satellites are kept in orbit

What is mass?

The amount of matter in an object

The more mass an object has, the larger…

the weight force it will experience

What is weight measured in?

Newtons

What two things does the weight an object experience depend on?

• object’s mass

• mass of the planet attracting it

What equipment is mass commonly measured with?

A top pan balance, with it you can indirectly find weight

What equipment can weight directly be measured with?

A calibrated spring-balance (aka a newton-meter)

Describe briefly a newton-meter

It consists of a spring fixed at one end with a hook to attach an object at the other

What is mass measured in?

KG

What is the centre of mass of an object defined as?

The point through which the weight of an object acts

For a symmetrical object of uniform density, where is the centre of mass located?

The point of symmetry (e.g. for sphere, it is the centre)

How can the centre of mass be found for an irregular object?

By locating its balance point

What is weight?

The gravitational force exerted on an object with mass

How proportional are the weight and mass of an object?

Directly proportional

What is the equation for calculating weight?

Weight = mass x gravitational field strength (w = mg)

What is the unit of measurement for g and also what is g also known as other than the gravitational field strength?

• N/Kg   or     m/s²

• acceleration due to gravity

What is free fall?

At what rate do objects on earth accelerate at a rate of in free fall?

• When an object falls solely under the influence of gravity

• 9.81m/s²

Name 2 things the gravitational field strength of planets depend on

• mass

• radius

What is a resultant force?

A single force that describes all of the forces operating on a body

When many forces are applied to an object and they are combined to produce one final force, what does this describe?

the combined action of all of the forces

What 2 things does the single resultant force determine?

• the direction in which the object will move as a result of all of the forces

• the magnitude of the final force experienced by the object

What is the resultant force also sometimes called?

Net force

What 2 types of forces can forces combine to produce?

• balanced forces

• unbalanced forces

What are balanced forces?

When forces combine in such a way that they cancel each other out, so no resultant force acts on a body

Explain then for example how the weight of a book on a table is balanced?

By the normal force of the desk which is equal to the weight of the book so there is no resultant force

What then are unbalanced forces? (+ example)

The forces have combined in such a way that they do not cancel each other completely and there is a resultant force on the object. (e.g. tug of war and one person pulls with greater force)

What is tension defined as?

The force experienced by a cable, rope, or string when pulled, hung, rotated or supported.

What symbol is tension normally labelled as on free body diagrams?

T

What direction does tension always act?

Away from the mass

What is the Normal Contact Force?

What is it sometimes also referred to as?

What is it normally labelled as on free body diagrams?

• The force arising when an object rests against another object acting at a 90 degree angle to the plane of contact

• The reaction force

• N or R

What law is the normal contact force arisen from?

Newton’s third law

What is upthrust and where can it occur?

What is it normally labelled as?

What direction does it always act in?

• The upward buoyancy force acting on an object when it is in a fluid

• Can occur in liquids and gases

• U

• Upwards

What is friction?

What direction does it always act?

What is it normally labelled as in free body diagrams?

What point does it always act at?

• The force arisen when two surfaces are in contact with each other

• Always opposes the motion

• F or Fr

• Where the two objects are in contact

What are free body diagrams useful for?

Modelling the forces acting on an object

What is each force represented as?

A vector arrow

Name 3 things about each arrow

• It is scaled to the magnitude of the force it represents

• Points in the direction that the force acts

• Is labelled with the name of the force it represents

What 2 things can free body diagrams be used for?

• To identify which forces act in which plane

• To resolve the net force in a particular direction

What may happen to a force acting at an angle?

It could be broken down or resolved

How many and which components can a single force be resolved?

• a horizontal component

• a vertical component

When several forces act on an object in different directions, how can the resultant force be found

by adding the arrows together

With two component arrows what method do you use to find the resultant force?

Put them head to tail, so single force starts at tail of first vector and ends at head of second

What does resolving vectors into components allow for in terms of the calculation of resultant forces?

More accurate calculations

What type of diagram can we use to construct a final resultant force vector from resolving involved forces into their components and adding or subtracting?

A scale diagram

What 2 methods are there to combine vectors using a scale diagram?

The triangle method and the parallelogram method

What are the steps to the parallelogram method? 

1. Link vectors tail to tail

2. Complete the resulting parallelogram

   1. The resultant vector is the diagonal of the parallelogram

What are the steps to the triangle method?

1. Link vectors head to tail

2. Form resultant vector by linking tail of a to head of b