PHY1: FORCES

This unit is about force and the types it has.

FORCES

Definition

It is something that changes the body’s state of rest or uniform motion in a straight line.

EFFECTS OF FORCES

– It causes moving bodies to reduce speed or stop moving

– It deforms objects

– It causes a body at rest to move

– It causes increment in the spies of moving objects

– It causes a body to change direction

Force is measured in new tons [N]

THIS VIDEO BRIEFLY DESCRIBES FORCE

CALCULATION OF FORCE

Force is calculated by using formulae

F = ma, where m is the mass of the body and a is the acceleration of the body.

Note:

the mass should be measured in kg;

EXAMPLES

Find the force acting on a body of mass 4kg  moving  with acceleration of 2m/s2

F = m x a

= 4 x 2

= 8N

What force does a 620g body exert when with an acceleration of 2m/s2?

F = m x a

620g to kg=(620/1000)kg

= 0.6 x 2

= 1.24N

TYPES OF FORCE

  1. Up thrust force
  2. Gravitational force
  • Friction force
  1. Centripetal force
  2. Magnetic force
  3. Molecular force
  • Electrostatic force
  • Elastic force

GRAVITATIONAL FORCE

This is a force the earth exerts on bodies by putting them towards its centre

The gravitational force is experienced in the following;-

– climbing a hill

– A dry leaf falling from a tree

– An object thrown up in space / our falling down etc

– jumping

This force is experienced in the gravitational force. 

WEIGHT

This is the force a body exerts on anything that freely supports it in the gravitational field.

Weight is obtained from; w = mg where m is mass of the body and g the acceleration due to gravity

 

COMPARISON BETWEEN MASS AND WEIGHT

MassWeight
1.      Quality of mattering bodyForce a body exerts on anything that freely supports in the grow field
2.      Mass is constantChanges from place to place
3.      Mass is scalar qualityWeight is a sector quality
4.      Measured using beam balanceMeasured using spring balance
5.      Measured in kilo gramsMeasured in newtons

 

Note:

Weight values changes slightly from place to place on earth due to;

  1. The shape of the earth i.e. the earth bulge at the equator
  2. The earth’s rotation

 

CALCULATION OF WEIGHT

Weight is measured in newtons.

W = mg, the acceleration of gravity is constant on earth: g =10 m/s2

EXAMPLES

  1. Calculate the weight of a body with mass of 0.8kg

W=mg

= 0.8 x 10

= 8N

  1. Mass of 500g

W= mg

= 0.5 x 10

= 5N

  1. Calculate the mass of a body whose weght is 20N

W= mg

20N = M x 10

= 20/10 =10m/10

M = 2kg

NOTE:

1kg = 10N

  1. What is the weight of a 1kg body

W= mg

=1 x 10

= 10N

1kg = 10N 

EXERCISE

Convert the following;-

  1. 2kg to newtons

1kg = 10N

0.2 kg = 0.2 x 10

= 2N

 

  1. 4 N to kg

10N = 1kg

1N =  1/10kg

4N=1/10kg x4

=0.4kg.

  1. 0043kg to N

1kg = 10N

0.0043kg =( 10×0.0043)N

= 0.043N.

 

CENTRIPETAL FORCE

This is the force that keeps bodies moving in a circular path. This force moves bodies to the centre of the circle.

force

EXAMPLES

  1. A car negotiating in a corner
  2. The moon in a bit with a planet
  • Pendulum

ELECTROSTATIC FORCE

Force extorted on a charged body in an electric field. It may be repulsive or attractive.

– Like charge repel and unlike charges attract

– The region around the charged body or a charge where electric fore is experienced.

MOLECULAR FORCE

Force of attraction between molecules of same substances. These include;-

  • cohesion

Force of attraction between molecules of the same substance

– Adhesion

Force of attraction between molecules of different substances

UP THRUST FORCE

Upward force experienced by objects in fluids

force 1

MAGNETIC FORCE

This is the force extorted by a magnet on a material. It is either repulsive or attractive

A magnet is a piece of metal or alloy which has ability to repel. The north and South Pole

A magnetic pole is the area near the ends of a magnet were the resultant  attractive force appears to concentrate.

force 2

A magnetic field is the area around the magnet where the magnetic force is applied.

Magnetism is the ability of a metal or alloy. [Magnet] to attract or repel other materials.

We use lines and arrows to show a pattern of a magnetic field. The arrows indicated the direction in which the magnetic force is acting.

When drawing magnetic field patterns, the following as properties of magnetic field lines must be considered;

– The lines run from the north to south pole so arrows should be drawn showing the direction of north to south.

– Magnetic field lines do not meet i.e. they are parallel.

Note

Like poles repel where as unlike poles attract

SKETCHES OF MAGNETIC FIELD PATTERNS

A magnetic field due to a single isolated magnet.

force 3

Two magnets with unlike poles near

force 4

Two magnets with like poles near

 

X is the neutral point

The neutral point in a magnetic field represents the region where the resultant magnetic flux is 0.

 

Attachments6

ASSIGNMENT : FORCES assignment MARKS : 10  DURATION : 1 week, 3 days

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