Force, Work & Energy

Marie Jessica B. Alumaga

Objectives:

• To cite examples of forces encountered in day to day activities

• To describe and discuss the four fundamental forces

• To state the scientific definition of work and solve work problems

• To describe the different forms and transformations of energy

• To explain how energy is related to work

Force

Force - push or pullcontact forces – exerted between objects

that are in close contactnoncontact forces – can be exerted

between objects that are not even touching

Fundamental forces• Gravitational force - affects anything that has mass• Electromagnetic forces – exerted on charged

objects or magnetic objects Like charges repel, unlike charges

attract.Like poles repel, unlike poles attract.

• Weak force – exerted between leptons and hadrons; active only at distances less than 10-18 m

• Strong forces – acts only on hadrons; strongest of the four but effective only at very short separations (10-

15m)

WorkWork - product of the force and displacement

(displacement – distance moved by an object as a result of the force’s action) - being done when all of the following conditions are satisfied

a. A force is exerted on the object.b. The object moves.c. The motion is in the direction of the force.

WorkSample problem 1:How much work is done when a 100-N cart is pulled over a distance of 1 meter? Given: F = 100N d = 1mFind: WSolution: W = Fd

W = 100N(1m) = 100Nm or 100J

Work

To get the maximum amount of work , the force must be parallel to the motion.

Work

The perpendicular component of the force does no work, if there is no motion in that direction .

Work: Sample problem 2

A box having 2 kg mass, under the effect of forces F1, F2, and F3, takes distance 5 m. Which of the forces do work? What is the work done by that force? W3 = F3d = 30N(5m) = 150J

Power Power – refers to the rate at which work is done

P = Wdone/time

Unit of power: • joule/second (J/s) 1J/s = 1 watt (W)• Horsepower (hp) 1hp = 746 W - used to specify the power output of motors and engine

Machine

Machine – a device that transfers a force from its point of application to some other point to where it is used

- may multiply force or distance or simply change the direction of the force

Simple Machines

Six Simple Machines 1. Lever – a rigid object that is free to rotate

about a fixed point called the fulcrum2. Wheel and axle – rotating lever consisting of

a large wheel attached to a smaller axle, which is just a rod or cylinder3. Pulley – modified lever; grooved wheel that is

free to turn about an axle

Simple Machines

Six Simple Machines 4. Inclined plane – a flat surface with one end

placed higher than the other5. Wedge – two inclined planes set back to back6. Screw – an inclined plane that is wound

around a cylinder

Concept of Energy

Energy - capacity to do work• If something has energy it can do work. • If something can do work, then it has energy.• Work and energy have the same unit, (joule,J)• It cannot be seen but evidences prove its

existence.

Different Forms of Energy

Potential Energy (PE) - stored energy due to the object’s position or internal arrangement of moleculesKinetic Energy (KE)- energy of moving object

Different Forms of Potential Energy1. Gravitational potential energy - energy is due to the object’s position or height

from the earth The higher the position, the greater is the gravitational PE 2. Magnetic potential energy - energy from magnets3. Chemical energy - energy due to the arrangement of the atoms and

molecules in matter

Different Forms of Kinetic Energy 1. Electrical Energy – due to the motion of charged particles through conducting wires2. Nuclear Energy – energy derived from the nuclear reaction nuclear fusion – nuclei join to form a nucleus nuclear fission – nucleus is broken into smaller nuclei3. Radiant Energy – energy transported by electromagnetic waves4. Thermal Energy - sum of the KE’s of the molecules due to their linear motion

Energy Transfer

Heat can be transferred through • Radiation – transfer by rays or

electromagnetic waves• Conduction – transfer by direct contact• Convection - transfer involving difference in

density

Energy transformation

Energy cannot be created nor destroyed, it can only be transformed from one form to another. (Law of Conservation of Energy)• Electrical energy chemical energy• Chemical energy electrical energy• Radiant energy chemical energy

Sources of Energy

• Renewable sources- energy sources that can be replenished

• Nonrenewable sources- energy sources that has undergone long process of formation

• Geothermal energy - heat energy from deep within the earth

• Hydropower - energy from moving water• Tidal energy - energy from the regular rise and

fall of sea water • Wind energy - energy from wind mills• Solar energy - energy rom the sun• Biomass – energy resource that is derived from

plant or animal matter

Sources of Energy – Renewable Sources

• Fossil fuels - derived from the remains of plants and animals that died millions of years ago

petroleum – also called crude oil; complex mixture of hydrogen and carbon compounds

coal – most abundant and dirtiest fossil fuel natural gas – mainly methane and the

cleanest of all the fossil fuels• Nuclear energy – energy from fission or fusion

reaction

Sources of Energy – Nonrenewable Sources

Problems Related to Energy

1. Combustion of fossil fuels release gases and solid pollutants that are harmful to living and non-living things.2. Nuclear energy production a) release radioactive substances harmful to living things b) waste disposal c) poses risk of reactor accidents d) causes thermal pollution 3. Tidal and hydroelectric dams create ecological froblems. 4. geothermal energy emits H2S

Energy Utilization and Consumption

• All people are users of energy.• Energy sources are popularly used for the

generation of electricity• Factors that will affect the selection of energy for

electricity include- availability in the country- cost of production- availability of local manpower to make and repair the plant- ecological impact during extraction, utilization and waste disposal

Energy conservation

- refers to the practice of extending the useful life of the earth’s energy resources- should be taught early in life- influenced by the people’s attitudes, mores and

beliefs

Small things make a big difference.

MJBA

References

• McLaughlin, Charles W., Thompson, Marilyn, and Zike, Dinah. (2012). Glencoe Physical Science. McGraw-Hill Education

• Serway, Raymond A., Vuille, Chris and Faugnn, Jerry S.(2009). College Physics (Volume 2) 8th ed. Brooks/Cole Cengage Learning

• Tillery, Bill W. (2011). Physical Science McGraw-Hill Publishing Co.