FORCESThey keep us grounded

WHAT IS FORCE?A force is simply a push or a pull.F = ma (mass x acceleration)Forces are measured in NewtonsForce has direction (whichever way it is being pushed or pulled)

Forces in the same direction can add to make a greater force.

5 N 5 N=

10 N

Two forces can be added to produce a greater net force in the same direction.

Net Force

5 N 15 N=

10 N

Two forces can be added to produce a greater net force in the same direction.

NET FORCEForces in opposite

directions will subtract to make lesser force.

BALANCED FORCESIf forces going in opposite directions are equal, they are balanced and no movement happens.

UNBALANCED FORCESIf two forces are pushing in the same direction, movement happens in that direction.

UNBALANCED FORCESIf unequal forces are moving in the opposite direction, movement happens in the direction of the greater force.

TYPES OF FORCESIn nature the two main types of forces we will talk about are:Electric and MagneticGravitational

GRAVITATIONGravity - The pull to the center of an object with mass.

Interesting fact:The force of gravity from Earth on the moon is equal to the force of gravity from the moon on Earth.

GRAVITATIONScientists do not know what causes gravity.However, they know a lot about it.

Larger masses have a larger gravitational force.Example: If Earth had twice as much mass and the same volume you would weigh twice as much.

Example: If Earth had half as much mass and the same volume you would weigh half as much.

Mass of Earth m = 6 Yg

(Yottagrams)

Mass of 2xEarth m = 12 Yg

(Yottagrams)

440 lb2000 N

220 lb1000 N

GRAVITATIONThe closer the object, the more the force of gravity.

Example: If you built a tower 6,371 km high you would weigh ¼ of your weight because you’re twice the distance from the center of the Earth.

Example: If Earth’s radius was halved, but maintained the same mass you would weigh 4 times as much.

Earth’s radiusr = 6,371 km

220 lb1000 N

2xEarth’s radius

r = 12,742 km

55 lb250 N

ELECTRIC AND MAGNETICA force that is created by charged (- or +) particles.Remember that opposites charges attract!Same charge repel!

WORK AND POWER

WHAT IS WORK?In science, the word work has a different meaning than you may be familiar with.

The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)

WORK OR NOT?According to the scientific definition, what is work and what is not?

a) a teacher lecturing to her class

b) a mouse pushing a piece of cheese with its nose across the floor

WHAT’S WORK?A scientist delivers a speech to an audience of his peers.

A body builder lifts 350 pounds above his head.

A mother carries her baby from room to room.

A father pushes a baby in a carriage.A woman carries a 20 kg grocery bag to her car?

FORMULA FOR WORK

Work = Force x Distance

The unit of force is NewtonsThe unit of distance is metersThe unit of work is Newton metersOne Newton meter is equal to one Joule

So, the unit of work is a Joule

W=FDWork = Force x distance

Calculate: If a man pushes a concrete block 10 m with a force of 20 N, how much work has he done?

POWERPower is the rate at which work is done.

Power = Work*/Time (J/s) *(force x distance)

The unit of power is the Watt.Joule/second = Watt

CHECK FOR UNDERSTANDING

1.Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds.

Which student does the most work? Which student delivers the most power?Explain your answers.

Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads.

Yet, Ben is the most powerful since he does the same work in less time.

Power and time are inversely proportional.

 

2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s² a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order.

Force=Mass x Acceleration

Work=Force x Distance

Power = Work/Time

How much power will it take to move a 10 kg mass at an acceleration of 2 m/s² a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order.

Force=Mass x Acceleration Force=10 kg x 2 m/s²

Force=20 N

Work=Force x DistanceWork = 20 N x 10 mWork = 200 Joules

Power = Work/TimePower = 200 J / 5 sPower = 40 watts

How much power will it take to move a 40 kg mass at an acceleration of 1 m/s² a distance of 100 meters in 2 minutes? This problem requires you to use the formulas for force, work, and power all in the correct order.

F = ma

F = 40 kg x 1 m/s²

F = 40 N

W = Fd

W = 40 N x 100 m

W = 4000 J

P = W/t

P = 4000 J/120 s

P = 33.3 W