Forces and Free-body diagrams

By Corina Bot

A force is a vector quantity (=> has both magnitude and direction). To fully describe the force acting upon an object, you must describe both the magnitude (size or numerical value) and the direction.

It is common to represent forces using diagrams in which a force is represented by an arrow. The size of the arrow is reflective of the magnitude of the force and the direction of the arrow reveals the direction which the force is acting. (Such diagrams are known as free-body diagrams)

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/newtlaws/u2l2a.html

2111

s

mkgNewtonN

If we push the block harder and harder, the frictional force will increase, until it reaches a

maximum (in this case, 2.5N). If we push harder still, (say, 2.6N), the block will

start to move, because we're now pushing harder than the frictional force.

Frictional Force Nkk FF

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/newtlaws/u2l2a.htmlhttp://home.clara.net/darvill/enforcemot/friction.htmhttp://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Dynamics/Forces/Tension.htmlhttp://en.wikipedia.org/wiki/Force

Tension Force

Spring Force

Contact Forces

Normal Force

Applied Force

Frictional Force

Tension Force

Spring Force

Normal Force

Gravitational Force gmFg

nF

gF

Action-at-a-Distance Forces

Gravitational Force

Electrical Force

Magnetic Force

Iron filings and compasses show the shape and direction of the magnetic field

http://en.wikipedia.org/wiki/Forcehttp://en.wikipedia.org/wiki/File:EfieldTwoOppositePointCharges.svghttp://www.school-for-champions.com/science/magnetic_detection.htm

Electric vector field surrounding two opposite point charges. Red is positive, green is negative.

An initially stationary object - allowed to fall freely under gravity drops a distance which is proportional to the square of the elapsed time.

gmFg Gravitational Force

x

y

0

A mini-car is pulling a block, with a force of 190N, as shown in figure. The block has a mass of 15 kg and the coefficient of kinetic friction between the bottom of the block and the surface is 0.6. Find the acceleration of the cart.

Fapp=190 N

m = 15 kg

k 0.6

ax = ?

15 kg

a

kF nF

appF

gF

aBy definition:

gmF

FF

g

Nkk

20

1

,

,

yyynet

xxxnet

amFF

amFF

From equation (1):

3156.0190

15190

15190

xN

xNk

xk

xkappx

akgFN

akgFN

akgFN

amFFF

From equation (2):

Ns

mkggmFF

FF

amF

gN

gN

yy

1478.915

0

0

2

From equation (3):

278.6

15

8.101

158.101

152.88190

151476.0190

s

m

kg

Na

akgN

akgNN

akgNN

x

x

x

x

Fapp=190 N

m = 15 kg

mk = 0.6

ax = ?

x

y

0kF nF

appF

gF

a

20

1

,

,

yyynet

xxxnet

amFF

amFF

A descendant of the original apple tree can be seen growing outside the main gate of Trinity College, Cambridge, UK, below the room Newton lived in when he studied there.