Forces
Examples
– A force causing an object to
start moving
– A force causing an object to
stop moving
– A force causing an object to
change direction
Force: The cause of an acceleration, or
the change in an object’s velocity.
Newton (N): The amount of force that,
when acting on a 1kg mass, produces an
acceleration of 1m/s2
1 N = 1kg x 1m/s2
Two Types of Forces
Contact Forces: Physical contact
between two objects, results in the change
in velocity (that is – acceleration)
– Example: pull on a spring, the string will move
– Pull on a wagon, the wagon will move
– Catching a football, the ball will stop
– Kick a soccer ball, the ball will move
– Your turn – let’s come up with 2 more
examples.
How do you know that these are forces?
Field Forces: Force that can exist between
objects, even in the absence of physical contact
between the objects.
– Example: gravity
magnets
static electricity
Force Diagrams
The effect of a force depends on its
magnitude and direction (sound familiar?)
Forces are vector quantities
Force Diagrams: a diagram of the objects
involved in a situation and the forces exerted
on the objects.
– Arrows represent forces
– Attached to the object on which the force is acting
Force diagram vs. Free body
diagram
Force diagram
shows the forces
exerted by the
car on other
objects
(b) Free Body Diagram: only shows the car
and the forces acting on it
Free Body Diagrams
Free Body Diagram (FBD): shows forces
affecting the motion of a single object.
– A free body diagram of the car will show all the forces
acting on the car as if the forces are acting on the
center of the car.
-- To draw the free-body
diagram, you must first
isolate and identify all the
forces acting on the car.
How to draw a free body diagram.
Force
exerted on
the car by
the tow truck
Road exerts
upward force
on the car
5800 N
775 N
14,700 N
The
interaction
between the
road and the
car’s tires,
the road
also exerts a
backward
force of
friction on
the car
Give it a try!
Draw a free-body
diagram (FBD) of
a football being
kicked. Assume
only forces acting
on it are gravity
and the force of
the kicker.
Draw a diagram of a
crash-test dummy in a
car at the moment of
collision. Assume that
the forces acting on
the car are 19600 N
downward, 17800 N
forward, 2500 N
backward. The forces
on the dummy are
585 N downward, 175
N backward, and 585
N upward.
Now make 2 FBD –
one for car, one for
dummy
Resolving
Derek leaves his
physics book on top
of a drafting table that
is inclined at a 35
degree angle. The
free body diagram to
the right shows the
forces acting on the
book. Find the net
external force acting
on the book, and
determine whether
the book will remain
at rest in this position.
F table on book = 18 N
F friction = 11 N
F gravity on book = 22 N
Select a coordinate
plane to work with.
Try to get the most
forces in the x & y
plane, then resolve
the ones left on an
angle.
+ & - direction is
important
y
x
18 N
11 N
22 N
Find the x and y components of all vectors
y
F table on book (normal):
x
F gravity on book :
X=0N
X=?
Y = 18 N
F friction:
Y=?
θ
θ=?
X = -11 N
Y=0N
Θ =35o
y
Find the net external force
in both the x & y directions
x
18 N
11 N
18 N
22 N
13 N
X
Y
ΣFx = Fg,x + Ffriction
ΣFy = Fg,y + Fnormal
ΣFx = 13N+ -11N
ΣFy = -18N + 18N
ΣFx = 2 N
ΣFy = 0 N
The net external force is 2 N
Evaluate your answer
The book experiences an acceleration in
the downhill direction and it will slide off
the table
Give it a try - Give it a try p. 133 #1-4