FORCES AND
LAWS OF
MOTION
FORCE
(push)
(pull)
EXAMPLES OF FORCES:
Contact
Pulling the handle of
the door
Pushing a stroller
Hitting a tennis ball
with a racquet
Bouncing a soccer
ball
etc, etc, etc.
Field
Force of gravity
Attraction of
negative and
positive particles
FREE BODY DIAGRAM
FREE BODY DIAGRAM
A free-body-diagram is a diagram that
represents the object and the forces that
act on it.
The net force on an object is the vector sum
of all forces acting on that object.
The SI unit of force is the Newton (N).
Individual Forces
4N
10 N
Net Force
6N
NET FORCE
100 +100 = 141(N )
2
2
Individual Forces
Net Force
5N
64
3N
4N
Fr = F + F
2
rx
2
ry
Fx = F1x + F2 x
Fy = F1y + F2 y
1) A HOCKEY PUCK IS SLIDING ON A
FRICTIONLESS ICE.
2) A HOCKEY PUCK IS SLIDING ON A
NORMAL ICE (WITH FRICTION)
FBD - PRACTICE
Many orangutans spend their entire lives
among the trees and are well adapted to
move in this arboreal habitat. They have long
arms (about two-thirds of their body height)
and powerful chest muscles. Suppose an adult
orangutan is hanging by its arms from a tree
branch. The angle between each of the
animal’s arms and the vertical is 15° with each
arm exerting a force 430 N. The gravitational
force acting on it is 830 N. Draw a free-body
diagram of the animal.
FBD - PRACTICE
Rock wall climbing is a challenging activity.
Suppose that on one ascent, a climbing piton
is wedged into a crevice on the rock wall. A
climber negotiating the wall exerts a force of
350 N at an angle of 65° below the horizontal
on a rope secured to the piton. The rock
surrounding the piton exerts an upward force
of 320 N and a horizontal force of 150 N to the
left on the pin. Draw a free-body diagram of
the pin.
FBD - PRACTICE
After a skydiver jumps from a plane, the only force
initially acting on
the diver is Earth’s gravitational attraction. After about
ten seconds of falling, air resistance on the diver will
have increased so that its magnitude on the diver is
now equal in magnitude to Earth’s gravitational force
on the diver. At this time, a diver in a belly-down
position will be falling at a constant speed of about
190 km/h.
a. Draw a free-body diagram of the skydiver when the
diver initially leaves the plane.
b. Draw a free-body diagram of the skydiver at the
tenth second of the falling.
FBD - PRACTICE
A chef places an open sack of flour on a kitchen
scale. The scale reading of 40 N indicates that the
scale is exerting an upward force of 40 N on the sack.
The magnitude of this force equals the magnitude of
the force of Earth’s gravitational attraction on the
sack. The chef then exerts an upward force of 10 N on
the bag and the scale reading falls to 30 N. Draw a
free-body diagram of the latter situation.
FBD - PRACTICE
A music box within the toy shown below plays tunes
when the toy is pushed along the floor. As a child
pushes along the handlebars with
a force of 5 N, the floor exerts a force of 13 N directly
upward on the toy. The Earth’s gravitational force on
the toy is 10 N downward while inter- actions between
the wheels and the floor produce a backward force of
2 N on the toy as it moves. Draw a free-body diagram
of the toy as it is being pushed.
Aristotle
384 –
322
BC
1564 - 1642
ISAAC
NEWTON
1643 - 1727