1. Force & Motion
Dynamics
 how things move

2. Aristotle (4-300 BC)
Believed everything moved to return to its
own. ie. rock to earth, smoke to air
Objects want to be in a state of rest.
Galileo (15-1600 AD)
Dropped objects from the Leaning Tower of
Pisa. Too quick, Too Fast  after 3s, v =
29 m/s (64 mi/hr), y = 44 m (1/2 football
field) per second

3. Slowed things down by rolling balls down various
inclines.
?
It always came to about the same height.
What would happen on a smooth horizontal
surface?
 Inertia  tendency of an object to maintain a
state of rest or motion

4. Newton (16-1700 AD)
Mass is a measure of inertia
The more mass you have, the less it wants
to change its state of motion.
1st Law (Law of Inertia)
 An object in motion tends to stay in
motion with a constant velocity and an
object at rest tends to stay at rest, unless
acted on by an unbalanced force.

5. Force
 a push or pull on an object
(measured in Newtons)
Net Force (Fnet)
 is the sum of all the forces acting on
an object
Balanced Unbalanced
F1 = 2 N F2 = -2 N F1 = 2 N F2 = -5 N
Fnet = 2 + -2 = 0 N Fnet = 2 + -5 = -3 N

6. Force Diagrams (Free Body Diagram)
 a vector diagram of all the forces acting on
1. Draw the object as a single dot.
an object
2. Draw the force vectors as coming out of the
dot. Label each of the vectors.
F1  Force of object 1 on object 2
3. Check to make sure that all forces are
accounted for.
2 Types of forces
Contact – Look at the objects actually touching
Noncontact – look at gravity, electricity, and magnetism

7. Normal Force
 force perpendicular to the ground pushing up
Samples
Draw a force diagram of
you sitting in your desk.
Accelerating out of the parking lot, you put your foot on
the gas and the engine applies 2500 N of force forward
on your 10,000 N car. If the ground puts 1000 N of
friction back on the car:
a. draw a free body diagram
b. what is the net force?

8. Newton’s 2nd Law
If all objects resting want to stay resting,
how do they move?
If all objects moving want to stay moving,
how do they stop?
Force!!!
F↑a↑ m↓a↑
Fαa 1/m α a
Force is directly mass is inversely
proportional to accel. proportional to accel.

9. 2nd Law (Law of Acceleration)
Force makes an object want to accelerate.
Mass makes an object not want to accelerate.
a α Fnet/m
a = k Fnet / m Const = 1
a = F/m
F = ma
1 Newton = amount of force necessary to
accelerate a 1 kg mass at 1 m/s2
1 N = 1 kg m/s2
This would be like holding 100 g in your hand.
1 N = (.1 kg) (9.8m/s2)
1lb is about 4 N

10. Net force in the
No Angle y direction Angle
Fnety = 0 usually cancels Fnety = 0
FN – Fg = 0 out to zero: FN + Fy = Fg
FN = F g
FN FN Fyou
FyouY
FF Fyou FF
FyouX
Fg Fg
Net force in x direction
causes acceleration: Constant Velocity
Accelerating Fnetx = 0
Fnetx = ma
Fx – Ff = 0
Fx – Ff = ma
F =F

11. To find the mass,
F1 = 2 N 196 N F2 = -5 N take the weight,
divide by 9.8.
Fnet = 2 + -5 = -3 N
Weight  force of
gravity
F = ma
Fg = m g
F = ma
196 N= m (9.8 m/s2)
m = 20 kg -3 = 20 a
a = -0.15 m/s2

12. Use vf, x, or t to find a, then a to find Fnet, or use Fnet to find a then a to find vf, x,
or t
vf
x
t x = vit + ½ at2 a Fnet = ma
Fnet
vf = vi + at
vi
vf2 = vi2 + 2ax

13. You pull a 50 N block with a force of 15 N at 26 °.
a. draw a force diagram
b. find the mass of the block.
c. find the components of your force.
d. find the normal force
e. find the acceleration of the block if friction is 4 N?
f. find the block’s final velocity after 3 s?
Givens

15. You pull a 50 N block with a force of 15 N at Y direction
26°. Fnet = m a
a. draw a force diagram
b. find the mass of the block. Fnet = 0
c. find the components of your force. FN + Fy – Fg = 0
d. find the normal force FN + 6.58 – 50 = 0
e. find the acceleration of the block if friction FN = 43.4 N
is 4 N?
f. find the block’s final velocity after 3 s?
FN
Givens X direction
Fx = 15 cos 26 = 13.5 N
15 N vi = 0 Fnet = m a
Fy = 15 sin 26 =a = 1.86 m/s2
6.58 N
Fx = 5.10 a
t=3s
13.5 -4 = 5.10 a
vf = ?
a = 1.86 m/s2
Fg = m g
g a vf = vi + at
50 = m(9.8) vf = 0 + (1.86)(3)
m = 5.10 kg vf = 5.59 m/s

17. Newton’s Third Law

18. Newton’s 3 Law rd
Every Action has an equal and opposite reaction.
∴Every interaction happens in 2’s.
Action Force and Reaction Force
How do we know what action & reaction are?
Action: Object A exerts a force on object B
Reaction: Object B exerts a force on object A
Notice that these forces are acting on two
different objects.

19. example: Earth exerts a force on Boulder
Boulder exerts a force on Earth
If the boulder exerts a
force on the earth that is
Force of equal, why doesn’t the
Earth on earth accelerate to the
Boulder boulder?
Force of
Boulder
on Earth

20. Mass! 2nd Law!!!
F=F
m AM = a
So the earth moves, but it is soooo small that we
can’t tell!
Another example… horse and cart.
Are the equal and opposite forces acting on the
same object?