1. CHAPTER 4 FORCES and the LAWS of MOTION

2. THE NORMAL FORCE Fn
a force that acts on a surface in a direction perpendicular to the surface

3. THE NORMAL FORCE Fn
the television is in equilibrium, so the normal force from the table must be equal in magnitude and opposite in direction to the gravitational force exerted on the television.

4. THE NORMAL FORCE
Fn is ALWAYS perpendicular to the surface, but NOT ALWAYS opposite to the Fg
Fn = mg ∙ cos 𝛉

5. The Net Force Is the vector sum of all forces acting on the body
4 forces acting on the car
Add them as vectors
The resultant is then the NET FORCE
If the NET FORCE is 0, then you know that the car is travelling at constant velocity (zero acceleration)

6. Free Body Diagrams (FBDs)
are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation.

7. Free Body Diagrams (FBDs)
4 forces acting on this object, but there MIGHT be more or less forces
Represent the object as a SQUARE or a CIRCLE
You MUST show all the forces acting on the object

8. Free Body Diagrams (FBDs)
A puck is sliding on a sheet of ice to the right FA
Fapp
Ffriction
How many forces are
acting on the puck?
Fgravity
Label all of them!

9. Free Body Diagrams (FBDs)
A puck is sliding on a sheet of ice to the right FA
Fapp
Ffriction
If the object is moving,
you MUST show
the direction of acceleration
Fgravity

10. Free Body Diagrams (FBDs)
A puck is sliding on a sheet of ice to the right a FA
Fapp
Ffriction
If the object is moving,
you MUST show
the direction of acceleration
Fgravity

11. Interpreting FBD (Quantitative = with numbers)
NET FORCE?? a FA
Fapp
Ffriction
Fgravity

12. Interpreting FBD (Quantitative = with numbers)
NET FORCE?? a
Fnet = 0 N in
vertical direction
40 N
10 N
25 N
Fnet = + 15 N in
horizontal direction
40 N

13. Interpreting FBD (Quantitative = with numbers)
Fnet = 0 N in
vertical direction
100 N
100 N
Fnet = 0 N in
horizontal direction
47 N
If the Net Force is 0 (Fnet = 0 N), the object is in EQUILIBRIUM

14. 4.1 The Newton’s FIRST Law:
Tells us what happens when all existing forces acting on a body are balanced
If the Net Force is 0 (Fnet = 0 N), the object is in EQUILIBRIUM
47 N
If the forces acting on a body are balanced, then the acceleration of the body will be m/s2
100 N
100N
47 N

15. 4.2 The Newton’s Second Law:
Tells us what happens when an unbalanced force acts on a body
If the Net Force is NOT 0 (Fnet ≠ 0 N), the object is NOT in EQUILIBRIUM a
47 N
If an UNBALENCED force acts on a body, the body will ACCELARATE
100 N
200N
47 N

16. 4.2 The Newton’s Second Law:
Tells us what happens when an unbalanced force acts on a body
The rate at which the body will ACCELERATE depends DIRECTLY on the UNBALANCED FORCE (Fnet) and INVERSELY on the mass of the body. ∝
a ∝ Fnet
a ∝ 1/m

17. 4.2 The Newton’s Second Law:
Tells us what happens when an unbalanced force acts on a body
The rate at which the body will ACCELERATE depends DIRECTLY on the UNBALANCED FORCE (Fnet) and INVERSELY on the mass of the body. ∝
a = Fnet / m
Fnet = a · m

18. First Law vs. Second Law

20. Fnet = a · m Fg = m · g Fk = µk · FN
A 45.0 N block is being pushed along a floor, where the coefficient of kinetic friction is If a horizontal force of 25.0 N is applied, at what rate will the block accelerate?
FBD
Known?
Unknown?
Formula(s)
Fg = 45.0 N
µk = 0.333
Fapp = 25.0 N
Fk = ???
FN = ???
Fk = ???
Fapp = 25.0 N
Fnet = a · m
m = Fg/g
= 45.0/9.81
= 4.59 kg
Fg = 45.0 N
Fg = m · g
Fk = µk · FN

21. A 45.0 N block is being pushed along a floor, where the coefficient of kinetic friction is If a horizontal force of 25.0 N is applied, at what rate will the block accelerate?
FBD
Known?
Unknown?
Formula(s)

22. Classwork
Page: 112
Problems: all problems

23. QUIZ
On Monday: 15 minutes
Problems: from 4.1 and 4.2