1. Objectives Chapter 4 Section 4 Everyday Forces
Explain the difference between mass and weight.
Find the direction and magnitude of normal forces.
Describe air resistance as a form of friction.
Use coefficients of friction to calculate frictional force.

2. Section 4 Everyday Forces
Chapter 4
Weight
The gravitational force (Fg) exerted on an object by Earth is a vector quantity, directed toward the center of Earth.
The magnitude of this force (Fg) is a scalar quantity called weight.
Weight changes with the location of an object in the universe.

3. Weight, continued Fg = mag Chapter 4
Section 4 Everyday Forces
Chapter 4
Weight, continued
Calculating weight at any location:
Fg = mag
ag = free-fall acceleration at that location
Calculating weight on Earth's surface:
ag = g = 9.81 m/s2
Fg = mg = m(9.81 m/s2)

5. Section 4 Everyday Forces
Chapter 4
Normal Force
The normal force acts on a surface in a direction perpendicular to the surface.
The normal force is not always opposite in direction to the force due to gravity.
In the absence of other forces, the normal force is equal and opposite to the component of gravitational force that is perpendicular to the contact surface.
In this example, Fn = mg cos q.

6. Section 4 Everyday Forces
Chapter 4
Friction
Static friction is a force that resists the initiation of sliding motion between two surfaces that are in contact and at rest.
Kinetic friction is the force that opposes the movement of two surfaces that are in contact and are sliding over each other.
Kinetic friction is always less than the maximum static friction.

7. Friction Forces in Free-Body Diagrams
Section 4 Everyday Forces
Chapter 4
Friction Forces in Free-Body Diagrams
In free-body diagrams, the force of friction is always parallel to the surface of contact.
The force of kinetic friction is always opposite the direction of motion.
To determine the direction of the force of static friction, use the principle of equilibrium. For an object in equilibrium, the frictional force must point in the direction that results in a net force of zero.

8. The Coefficient of Friction
Section 4 Everyday Forces
Chapter 4
The Coefficient of Friction
The quantity that expresses the dependence of frictional forces on the particular surfaces in contact is called the coefficient of friction, m.
Coefficient of kinetic friction:
Coefficient of static friction:

9. Section 4 Everyday Forces
Chapter 4

10. Practice problems Page 137
A 24kg crate initially at rest on a horizontal floor requires 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.

11. Practice problems 1-3 page 137

12. Chapter 4 Sample Problem Overcoming Friction
Section 4 Everyday Forces
Chapter 4
Sample Problem
Overcoming Friction
A student attaches a rope to a 20.0 kg box of books.He pulls with a force of 90.0 N at an angle of 30.0° with the horizontal. The coefficient of kinetic friction between the box and the sidewalk is Find the acceleration of the box.

13. Sample Problem, continued
Section 4 Everyday Forces
Chapter 4
Sample Problem, continued
1. Define
Given:
m = 20.0 kg
k = 0.500
Fapplied = 90.0 N at  = 30.0°
Unknown:
a = ?
Diagram:

14. Sample Problem, continued
Section 4 Everyday Forces
Chapter 4
Sample Problem, continued
2. Plan
Choose a convenient coordinate system, and find the x and y components of all forces.
The diagram on the right shows the most convenient coordinate system, because the only force to resolve into components is Fapplied.
Fapplied,y = (90.0 N)(sin 30.0º) = 45.0 N (upward)
Fapplied,x = (90.0 N)(cos 30.0º) = 77.9 N (to the right)

15. Sample Problem, continued
Section 4 Everyday Forces
Chapter 4
Sample Problem, continued
Choose an equation or situation:
A. Find the normal force, Fn, by applying the condition of equilibrium in the vertical direction:
Fy = 0
B. Calculate the force of kinetic friction on the box:
Fk = kFn
C. Apply Newton’s second law along the horizontal direction to find the acceleration of the box:
Fx = max

16. Sample Problem, continued
Section 4 Everyday Forces
Chapter 4
Sample Problem, continued
3. Calculate
A. To apply the condition of equilibrium in the vertical direction, you need to account for all of the forces in the y direction:
Fg, Fn, and Fapplied,y. You know Fapplied,y and can use the box’s mass to find Fg.
Fapplied,y = 45.0 N
Fg = (20.0 kg)(9.81 m/s2) = 196 N
Next, apply the equilibrium condition,
Fy = 0, and solve for Fn.
Fy = Fn + Fapplied,y – Fg = 0
Fn N – 196 N = 0
Fn = –45.0 N N = 151 N
Tip: Remember to pay attention to the direction of forces.
In this step, Fg is subtracted from Fn and Fapplied,y because Fg is directed downward.

17. Sample Problem, continued
Section 4 Everyday Forces
Chapter 4
Sample Problem, continued
B. Use the normal force to find the force of kinetic friction.
Fk = kFn = (0.500)(151 N) = 75.5 N
C. Use Newton’s second law to determine the horizontal acceleration.
a = 0.12 m/s2 to the right

18. Section 4 Everyday Forces
Chapter 4

19. Air Resistance Chapter 4
Section 4 Everyday Forces
Chapter 4
Air Resistance
Air resistance is a form of friction. Whenever an object moves through a fluid medium, such as air or water, the fluid provides a resistance to the object’s motion.
For a falling object, when the upward force of air resistance balances the downward gravitational force, the net force on the object is zero. The object continues to move downward with a constant maximum speed, called the terminal speed.

20. Fundamental Forces Chapter 4 There are four fundamental forces:
Section 4 Everyday Forces
Chapter 4
Fundamental Forces
There are four fundamental forces:
Electromagnetic force
Gravitational force
Strong nuclear force
Weak nuclear force
The four fundamental forces are all field forces.