1. Force and Its Representation
Newton’s Laws Lesson 2
Force and Its Representation
Physics
Fall 2012

2. The Meaning of Force
Force: push or pull upon an object resulting from the objects interaction with another object
Whenever there is an interaction between two objects, there is a force upon each of the objects
When the interaction ends, the two objects no longer experience the force

3. The Meaning of Force
Forces only exist as a result of an interaction

4. The Meaning of Force
Contact forces: forces that result when the two interacting objects are perceived to be physically contacting each other
Friction
Tension
Normal
Air resistance
Applied
Spring

5. The Meaning of Force
Action-at-a-distance forces: forces that result even when the two interacting objects are not in physical contact, but can still exert a push or pull
Gravitational
Electrical
Magnetic

6. The Meaning of Force Newton: metric unit for force
10 N means 10 Newton of force
1 N is the amount of force needed to give a 1kg mass an acceleration of 1m/s2
Force is a vector quantity so must specify direction  10 N downward

7. The Meaning of Force Forces are represented using arrows in a diagram
Balanced force:
Forces are represented using arrows in a diagram
Size of the arrow is reflective of the magnitude of the force
Direction of the arrow revels the direction that the force is acting
Unbalanced force:

8. Types of Forces- Contact Forces
Applied Force (Fapp)
Force that is applied to an object by a person or another object

9. Types of Forces- Contact Forces
Normal Force (Fnorm)
The normal force is the support force exerted upon an object that is in contact with another stable object
Ex. If a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book
Special case: horizontal normal force when a person leans against a wall

10. Types of Forces- Contact Forces
Friction Force (Ffrict)
Force exerted by a surface as an object moves across it or makes an effort to move across it
Usually friction force opposes the motion of an object
Two types: sliding friction and static friction
Friction depends on the nature of the two surfaces in contact and the degree to which they are pressed together
Ffrict = (u)(Fnorm)

11. Types of Forces- Contact Forces
Air Resistance Force (Fair)
Special type of frictional force that acts upon objects as they travel through the air
Often opposes the motion of an object
Most noticeable with objects traveling at high speeds, or objects with large surface area

12. Types of Forces- Contact Forces
Tension Force (Ftens)
Force that is transmitted through a string, rope, cable, or wire when it is pulled tight by forces acting from opposite ends
Tension force is directed along the length of the wire and pulls equally on the objects on the opposite end

13. Types of Forces- Contact Forces
Spring Force (Fspring)
Force exerted by a compressed or stretched spring upon any object that is attached to it
An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position
For most springs (Hook’s law) the magnitude of the force is directly proportional to the amount of stretch or compression of the spring

14. Types of Forces- Action-at-a-Distance Forces
Gravitational Force (Fgrav)
AKA weight
Force with which the earth, moon, or other massively large object attracts another object towards itself
This is the weight of an object
Fgrav = m x g
m = mass (kg)
g = 9.8 N/kg (on earth)

15. Mass vs. Weight
Mass: the amount of matter that is contained by the object Kg
Mass will be the same no matter where in the universe an object is located
Weight: the force of gravity acting upon the object
N, weight = m x g
Weight varies according to the universe the object is in
Depends upon gravitational field strength

17. Sliding vs. Static Friction
Sliding Friction: results when an object slides across a surface
Can be calculated from knowledge of the coefficient of friction and the normal force exerted upon the object by the surface it is sliding across
Sliding Ffrict = (u)(Fnorm)

18. Sliding vs. Static Friction
Static Friction: results when the surfaces two objects are at rest relative to one another and a force exists on one of the objects to set it into motion relative to the other object

19. Static Friction Example
Push with 5-Newton of force on a large box to move it across the floor.
The box might remain in place.
A static friction force exists between the surfaces of the floor and the box to prevent the box from being set into motion.
The static friction force balances the force that you exert on the box such that the stationary box remains at rest.
When exerting 5 Newton of applied force on the box, the static friction force has a magnitude of 5 Newton.

20. Static Friction Example
Suppose that you were to push with 25 Newton of force on the large box and the box were to still remain in place.
Static friction now has a magnitude of 25 Newton.
Then suppose that you were to increase the force to 26 Newton and the box finally budged from its resting position and was set into motion across the floor.
The box-floor surfaces were able to provide up to 25 Newton of static friction force to match your applied force.
Yet the two surfaces were not able to provide 26 Newton of static friction force. The amount of static friction resulting from the adhesion of any two surfaces has an upper limit.
In this case, the static friction force spans the range from 0 Newton (if there is no force upon the box) to 25 Newton (if you push on the box with 25 Newton of force).

21. Static Friction Example
This relationship is often expressed as follows:
Ffrict-static ≤ μfrict-static• Fnorm
In general, values of static friction coefficients are greater than the values of sliding friction coefficients for the same two surfaces.
Thus, it typically takes more force to budge an object into motion than it does to maintain the motion once it has been started.

22. Drawing Free Body Diagrams
Free-body diagrams are used to show the relative magnitude and direction of all forces acting upon an object in a given situation.
The size of the arrow in a free-body diagram reflects the magnitude of the force.
The direction of the arrow shows the direction that the force is acting.
Each force arrow in the diagram is labeled to indicate the exact type of force.

23. Drawing Free Body Diagrams

24. Determining the Net Force
An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force
If either all the vertical forces (up and down) do not cancel each other and/or all horizontal forces do not cancel each other, then an unbalanced force exists.

25. Determining the Net Force
The net force is the vector sum of all the forces that act upon an object

26. Determining the Net Force

27. Net Force?
Yes No
Yes
Yes No
Yes