1. Forces and Newton’s Laws of Motion

2. Forces
A force is a vector quantity (has magnitude and direction) that is typically described as a push or pull. Forces cause objects to accelerate (change velocities) if they are unbalanced. Forces that are in equilibrium are balanced and therefore cause no change in motion.
Balanced Forces
No Acceleration
Force (cow on chair}
Unbalanced Forces
Acceleration
Weight FN

3. Types of Forces
Contact Forces - two interacting objects are physically in contact with each other
Examples: friction, tension, and applied forces

4. Types of Forces
2. Field or (Action-at-a-distance) Forces - two interacting objects are not in physical contact with each other
Examples: gravitational forces (9.81m/s2 on Earth), electric and magnetic forces
electricity
magnetism

5. Free-Body Diagrams
Free-body diagrams (FBDs) are graphical representations of objects where all forces acting ON the object are represented. Forces exerted by the object on other objects are not included in the diagram.
FBDs are used to analyze situations. When you isolate an object, and identify all of the forces acting on it, you can predict how the object will move.

6. Video

7. Newton’s First Law of Motion
Newton’s First Law of Motion, also known as the Law of Inertia, states that a body in motion will remain in motion at a constant velocity, and a body at rest will remain at rest unless acted upon by an unbalanced force.
Inertia is the resistance of an object to changes
in its motion.

8. More on Inertia
Galileo, the premier scientist of the 17th century, developed the concept of inertia. Galileo reasoned that moving objects eventually stop because of a force called friction.
Mass is a measure of inertia.

9. The man continues to move forward because he has inertia.
Question: If you were the man, what could you do to stop yourself from flying over the hood of the car?
Answer: Wear a seatbelt. The seatbelt provides the unbalanced force to stop the body in motion.

10. Implications of Newton’s First
A body in motion at constant velocity does not necessarily require a force to keep it in motion at constant velocity.
If there were no friction, objects would continue to move at the same speed in the same direction indefinitely.

11. Implications of Newton’s First
Any forces acting on a body in motion at constant velocity MUST be balanced. Consider a bird carrying an egg descending at a constant velocity:
Force of Bird
Force of Bird = Weight
Weight

13. What happens if the forces were unbalanced?
Acceleration

14. Balanced and Unbalanced Forces
Balanced Force – forces on an object that have the same magnitude but acting in opposite directions cancel each other
Unbalanced Forces – forces that do not cancel each other. Unbalanced forces cause a change in the state of motion.

16. Newton’s Second Law
Forces
mass
acceleration
summation

17. SI Unit for Forces
The newton (N) is the SI unit for force… the pound is the English unit for force.
One newton is the force required to accelerate a
mass of 1 kg at a rate of 1 m/s2

18. How much force does the man need to apply to the car to accelerate it
How much force does the man need to apply to the car to accelerate it .05 m/s2?
F = ma
F= 1,000kg * .05 m/s2
F= 50 kg*m/s2
F= 50 N

19. Newton’s Second Law
F is the summation of forces acting on the object under consideration. F may also be called the net force, or the unbalanced force. Let’s consider the free-body diagrams below:

20. Unbalanced Forces
Since force is a vector, an unbalanced force exists whenever all vertical and/or horizontal forces do not cancel out.

21. Determining the Net Force
Ftens = 1200N
Fnet = 400 N, up
Fgrav = 800N
Fnorm = 50N
Fnet = 20 N, left
Ffric = 20N
Fgrav = 50N

22. If a net force is applied in the same direction as an object is moving, then the object’s speed will increase.

23. A NET force applied in the opposite direction of an object’s motion will decrease the speed of the object.

24. A NET force applied at an angle to the movement of an object will change the direction (and sometimes the speed) of the object.

25. Implications of Newton’s Second
Bigger forces yield bigger accelerations on the same mass.
10 N
F = ma
10 N = 5 kg (a)
a = 2 m/s2
m = 5 kg
50 N
F = ma
50 N = 5 kg (a)
a = 10 m/s2
m = 5 kg

26. Implications of Newton’s Second
Forces accelerate smaller masses more than larger masses.
F = ma
10 N = 2 kg (a)
a = 5 m/s2
10 N
m = 2 kg
10 N
F = ma
10 N = 4 kg (a)
a = 2.5 m/s2
m = 4 kg

27. Newton’s Third Law of Motion
For every action, there is an equal and opposite reaction.
Rocket landinghttp://phet.colorado.edu/en/simulation/lunar-lander
Video

28. Implications of Newton’s Third
If you push/pull on something, it pushes/pulls back on you with an equal but opposite force.
Forces always occur in pairs, called action/reaction pairs.

29. Action- Reaction

30. Action: Object A exerts a force on object B
Reaction: Object B exerts a force on object A
Incorrect for situation: Apples falls to ground
Action: Earth pulls Apple
Reaction: Apple falls to the ground
Correct
Reaction: Apple pulls Earth

31. And Even More

32. The Forces Acting ON an Object Determine its Motion
The action/reaction pairs are forces that are exerted on two objects by each other.
Why does anything move? These forces seem to cancel.

33. Action/Reaction on different masses
If the force on the apple and the Earth is equal, why does the apple move but the Earth does not move?
Acceleration depends on force and mass. The force are the same but the masses are not.
The Earth has a large mass, so it has a very small acceleration.
The apple has a small mass and a large acceleration

34. The force the bug exerts on the car, and the force the car exerts on the car are EQUAL
Fbug = Fcar
mbugabug = mcar acar
These forces do not cancel each other out because the forces are exerted on objects with different masses .