1. Newton’s Laws
Forces and Motion

2. Laws of Motion formulated by Issac Newton in the late 17th century
written as a way to relate force and motion
Newton used them to describe his observations of planetary motion.

3. History Aristotle was an ancient Greek philosopher
Based on his observations the common belief was that in order for an object to continue moving, a force must be exerted in the direction of the motion
This lasted until Issac Newton proposed his “Laws of Motion” based on observations made of bodies free from earth’s atmosphere.

4. Newton’s 1st Law Inertia
An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity unless acted on by an unbalanced force.
This statement contradicted Aristotle’s teaching and was considered a radical idea at the time. However, Newton proposed that there was, in fact, an unrecognized force of resistance between objects that was causing them to stop in the absence of an applied force to keep them moving. This new unseen resistance force became known as “friction”.

5. Newton’s 2nd Law Fnet = ma
If an unbalanced force acts on a mass, that mass will accelerate in the direction of the force.
Since 8N is greater than 2N, the unbalanced force is to the right so the acceleration is to the right.
2 N
8 N a
Newton’s 1st Law says that without an unbalanced force objects will remain at constant velocity (a=0)…so it seems logical to say that if we apply a force we will see an acceleration.

6. Newton’s 3rd Law Action - Reaction
For every action force there is an equal and opposite reaction force.
Example: If you punch a wall with your fist in anger, the wall hits your fist with the same force That’s why it hurts!
Action-reaction forces cannot balance each other out because they are acting on different objects.

7. A Force is… A “push” or “pull”
Measured in Newtons (N) in the metric (SI) system and pounds (lbs) in the English system
A vector quantity requiring magnitude and direction to describe it
Represented by drawing arrows on a diagram

8. Types of Forces Weight - force of gravity
Normal force – surface pushing back
Friction - resistance force
Applied force - force you exert
Tension - applied through a rope or chain
Net force – total vector sum of all forces
Balanced forces – equal and opposite
Unbalanced forces – not equal and opposite

9. Weight The force of gravity acting on a mass. Weight always acts down!
Weight = mass (kg) * acceleration due to gravity
Weight is a force…so this is a special case of F=ma and the unit is a Newton.

10. Mass is… The amount of matter an object is made up of.
Measured in kilograms.
A universal value, independent of the influences of gravity.
NOT a force.

11. Normal Force (FN)
Defined as the force of a surface pushing back on an object.
Always directed perpendicular to the surface.
This is a contact force. No contact…no normal force.
NOT always equal to weight. FN
Examples: FN
Wa l l
Table

12. Friction
A resistance force usually caused by two surfaces moving past each other.
Always in a direction that opposes the motion.
Measured in Newtons
Depends on surface texture and how hard the surfaces are pressed together.
Surface texture determines the coefficient of friction (μ) which has no units.
Normal force measures how hard the surfaces are pressed together.

13. Types of friction
Static friction is the force an object must overcome to start moving.
Kinetic friction is the force an object must overcome to keep moving.
Static friction is always greater than kinetic friction!

14. Calculating the Force of Friction
Where f is the force of friction, μ is the coefficient of friction, and FN is the normal force
For kinetic friction:
For static friction:

15. May the Net Force be with you
Total force acting on an object
Vector sum of all the forces
The unbalanced force referred to in Newton’s Law of Motion
Net force is equal to the mass of an object times the acceleration of that object.

16. Force Diagrams
Force diagrams must include the object and all forces acting on it.
The forces must be attached to the object.
No other vectors may be attached to the object.
Components of forces, axis systems, motion vectors and other objects or surfaces may be included in force diagrams.

17. Force Diagram
Problem: A 10 kg crate with an applied force of 100 N slides across a warehouse floor where the coefficient of static friction is 0.3 between the crate and the floor. What is the acceleration of the crate.
Fapplied = 100 N
10 kg FN
f = μFN
Weight = Fg = (10 kg)*(9.81m/s2)

18. To Solve the Problem Now that you have the force diagram!
Problem: A 10 kg crate with an applied force of 100 N slides across a warehouse floor where the coefficient of static friction is 0.3 between the crate and the floor. What is the acceleration of the crate. FN
f = μFN
Fapplied = 100 N a
10 kg
Weight = Fg = (10 kg)*(9.81m/s2)
Write the Newton’s 2nd Law equation for the x- and y- directions.
Now plug in what you know and solve for what you don’t. Algebra!!