1. The Laws of Motion
Unit 3 Presentation 1

2. What is a force? A force is a push or a pull on an object.
Two types of forces:
Contact Force: Two objects are in contact with each other causing a force (ex: springs, physically pulling something, etc.)
Field Force: A force is induced by two objects that are not touching (ex: Electrostatic forces, magnetism, gravity)

3. Units of Force
SI Unit = Newton (N)
English Unit = Pound (lb)

4. Isaac Newton (1642 – 1727) English Physicist and Mathematician
Credited with inventing Calculus
Formulated the basic laws of mechanics, discovered the law of gravitation, and numerous smaller discoveries

5. Newton’s 1st Law of Motion
An object moves with a velocity that is constant in magnitude and direction, unless acted on by a nonzero net force.
The “Lazy Law” or the “Law of Inertia”
Inertia: The tendancy of an object to continue in its original state of motion.
“An object stays in motion (or stays at rest) unless acted upon by another net force.

6. Newton’s 2nd Law
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

7. Newton’s 3rd Law If object 1 and object 2 interact, the force
exerted by object 1 on object 2 is equal in magnitude but opposite in direction to the force exerted by object 2 on object 1.
“Every action force has an equal but oppositely-directed reaction force.”

8. Types of Forces
Gravity: The force of attraction between two massive bodies. Often called “weight” on Earth.
Normal Force: The force exerted by an object on another object in direct contact. Always perpendicular to the surface.
Tension: Force that is exerted along a rope
Friction: Force that always opposes motion

9. Net Forces
Net Force: The vector sum of all forces acting on an object.
Free Body Diagram: A pictoral representation of all of the vector forces acting on an object’s center of mass.

10. Free Body Diagram Example
Bob is standing in the street. Draw a FBD for Bob.
First, draw a dot representing Bob’s center of mass.
Next, consider every force that is acting on Bob and its direction.
Forces:
1) Gravity (downward)
2) Normal Force (upward)
Normal Force
Gravity
There are no ropes present (no tension) and Bob isn’t moving (no friction).

11. Another Free Body Diagram Example
Bob is getting pulled down the street (to the east) by his friend Joe with a rope. Draw a FBD for Bob.
Forces:
1) Normal force of street onto Bob
2) Gravity
3) Tension (towards the east)
4) Friction (opposing the tension, pointed to the west)
Normal Force
Friction
Tension
Gravity

12. Another Free Body Example
Bob is pulling a crate up an inclined plane by a rope. Draw a FBD for the crate.
Lets draw a picture.
Forces:
1) Gravity (downward)
2) Normal Force (perpendicular to surface)
3) Tension (up the incline)
4) Friction (down the incline)
Normal Force
Tension
Friction
Gravity

13. Using Newton’s 2nd Law
Calculate the weight of an object using Newton’s 2nd Law.
What is the appropriate acceleration? Use the acceleration due to gravity on the Earth’s surface! (g = 9.81 m/s2)
Hence, weight = mass * g

14. Weight Example Problems
Calculate the weight of a 5 kg bowling ball.
Calculate the mass of a 150 N doll.

15. Newton’s 2nd Law Example
An airboat of mass 3.50 x 102 kg, including passengers, has an engine that produces a net horizontal force of 7.70 x 102 N, after accounting for forces of resistance.
(a) Find the acceleration of the airboat
(b) Starting from rest, how long does it take the airboat to reach a speed of 12.0 m/s?
(c) After reaching this speed, the pilot turns off the engine and drifts to a stop over a distance of 50.0 m. Find the resistance force, assuming its constant.

16. Newton’s 2nd Law Example Cntd.
First, apply Newton’s 2nd Law to find the acceleration.
Now, use this acceleration and a kinematics equation to find the time. Remember, the boat starts from rest.

17. Newton’s 2nd Law Example Cntd.
Now, lets find the resistance force. First, lets find the resistance acceleration (or deceleration) using kinematics.
Now, lets use the deceleration and mass to find the resistance force using Newton’s 2nd Law: