1. Force and Motion Section 6.1
Physics

2. Let’s Get Started
Your assignment is to define ALL vocabulary words on page 144.
This is due at the end of class today.

3. Objectives
Define a force and differentiate between contact forces and long-range forces.
Recognize the significance of Newton’s second law of motion and use it to solve motion problems.
Explain the meaning of Newton’s first law and describe an object in equilibrium.

4. Types of Forces
Two Types:
Contact Forces
Long-Range Forces

5. Contact Forces A contact force acts on an object only by touching it.
Give an example of a contact force.

6. Long-Range Forces A long-range force is exerted without contact.
Magnets
Gravity
The force of gravity is an attractive force that exists between any two objects and is dependent on distance and mass of the objects.

7. Forces and Their Agents
Each force has a specific, identifiable, immediate cause called the agent.
When solving problems, you must identify and draw the agents.
Example: Hand holding a ball.
FHand on Ball
Ball
FEarth’s mass on Ball

8. Practice Problems
P 119 #1

9. Newton’s Second Law of Motion
Force = Mass x Acceleration
F = m(a) F m a

10. How are All Three Related?
How are force, mass, and acceleration related?
Explain the equation: F = m(a)

11. What if there is more than one force?
Often times, more than one force acts on an object.
The forces need to be combined to form the vector sum of the different forces.
A free-body diagram is drawn to show the different forces that act on an object.

12. Free-Body Diagrams Objects are represented by dots.
Forces are drawn on the dot in the direction and magnitude of the force.
Because forces are vectors, the total force on an object is the vector sum of all forces exerted on the object.
You are looking for the net force on the object.

13. Strategy First identify all forces acting on the object.
Draw the free-body diagram showing the direction and relative magnitude of each force acting on the system.
Use Newton’s second law to calculate the acceleration.
Use kinematics to find the velocity and position of the object.

14. Units of Force
If Force = Mass(Acceleration), what are the units of force?
The SI Unit of force is the Newton.
1 N = 1 kg (m/s²)

15. Practice Problems
Pg 122
2-6

16. Newton’s First Law of Motion
An object at rest will stay at rest or an object in motion will stay in motion if the net force acting on that object is zero.
Net Force: The sum of all the forces on an object.
Newton’s first law is often called the law of inertia.
Inertia: the tendency of an object to resist change. NOT a FORCE!

17. Newton’s First Law of Motion Cont.
If the net force on the object is zero, what will be the object’s motion?
It will not move or continue with unchanging velocity.
When the net force on an object is zero, the object is said to be in equilibrium.

18. Practice Problems
Pg 124
7-11

19. Common Misconceptions about Forces
A force is needed to keep an object moving.
If there is no net force, the object will keep moving.
When a ball has been thrown, the force of the hand that threw it remains on it.
NO! It is only a contact force.

20. Common Misconceptions about Forces Cont.
Inertia is a force
No, it is just a tendency of an object to resist a change.
Air does not exert a force.
Air exerts a huge force, there is just no net force. All the forces are balanced.