1. Force

2. Goal of the class To understand what forces are and how they interact
Question of the day: How does an unbalanced force act on an object?

3. Forces A force is a push or a pull
Forces are what cause objects to move (dynamics)
A force has a magnitude and a direction. Force is a vector.

4. Forces The SI derived unit of force is the Newton (N) 1 N = 1 kg·m/s2
A force of 1N acting on a mass of 1kg will produce an acceleration of 1m/s2
Sir. Isaac Newton

5. Forces
Forces can act through a distance or through contact: Distance Contact Gravity Pushes + Pulls Electric Force Friction Magnetic Force

6. Fundamental Forces

7. Force Diagrams
A force diagram or free-body diagram is used to show forces to help analyse a problem

8. Combining Forces
Forces are vectors and can be combined in different ways. This combined force is the addition of all the forces and is called the net force. 5N 5N
10N = 5N
10N 5N = 5N 5N
= 0

9. Combining Forces
안녕!
Draw a free body diagram for the baby on the sled. The dog pulls horizontally with a force of 100N. The weight of the baby and sled is 150N.

10. Net Force
A book is placed on a table with an incline of 35 degrees. The free-body diagram shows the forces acting on the book. Find the net force
N = 18N
Set up coordinate system
Find all x&y components
Solve
Ffriction = 11N
mg = 22N

11. Newton’s First Law
An object with no resultant force is not moving

12. Newton’s First Law
An object will remain at rest, or with a constant velocity unless it experiences an external net force.
Sometimes called the Law of inertia
Inertia is the resistance to a change in velocity
Car example going around corner

13. Equilibrium Equilibrium means to be in balance.
When the net force is zero an object is in equilibrium.
Example: A lift stopped on the 11th floor.

14. Newton’s Second Law
The acceleration of an object is directly proportional to the net force and inversely proportional to the mass.
ΣF = ma
Net force = mass x acceleration
1N = 1kg x 1m/s2
momentum

15. Newton’s Second Law
A ball is kicked with a force of 15N and accelerates at 5m/s2 to the right. What is the balls mass?

16. Newton’s Third Law
For every action there is an equal and opposite reaction.
Rocket in outer space
This does not mean that the net force is always 0
Imagine pushing a door

17. Questions

18. Homework set #5 Read Chapter 5 Answer questions from page 143 on
Question numbers 19, 29, 33, 53, 61, 75