1. Unit 2: Dynamics: Chapter 3: Forces
deals with the effects of forces on objects!
Chapter 3: Forces
3.1 The Nature of Force
3.2 Newton’s First law
3.3 Newton’s Second Law
3.4 Newton’s Third Law
3.5 Friction Affects Motion
May the force Be with You.....

2. Chapter 3:Forces can change velocity
Key Terms and Concepts:
inertial mass
net force
Newton’s laws of motion
Normal force
reaction force
static and kinetic friction
vector addition
action force
coefficients of static and kinetic friction
dynamics
force
free-body diagrams
friction
inertia

3. Check out this small ass!
What is a Force?
A force is just a push or pull!
Examples:
-an object’s weight
-tension in a rope
-friction
-attraction between an electron and proton
-Bodies don’t have to be in contact to exert forces on each other, e.g., gravity.
Check out this small ass!

4. More About Forces: a force can cause a change in shape, distortion
forces can cause a change in velocity, speeding up or slowing down
force is a vector, units: Newton’s, N
We measure force with a Spring Scale
(Try Quick Lab- 3.2 on page. 128)

5. What are Newtons- they’re not just a cookie!
Fnet = m a
1 N = 1 kg m/s2
The SI unit of force is the Newton.
One Newton is the amount of force required to move a 1-kg object with an acceleration of 1 m/s²
1 Newton is approximately equal to the weight of 1 apple or 2 golf balls.

6. What are the types of Forces we will look at in Physics 20?
Fapp- Force Applied, push or pull
Ff - Force of Friction, opposite direction (-) of moving object.
Fg - Force due to Gravity, always down, the weight of an object
FN - Force Normal, is the force between the object and the surface, in most cases is up
FT – Force Tension is the force exerted on a object due to a rope or cable pulling it.
NOTE: sometimes FT is FS if it is an elastic!

7. A closer look at Fg! Near the surface of the Earth, g = 9.81 m/s2.
Weight - is the force due to gravity on an object. It is not mass! It is both mass X the acceleration due to gravity.
Fg = m (kg) X g(m/s²)
(Weight and g are always considered positive.)
Near the surface of the Earth, g = 9.81 m/s2.
Example 0: Homer has a mass of 120 kg, what is his weight?

8. Free- Body Diagrams (FBD)
An FBD is a vector diagram showing all the forces acting on an isolated object!
Steps to making an FBD:
Identify the object and make a box or dot on your paper to represent the object.
Identify and label all the forces on the object with the correct arrow placement and magnitude of the vectors.
Draw forces proportionally extending out form the dot at the center of the object.

9. Sample FBD’s!

10. Lets Practice!
1. A book is at rest on a table top. Diagram the forces acting on the book.
2. A girl is suspended motionless from the ceiling by two ropes. Diagram the forces acting on the combination of girl and bar.
3. A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.
4. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.
5. A force is applied to the right to drag a sled across loosely-packed snow with a rightward acceleration. Diagram the forces acting upon the sled. Include a bit of friction from the snow.

11. More Practice Cont’d:
A car moving at a constant velocity starts to speed up. The weight of the car is N [down]. The force of air resistance is 3600 N [backwards]. With the engine engaged, the force applied by the friction from the road and the engine is 7200 N [forward]. Draw a free- body diagram for this situation.
You are trying to push a heavy box across the floor. It’s causing you some trouble because the floor is not very smooth. Sketch a free body diagram of the box. The box is not moving.
A sled is sliding down a slopped hill. Sketch a free body diagram of the sled. Hint! What do we not have as a force in this question but we had in #7. Include friction in your diagram. Why?

12. Using Free- Body Diagrams to Find the Net Force
The NET FORCE is the vector sum of all the forces acting simultaneously on an object. We find the net (resultant) vectors in the same way we did during kinematics. We always add up the vectors in the x- axis (E/W) and then add the components in the y- axis (N/S) and then find the resultant vector to get the F net.
Can the net force on an object ever be zero?
There are two different question types to calculate the net force:
Collinear Vectors: Vectors that are parallel are collinear even if they have opposite directions. AKA: 1-Dimensional Problems!
Non-Collinear Vectors: Vectors that are at angles initially. We then have to calculate the components like we did in kinematics! Often called 2-Dimension Problems.

13. Never round your answers to the end of the question!!
Addition of Forces: Calculating Fnet!
Examples
1. Rose is using a force of 120 N to ride her bicycle east and the wind is blowing and exerts a force of 90 N west.
a.Draw a FBD diagram
b.What is the net force on the bicycle?
Marty is canoeing with his friends north across a river using a force of 85 N,
the river exerts a force of 65 N west.
a. Draw a FBD diagram
b. What is the net force on the canoe?

14. 3. Three zoo keepers are using ropes to move a snow leopard into a new pen. The zoo keepers use forces of 75 N 120°, 80 N 180°, and 100 N 220°.
a. Draw a FBD diagram
b. What is the sum of all of the forces?
75 N
80 N
100N

15. 4. As Maryn and Malayna pull their sled, they notice a patch of ice on the sidewalk directly in front of them. To keep on pulling the sled without slipping on the ice, they must begin to pull at an angle as they walk around the ice. Maryn pulls with 60 N at 20º N of E and Malayna pulls at 85 N at 20º S of E. The friction on the sled is 40N. Sketch a free body diagram and determine the new net force. HINT: WE WILL NEED TO DO COMPONENTS AGAIN!