1. Forces

2. Newton’s 1st Law Review Question
Use Newton’s 1st Law of Motion to explain why the zebra’s stripes in the comic below ended up all on his head…

3. Inertia
An object’s resistance to changing its velocity

4. Quick Recap Newton’s First Law of Motion: Inertia: Mass:
An Object at rest stays at rest unless acted upon by an unbalanced force
An Object in motion stays in motion at the same velocity unless acted upon my an unbalanced force
Inertia:
An Object’s resistance to changing it’s motion
More Mass=More Inertia
Less Mass=Less Inertia
Mass:
How much stuff makes up an object
Book vs. Piece of Paper
Say these aloud as a class. This is the exact phrasing you should use when I ask you to explain how Newton’s First Law applies.
More mass=more inertia the more mass something has the more resistant it is to changing its motion
Less mass=Less inertia  the less mass something has the more resistant it is to changing its motion

5. Force A force is a type of push or pull Force is measured in Newtons
1 Newton is the force is takes to accelerate a 1kg object 1m/s2
Force is a vector- They have a magnitude and direction

6. Free Body Diagrams
These help us see all of the forces acting on an object
Objects are represented by a box
Forces are represented by arrows coming from (pulling) or going into (pushing) the box (they are vectors)
The direction of the arrow is the direction of the force
The length of the arrow represents how big the force is
The diagram to the right would be an object with a
force of 2 Newton’s upward and 4 Newton’s downward 2N 4N

7. Net Force Net force is the overall force acting on an object
Right and up are positive, left and down are negative
An object will accelerate in the same direction as the net force
Balanced forces have a Net Force of 0 Newtons
This will result in NO change in velocity
Unbalanced forces will change an object’s velocity
An unbalanced force is REQUIRED in order to change an object’s velocity (accelerate)

8. What is the net force? Example 1:
Roy pushes a box to the left with a force of 30 N and Sally pushes the box to the left with a force of 50 N
Step 1: write down givens (draw a FBD)
ForceRoy= -30N
ForceSally= -50N
Step 2:
Total Force=(ForceRoy)+(ForceSally)
Step 3:
Total Force= (-30)+(-50)
Step 4:
Total Force= -80N (OR 80N to the left)
Sally (50N)
Roy (30N)
*Remember: forces to the LEFT or DOWN are negative

9. What is the net force? Example 2:
Roy pulls a box to the left with a force of 30 N and Sally pulls the box to the left with a force of 50 N
Step 1: write down givens (draw a FBD)
ForceRoy= -30N
ForceSally= -50N
Step 2:
Total Force=(ForceRoy)+(ForceSally)
Step 3:
Total Force= (-30)+(-50)
Step 4:
Total Force= -80N (OR 80N to the left)
Sally (50N)
Roy (30N)
*The math works out the same! HOWEVER, note the arrows for the Free Body Diagram are pointing outward, not inward

10. Check YOUR Understanding: What is the net force?
1) Roy pulls a box to the right with a force of 30 N and Sally pushes the box to the left with a force of 50 N
2) Roy pulls a box to the left with a force of 30 N and Sally pushes the box to the right with a force of 50 N

11. Check YOUR Understanding What is the net force?
Al pushes a box to the left with a force of 40 N, Bob pushes the box to the right with a force of 60 N and Carl pushes the box to the left with a force of 30 N

12. Types of Forces Compression Force (FCompression) a push force
Think about a spring
Applied Force (FApplied) a pushing force that you would put on an object to make it move
Tension Force (FTension) a pull force
Think about tug o’ war
Gravitational Force (FGravity) pulling force that exists between any two objects that have mass

13. Frictional Force (FFriction) opposes the motion of an object that’s in contact with another object
Kinetic Friction (FKineticFriction ) Friction felt by an object when it is moving
Static Friction (FStaticFriction )  Friction felt by an object when it is not yet moving
FFriction
FTension

14. Normal Force (Fnormal) The force acting on an object, perpendicular to the surface where the object rests
This is the force that keeps gravity from pulling you through your chair and the floor and the ground to the center of Earth!
FNormal
FWeight

15. If you aren’t moving are you exerting a force?

16. YES… standing on Earth anyway
We are always exerting a force standing on Earth because we are always encountering another object (at the very least the ground)
Gravity causes us to put a force on the ground or our chairs. This means we also have forces acting on us, the normal force.