1. Forces cause changes in motion!
Newton’s Laws
Forces cause changes in motion!

2. Review Force is: Push or pull Vectors vs. scalars Vector diagrams
Adding vectors
Balanced and unbalanced forces
Speed and Acceleration
Newton’s 1st Law
SI Units: mass and force

3. Newton’s 1st Law
All objects remain at rest OR continue in a straight line path at constant speed
UNLESS acted on by a net force
The law of inertia
Its all about equilibrium (balanced forces)

4. Inertia
“inertia” = an object’s tendency to keep doing what it was already doing
inertia = mass
Inertia is measured by an object’s mass
Mass is the amount of matter contained in an object
More mass = more resistance to changes in motion = more inertia

5. Mass is not Weight Mass = amount of matter contained within an object
SI unit: kilogram (kg)
Weight = amount of gravitational pull on an object
SI unit: Newton (N)
Due to amount of mass: proportional but not equivalent
1 N = (1 kg)*(1 m/s²)
w = mg
Derived from Newton’s 2nd Law
Acceleration due to gravity: 10m/s²
mass
weight

6. Example: mass to weight
How much does a 0.02 kg fish weigh on Earth?
w = mg
w = (0.02 kg)*(10 m/s²)
w = 0.2 N
1 N = 1 kg*m/s²

7. Example: weight to mass
How much mass is contained in a 2 N apple?
w = mg

8. Newton’s 2nd Law: WHY things accelerate
Its all about net force (unbalanced forces)
Resultant of all forces acting on the object
Rate at which an object speeds up or slows down
Mass of the object

9. Units

10. Free-Body Diagrams AKA vector diagrams
Used to figure out what the net force on an object is
It’s just a diagram of the forces acting on the object
Draw the object
Draw the forces: diagram the vectors
Each force = an arrow
Length of arrow = magnitude of the force
Direction of arrow = direction of the force

11. Free Body Example
Imagine you are pushing a huge box across the floor
But the box is too heavy, so your friend comes to help
You push with a force of 25 N, and your friend pushes with a force of 15 N.
The crate weighs 100 N.

12. Start identifying forces: Your push: 25 N Your friend’s push: 15 N
The crate is a dot
Start identifying forces:
Your push: 25 N
Your friend’s push: 15 N
The weight of the crate: 100 N
The normal force on the crate: 100 N
New vocabulary:
Normal force = support force

13. Example: Newton’s 2nd
You push a crate of oranges across a frictionless floor with a force of 25 N.
If the crate has a mass of 5 kg, what will be its resulting acceleration (while you push it)?

14. 2nd step: Newton’s 2nd Law equation
1st step: free body diagram to determine net force
Support force:
50 N
3rd step: plug what you know into the equation
Your push: 25 N
Weight of crate:
w = mg
= 50 N
4th step: evaluate

15. Example 2: Newton’s 2nd Law and net force
Imagine you are pushing a huge box across the floor
But the box is too heavy, so your friend comes to help
You push with a force of 25 N, and your friend pushes with a force of 15 N.
The crate has a mass of 10 kg.
What is the acceleration of the crate?

16. 2nd step: Newton’s 2nd Law equation
1st step: free body diagram to determine net force
Support force:
100 N
Your push: 25 N
3rd step: plug what you know into the equation
Your friend’s push: 15 N
Weight of crate:
w = mg
= 100 N
4th step: evaluate

17. Extend your understanding
You and your friend push that box from rest for 4 seconds. How fast is it going at the end of your 4 second push?
v = (0 m/s) + (4 m/s²)(4 s)
v = 16 m/s
How far has it gone during the 4 second push?
x = (0 m/s)*(4 s) + ½(4 m/s²)*(4 s)²
x = 32 m

18. Solids: Friction Is a force: just like any other
Acts on materials that are in contact with each other
Always acts in a direction to oppose motion
Actual motion
Intent to move
Caused by irregularities between surfaces and weak chemical bonds
Amount of friction depends on 2 factors:
Type of materials
Amount of force holding the 2 surfaces together: Normal force!

19. Fluids: Friction Is a force: just like any other
Acts on materials that are passing through a fluid
Always acts in a direction to oppose motion
Amount of friction depends on many factors:
Roughness of the object
Surface area of the object, perpendicular to motion
Speed of the object
Density and viscosity of the fluid

20. Example: Newton’s 2nd law and friction
You are skydiving, and you weigh 600 N. At one moment in the dive, the force on you due to air resistance is 50 N. At that moment, what is your acceleration?
Air resistance:
50 N
Net force on you:
550 N (down)
Your weight:
600 N

21. Friction between the floor and the crate is 5 N
Imagine you are pushing a huge box across the floor
But the box is too heavy, so your friend comes to help
AND this is real life, and there is friction
You push with a force of 25 N, and your friend pushes with a force of 15 N.
Friction between the floor and the crate is 5 N
The crate has a mass of 10 kg.
What is the acceleration of the crate?

22. 2nd step: Newton’s 2nd Law equation
1st step: free body diagram to determine net force
Support force:
100 N
Your push: 25 N
3rd step: plug what you know into the equation
Friction: 5 N
Your friend’s push: 15 N
Weight of crate:
w = mg
= 100 N
4th step: evaluate

23. Newton’s 3rd Law
“For every action force there is an equal and opposite reaction force.”
Forces always occur in pairs!
If object A pushes on object B, object B pushes back on object A with an equal force but in the opposite direction
This happens immediately and every time any object exerts any type of force
Action force acts on object B; reaction force acts on object A.
the action/reaction forces never both act on object A (or both on object B)

24. Seems Easy… Action: Reaction:
Object A pushes on object B
Reaction:
Object B pushes back on object A
Switch the nouns!
Example: A baseball player hits a ball with a bat. What is the reaction?
Action: the bat pushes on the ball
Reaction: the ball pushes back on the bat

25. More examples
Its not so easy to see the reaction when the objects aren’t every-day things… use the formula!
Action: your weight
Whaaat?
Your weight = the Earth’s gravitational pull on you
Action: The Earth’s pull (downward) on you
Reaction: You pull (upward) on the Earth

26. Why it is confusing Force versus effect of the force (acceleration)
Example: A 5kg bat pushes with 5 N on a 0.2 kg ball
Action: Bat pushes on ball with 5 N force
Effect:
a = F/m
= (5 N)/(0.2 kg)
= 25 m/s²
The 0.2 kg ball accelerates at 25 m/s²

27. (Action: Bat pushes on ball with 5 N force)
Reaction: Ball pushes back on bat with 5 N force
Effect:
a = F/m
= (5 N)/(5 kg)
= 1 m/s²
The 5 kg bat accelerates (if you immediately dropped it) at 1 m/s²

28. The idea Same force but big mass = tiny accel.
Newton’s 2nd Law
Same force but big mass = tiny accel.
Same force but tiny mass = big accel. OR

29. An apple weighs 1 N
Action: the Earth pulls down on the apple with a force of 1 N
Effect:
a = F/m
= (1 N)/(0.1 kg)
= 10 m/s²
The 0.1 kg apple accelerates downward at 10 m/s²

30. Action: the Earth pulls down on the apple with a force of 1 N
Reaction: the apple pulls up on the Earth with a force of 1 N
Effect:
a = F/m
= (1 N)/(6,000,000,000,000,000,000,000,000 kg)
= m/s²
the 6,000,000,000,000,000,000,000,000 kg Earth accelerates upward at m/s²