1. Introduction to Newton’s Laws

2. Are heavier objects harder to stop when moving than light objects?
Why do you move to the side of a car when the car turns a corner?

3. Back in the day…
Aristotle tried to answer the question of why objects fall to Earth? By explaining that objects would seek their “natural position.”
-This also explain why Earth didn’t move.
It was already in its natural resting place at the center of the universe.
This was, until Copernicus
came along and explained that the Earth orbited the sun.
This was later backed up by Galileo and Newton when they explained forces and motion in the universe.

4. Comparison between Aristotle and Galileo
Things at rest were at their natural resting place.
Objects that are moving away from their natural resting place need to be continuously forced to keep them moving
Galileo
Objects at rest stay at rest until something forces them to do otherwise.
Objects in motion stay in motion unless something forces it to do otherwise

5. Newton’s Laws
What are Newton’s 3 Laws of motion?
1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. (Law of Inertia)
2nd Law – Force equals mass times acceleration.(F = ma)
3rd Law – For every action there is an equal and opposite reaction. (All forces come in pairs)

6. Newton’s 1st Law Law of Inertia
An object at rest will stay at rest, and an
object in motion will stay in motion at constant
velocity, unless acted upon by an unbalanced force.
What does this all mean?

7. What gives an object Inertia?
Volume is the amount space an object takes up.
Not what gives an object inertia.
Weight is the affect that gravity has on an object.
Not what gives an object inertia.
Mass is the amount of matter an object possesses.
(the amount of “stuff” something is made of)
This is what gives an object inertia.

8. Inertia Inertia is an objects ability to resist a change in motion.
Mass is directly related to Inertia. The more mass you have, the more inertia you have.
Some Inertia Demos

9. Newtons’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 km/hour.

10. If Newton’s 1st law is true:
Why then, do we observe every day
objects in motion slowing down and
becoming motionless seemingly
without any outside influence?

11. Forces! In the absence of friction, an object you
A force is a “push or pull.”
-force is a vector quantity (it has a size and direction)
Friction is a type of force. It is a force that opposes motion.
Acts in the direction opposite motion
In the absence of friction, an object you
push (like a book on a table top) would continue in
motion with the same speed and direction - forever!
(Or at least to the end of the table top.)

12. Up until this point, we have only been dealing with objects at rest or moving with a constant velocity.
Can we describe the motion of objects that don’t move at a constant velocity?
Yes!!!
Things that aren’t moving at a constant velocity are accelerating.

13. Acceleration Review
Acceleration is how much velocity changes in a certain amount of time.
Acceleration is a vector quantity.
The units for acceleration are meters per second per second, or m/s2

15. F = ma Newton’s 2nd Law The net force of an object is equal to
the product of its mass and
acceleration

16. Newton’s 2nd Law
If F = ma then the units for force are kgm/s2
We call kgm/s2 Newtons (N)
How much force is needed to accelerate a 1400 kilogram
car 2 meters per second/per second?

17. Another form of F = ma We can write this as
What does this mean in English?

18. What is a net force?
The sum of all forces acting on an object in a given direction.
1 N
3 N
2 N

19. Unbalanced forces vs balanced forces
Forces are unbalanced when the net force on an object is not at equilibrium (or at zero).
3 N
2 N
1 N =
Forces are balanced when the net force on a object is at equilibrium.
3 N =
No net force

20. So why does everything fall with the acceleration due to gravity?
Little F over little m
Big F over big m F m F m g =
9.8 m/s2 a = g =
9.8 m/s2 a =

21. For every action there is an equal and opposite reaction
Newton’s 3rd Law
For every action there is an equal and opposite reaction OR
All forces come in pairs
Whenever an object exerts a force on a second object, the second object will exert an equal force in the opposite direction on the first object…
If “A” pushes on “B” then “B” also pushes on “A”

22. Consider the propulsion of a fish through the water
Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.
The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

23. Can you think of other examples of Newton’s 3rd Law?

24. Horse and Cart
A horse can pull a cart with a force of 3120N. According to Newton’s 3rd, what is the force that the cart pulls back on the horse with?
If this is true, how can the horse ever move the cart if the forces cancel each other out?

25. Tug of War
How much force does the bird on the right pull with if the bird on the left pulls on the worm with a force of 2 N and it wins the tug of war? What is the tension in the worm?

26. Free Body Diagram (Force Diagrams)
Lets draw a picture of the force of a man pushing on a Physics Box
15N

27. Free Body Diagrams
When we draw forces acting on an object (in the form of a free body diagram), we should draw all the forces so that they are pulling from the “center of mass.”
15N

28. Other types of forces
Normal force is the force that is always perpendicular to the surface of what ever an object is sitting on. It generally opposes the downward force of gravity.
Gravitational force: Force due to gravity is the force that an objects exerts because of gravity. On Earth, all objects gravitational forces act downward toward the center of the planet.
Weight is an object’s force due to gravity. They are interchangeable.
Tension: only observed when an object is under the influence of a “pull.”
Friction: a force that opposes motion (retarding force)

29. When you sit in your chair, your body exerts a downward force on the chair (your weight) and the chair exerts an upward force on your body (known as the Normal Force).

30. Question: For each question, make sure to draw an accurate free body diagram!
What is the net force on a 15 kg box that is sitting on a table?
What is the net force on a 15 kg box that is being pulled to the left with 40 N of force and 28 N to the right?

31. Question: For each question, make sure to draw an accurate free body diagram!
What is the net force on a 15 kg box on a frictionless surface that is moving at a constant velocity of 4 m/s to the right?
What is the net force on a 15 kg box on a frictionless surface that is accelerating to the right at a rate of 3 m/s2?

32. Question: Can you do this one?
Draw a free body diagram of a 20 kg box that is being pushed to the right with a force of 50 N and there is a 30 N frictional force. What is the net force? What is the acceleration?
Can you do this one?
What is the acceleration of a 15 kg box sitting on a frictionless surface if you pull it from a rope to the right at an angle of 30 degrees from the horizontal?
For this one we need TRIGONOMETRY!

33. Types of friction
Static Friction: The frictional force that exists between two surfaces when they are stationary
Kinetic Friction: The frictional force that exists between two surfaces when the objects are moving across each other (like a book sliding on a table)
Both of these types of friction can be calculated by using the coefficient of friction (either the coefficient of static friction or the coefficient of kinetic friction.)

34. Coefficient of Friction
A value that is different for any two surfaces.
- It is a constant that can be used to help calculate the force due to friction if you know the normal force of an object.
Ff = FN
Mu (like the pokemon)
 is usually in between zero and one

35. Coefficient of Friction
The coefficient of static friction tends to be greater than that of kinetic friction
Another way to think about what  is, is how “sticky” the interaction between two surfaces is.
- The greater the , the greater the friction.

36. Questions
If the coefficient of friction between your tires and the road is 0.2, and the car has a mass of 1700 kg, what is the frictional force between the tires and the road?

37. Questions
A car with a mass of 700 kg is breaking hard. If the car is traveling at an initial velocity of 15 m/s, and takes 3 seconds to stop, what is the coefficient of friction between the tires and the road? (draw a free body diagram)

38. Here’s a doozy
If this 15 kg box is sliding down the hill at a constant velocity, what is the frictional force between the box and the incline? FN Ff F
F Fg
30o

39. Thought question
Why is it impossible to make a piece of string completely horizontal and straight if you hang a mass from the center?