1. http://www.mrwaynesclass.com/freebodies/rea ding/index01.html
WARM-UP
We will take a free body diagram “quiz”. You don’t need to write the questions today, just answers. Then a “grade” Be honest!! It will just let you know how much you need to practice FBDs!
ding/index01.html

2. Newton’s Laws of Motion

3. Newton’s 1st Law of Motion
An object in motion tends to stay in motion and an object at rest tends to stay at rest, unless the object is acted upon by an unbalanced force.
Also called the Law of Inertia

4. Newton’s 1st Law of Motion

5. Inertia The resistance an object has to change its state of motion.
The more MASS something has, the more inertia it has!

6. We feel the effects of Newton's First Law every day, but usually don't notice them because other forces interfere. In space, the First Law is much more obvious. Objects will follow their natural path until they are stopped by an outside force. On Earth, the atmosphere will eventually slow down all moving objects, but in a vacuum (basically an empty space with no air or atmosphere), like space, it will be more obvious that objects obey Newton's Laws.

7. Some everyday applications
Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.
You amaze your friends by pulling a tablecloth out from under dishes on table
To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and thrusted downward at high speeds and then abruptly halted.
Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.
While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object that abruptly halts the motion of the skateboard.

8. How much force?
A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction?
Zero N
An object in motion will maintain its state of motion. The presence of an unbalanced force will change the velocity of an object.

9. Newton’s Second Law of Motion

10. Newtons’ Second Law Fnet = ma OR a = Fnet/m
The acceleration of an object is directly proportional to the net force acting on the object…
…and inversely proportional to the mass of the object.

11. Fnet vs. F
The distinction is important because not all forces cause acceleration only the NET FORCE on the object

12. NEWTON'S 2nd LAW OF MOTION

13. Graph of F vs. a
If various known forces are applied to an object - one at a time and the corresponding accelerations are measured. The data are plotted. Since F and a are directly proportional, the relationship is linear. F a

14. Slope of F vs. a
Since slope = rise / run = F / a, the slope is equal to the mass F a

15. More about mass Mass is an inherent property of an object
Mass is independent of the object’s surroundings
Mass is a scalar quantity
The SI unit of mass is kg

16. Mass vs. Weight …Mass and weight are different quantities
Weight is equal to the magnitude of the gravitational force exerted on the object
Weight will vary with location
Weight is not an inherent property of the object
Weight is a force so it equals m*a. In the case of weight, the acceleration is due to gravity. So weight = mg or 10m

17. For every action there is an equal and opposite reaction.
Newton’s 3rd Law
For every action there is an equal and opposite reaction.
Book to
earth
Table to
book

18. Think about it . . .
What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.
Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

19. Action - Reaction
If you hit a tennis ball with a racquet, the force on the ball due to the racquet is the same as the force on the racquet due to the ball, except in the opposite direction.
If you drop an apple, the Earth pulls on the apple just as hard as the apple pulls on the Earth.
If you fire a rifle, the bullet pushes the rifle backwards just as hard as the rifle pushes the bullet forwards.

20. a = m a m Apple’s Earth’s little mass big mass
The products are the same, since the forces are the same.
a = m m a
Apple’s
little mass
Earth’s
big mass
Earth’s little acceleration
Apple’s big acceleration

21. Important points that are often interpreted incorrectly
If an object is moving, there does NOT need to be some force making it move. It can be moving without accelerating. The force that set it in motion is in the past.
If v = 0, then a and Fnet do not necessarily need to be zero. Think of a projectile shot straight up at its peak.
An object does NOT need to move in the direction of the net force. It must accelerate that way but not necessarily move that way.

22. Misconceptions (cont.)
In a vacuum, heavy objects fall at the same rate as light ones.
When a big object collides with a little one, they hit each other with the SAME force. The smaller one accelerates more!

23. Example
Goblin 400 N
Ogre 1200 N
Troll 850 N
Treasure 300 kg
A troll and a goblin are fighting with a big, mean ogre over a treasure chest, initially at rest. Find:
Fnet a
v after 5 s
= 50 N left
= m/s2 left
= m/s left

24. Example
A 3 kg watermelon is launched straight up by applying a 70 N force over 2 m. Find its max height.
Phase I: the launch
Draw pic and find net force.
Calculate a during launch.
Calculate vf at the end of the launch (after 2 m).
40.6 N up
m/s2
m/s
Phase II: freefall
Draw pic and think about what a is now.
vf from phase I is v0 for phase II.
What is vf for phase II?
Calculate max height & add 2 m.
-9.8 m/s2
-9.8 m/s2
zero
4.76 m

25. Example
Consider an elevator moving downward and speeding up with an acceleration of 2 m/s2. The mass of the elevator is 100 kg. Ignore air resistance. What is the tension in the cable?
Identify Forces: Tension in cable, weight of the elevator
Draw freebody diagram
Chose coordinate system: Let up be the +y direction and down –y. Then :
Translate the FBD into an algebraic expression. T-W = m(-a) so
T-(100 kg)(9.8 m/s2) = (100 kg)(-2 m/s2) v T
W=Fg earthelevator. a

26. A scale measures normal force – not weight
A final note…..
A scale measures normal force – not weight
Another word for normal force is apparent weight