video to understand Newton's 3 laws of motion

here is another one:

Newton invented Calculus to explain motions. 
His most famous equation is referred as Newton’s second law. 
1) Could you find this equation knowing:
- Force (symbol F) causes the velocity  to change from an initial value (Vi) to a final value (Vf) . As the force increases, the change in velocity increases.
 (∆V = Vfinal - Vinitial  =change in velocity )
-The mass or inertia (m) opposes the change in velocity. As m increases, the change in velocity decreases.
- Increasing the time (∆t) during which the force is applied increases the change in velocity.
Call the change in velocity ∆V . (∆V = Vfinal -Vinitial)

Write: ∆V =  _________________                          Use F, m and ∆t

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2) The second law is now best known as:
The total force (net force)  acting on a body is equal to the mass times its acceleration
Or F = _________
a is the _____________  , m is the _____________, F is the ______________
a and F are vector quantities. They have magnitude and direction.
But we will only work in 1D in 10th grade. We will only consider 2 directions:
 right and left  or up and down. A negative sign shows the down or left direction.
For example: g = -9.8 m/s² means the acceleration due to gravity is pointing down.
Newton's formula tells us that  1N is the force necessary to accelerate a 1-kg mass at the rate of 1 m/s²
1N is about the force required to lift an apple
Units for F is newton (of kg m/s/s), units for a is m/s² , units for mass is kg.

The other way to write the Newton’s 2nd law helps you understand many situations:
F ∆T = m ∆V

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3)

watch that movie but:

try to answer the questions before he does. Do the experiments. HERE ARE THE DEMONSTRATIONS to do in class.
A)  Attach 2 toy cars by a spring. put 1kg  in one. stretch the spring. mark the middle. the spring exert the same force on both cars. 
Let go the cars. Do they get in the middle at the same time? Which one is faster ? Which one has the smaller inertia ?
(that is which one resists change in motion the least?

Newton's third law= the same force acts on both cars. But the one with the largest inertia M will resist a change in motion more.
 So more mass = less acceleration.
So the car with the least mass m will go faster and will reach the middle first. Same force but not the same acceleration.
Mathematically: M a = F = m a      

B)  Use a round spring scale. attach a mass of 1kg. Read the weight in Newton. weight = mg = ______ N. This is the gravitational force (pull) exerted by the Earth on the mass. This pull stretches the spring and moves the arrow.

You can call it the rest weight. Now quickly move the spring scale upward such as the spring scale is accelerated upward along the vertical.
Call the acceleration of the spring a. While you accelerate the scale read the it.  new weight = __________ N
 Do you read the same weight ?  less ? more ?  explain
Because of inertia the mass wants to stay at rest and resists motion. The spring stretches more and the scale reads more.
The pull on the spring is more than the rest weight mg. The pull is : new weight =  mg + ma = weight at rest + ma  .
with a the acceleration of the spring.  Next time you take an elevator, take a bathroom scale with you. As the elevator moves up, you weight increases by ma. You feel heavier because the bottom of the elevatory pushes you, while you want to stay at rest.  The force acts on the elevator, not on you.



source dessin:http://sol.sci.uop.edu/~jfalward/physics17/chapter4/chapter4.html
astronauts experiencing free-fall. They are said to be " weightless "
The force of gravity is still acting on them but because thy fall inside a plane
nothing (like the ground ) is pushing back on them. They don' t feel their weight.


C) same experience but accelerate the spring downward. Is the reading the same? less ? more ? new weight = __________N
Now the spring is less stretched, the reading is less than the rest weight. new  pull = new weight = mg - ma.
If you take an elevator to go down, as the elevator accelerates downward, you feel lighter. You push less on the bottom.

D) If you drop the spring scale. The scale is in free fall.  What is the reading ?
the spring is not stretch anymore, the mass and the spring are falling at the same rate, the mass is weightless.
a = g so pull = new weight = mg - ma = 0
If you could jump from the table with a bathroom scale attached to your feet, same thing. you would be weightless.
you are both falling at the same rate, you are not pushing on the scale.
Astronauts in space are weightless. 2 forces act on them. force of gravity and the centrifugal force. They balance eather other so astronauts feel
weightless. Because, they don't need to work against gravity, the experience a loss in muscle.
E) Here in an experiment to understand better the weighless phenonmenon. Get a styrofoam cup and make holes at the bottom.
Fill it with water. Of course the water will leak because of the pull of gravity. But now drop the cup full of water. (above a sink).
the cup and the water are in free-fall and the water does not leak !

F) get 2 spheres ith different mass. In free-fall, they both the ground at the same time. They have the same acceleration because:
small mass:   F1 = m a1  (newtons 2nd law) and F1 = m g (force = pull due to gravity)    so a1 = m g / m = g
large mass: F2 = m a2  and F2 = Mg so g2 = Mg / M = g

Do the experiment with a feather and a penniy in a vaccuum.

G) Imagine 2 little girls. they both weighs 70 pounds. one is standing up a hill and the other one at the bottom, ready to read a spring scale/

The top girl jumps on the spring scale. The reading is less than 70 ? more ? 70 pounds ?

H) 2 guys are hanging on each end of a rope. The rope goes through a pulley. One guy climbs on a rope. what happens to the other guy?


I) If you happen to float in space with an elephant next to you. Far from any gravity.
Would it be easier to push the elephant to get it moving in space than on EArth ?
Remember the mass, the quantity of matter , is the inertia and is the same wherever the elephant happens to be.

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3) You increase the speed of a ball you are throwing by keeping it longer in  your hand. Why ?

 

4) According to the second law, if the net force is zero (forces can balance each other), what is the change in velocity ? So what acceleration ?

5) Suppose a high-flying airplane is flying at a constant speed 900km/h and the thrust of the engines is 80,000 newtons. What is the acceleration of the plane ?

(hint: change in velocity is .. )Sketch the situation.

6) What kind of relationship you have between the net force and the acceleration?
So if you double the force, you ___________ the acceleration
If you triple the net force acting on a body, you ___________ the acceleration

If you want extra credits and get a better understanding, do graph2  from the menu.

7) A force F is acting on an object of mass m. If you double the mass but keep the force constant
(in magnitude and direction), what will happen to the new change in velocity ∆V?  acceleration?

8) An unbalanced force gives a 2.00kg mass an acceleration of 5.00 m/s². 
What is the force applied to the object ?

9) An unbalanced force of 25N , F, is applied to a 12-kg mass. What is the acceleration of the mass ?

10) A racing car undergoes a uniform acceleration of 8.00 m/s² . If the unbalanced causing the acceleration is 6.00 10³ N . What is the mass of the car ?

 

11) You drag a block of 30g  with a force of 5N  during 10 minutes. What is the change in velocity ?

hint: convert g to kg and minutes to seconds. Use a = change in velocity / change in time

12) You drag a block of 50g with a force of 5N (to the right) .
 A force of friction of 2N is acting to the left.
a) what is the net force ? (right = positive, left = negative)
b) What is the change in velocity of the block after 1 minute ?
hint: use Fnet = m a  and a = change in velocity / change in time.  converte minutes to seconds
and grams to kilograms.
c) What is the change in velocity of the bock after 1 hour ?
d) what is the change of velocity of the block after 1 year ?
e) AFTER 2 YEARS ?
G) if you keep pulling the velocity keeps _______________
h) so can you answer this question:
If one were able to produce and maintain a net force of only 1 Newton on the Queen Mary ocean liner what would be the final speed ?

13) You are pulling on a block with a force of 10N (no friction).
 The block was at rest but reaches a velocity of  5m/s. The time elapsed is 10s .
a) Find the mass of the block.
hint: use F = m (change in velocity / change in time)
b) A friction force of 4N@ left is now acting on the block in addition to the 10N@right.
The motion lasts 3 minutes. i) Find the net force acting on the block. ii)  Find the acceleration.
hint: use the mass found in a). convert minutes to seconds.

14) Albert the skater, 25kg total mass is propelled by a rocket power. The force F exerted by the rocket can vary.
A) Fill the folling table:

Thrust F	acceleration
100N	4 m/s/s
200N
	10m/s/s

B) Suppose now a 50N air resistance (constant)  force opposes the motion of Albert.(25kg)  F is to the right and the 50N resistance is to the left. Fill the TABLE:
(net force = thrust - air resistance = mass x acceleration)

Thrust F	acceleration
50N	0 ms/s
100N
200N

16)


A) A ball rolls down a uniform slope ramp. The acceleration is:
decreasing ? constant ? increasing?

If the ramp were steeper, the acceleration would be:
more? the same ? less?

When the ball reaches the bottom and rolls along the smooth level surface it:
continues to accelerate? does not accelerate ?

B)


This time a ball rolls down a ramp of varying slope .   
The ball's acceleration si greater: at the top ? at the middle? at the bottom?

the speed is greater at:

For this case the speed is greater when the acceleration is the : least ? greatest ?

C) can you draw a rampon which the acceleration of the ball will be very little at the top and progressively greater as it moves down the ramp.

17)
answer this question

18) answer this question
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19)   answer this question

20) extra credit




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