Newton's First Law LAB: INERTIA SCALE LAB2: INERTIA worksheet Newton's law, concepts. From Newton' s Principia: Every material continues in its state of rest, or of uniform motion ina straight line, unless it is compelled to change that state by forces impressed upon it. Objects retain the motion they have unless compelled to do so by a force
PART 1 - INTRODUCTION FORCES
1) read: Classical physics is sometimes called Newtonian physics in honor of Sir Isaac Newton who lived from 1642 to 1727 and formulated three laws of motion that summarize much of the behavior of moving bodies.
Forces may cause motion. Inertia tends to resist the influence of an applied force.
Forces, inertia, friction, and how they relate to motion are considered now.
2) Read
A force is a push or a pull that tends to cause motion or tends to prevent motion. According to Newton it is anything that can accelerate an object.
Force is a vector quantity and thus has both magnitude and direction. The unit is the newton. For example: The force due togravity, acting on 1kg mass,is 9.8N down 9.8N is the magnitude and down is the direction.
The force tends to produce an acceleration in the direction of its application. For example: A free-fall body accelerated downward because of the force of gravity.
Some force do not produce motion because they are balanced by other forces. In that case, the net force is zero.
3)
Draw the forces acting on:
·A book on a table ·A car moving at a constant speed ·A car accelerating ·A resting load on a string ·A rock falling. Neglect air resistance. ·A feather falling with a constant speed ·A sled pulled by you. Your sister is sitting on it. The speed is constant. wagon pulled by your friend. The wagon is accelerating.
4) read
Only 4 fundamental forces can explain any interaction between 2 objects:
the force of gravity
Newton found that the same force
(gravitational force) explain the motion of planets or other objects in
space and the motion of object on earth (like a projectile). He showed
that the moon falls toward the Earth the same way an apple falls toward
it. It was the first unification of 2 forces.
The electromagnetic force. Maxwell unified the electric force and the magnetic
force. He showed that electricity and magnetism were 2 aspects of the
same phenomenon. He wrote 4 elegant mathematical equations to show this
unification. More amazing: his equations lead to the value of
the speed of light. This time, mathematics showed the way to physicists.
The strong nuclear force.
This force was discovered in the 20th discovery what
physicists started to understand atoms and their structure. This force
hold the protons together despite their strong repulsion. That's why
this force is very strong but only over a very short distance.
The weak nuclear force.
This force acts inside the particles of the nucleus.
Physicist were able to unify this force with the electromagnetic force
leading to the electroweak force. The weak force is involved in
breaking apart some nuclei and changing neutrons into protons.
One of the challenge of Physics is to unify all the
4 fundamental forces of nature. That was the big problem that puzzled
Einstein during years. It is what string theory is about. Unification of the 4 forces. So far, string theory has not been proved right. Just ideas.
The units for measuring force are the
newton (N) in the metric system and the pound (lb) in the English
system. The conversion factor is 4.45N = 1 lb
1 kg mass is acted upon by a force of gravity of 9.8N on Earth or 2.2 lbs
PART 2: LAW OF INERTIA - NEWTON's FIRST LAW 0) READ If a large
object is at rest (car), it resists being moved. That is, you have to
produce a tremendous push to get it moving. Similarly, if a car is
moving, it takes a large force to stop it (the brakes). This property
of resisting a change in motion is called inertia.
Inertia
is the property of a body hat causes it to remain at rest if it is at
rest or to continue moving with a constant velocity unless an unbalanced force acts upon it.
Every
object continues in its state of rest or motion, in a straight line, at
a constant speed, unless compelled to change that state by forces exerted upon it. misconception:
" Motion requires force " WRONG. A puck initially hit will keep
moving on the ice until it is stopped by a wall or by a player. motion
does not require force. A net force is required to change the motion. to
speed up an object, to slow it down, to stop it, to get it to move, or to change the direction.
Mass is the quantity of matter in an object. Mass is a measure of the inertia of an object. (its resistance to change in motion, its laziness) Mass is measured in kilograms source: Paul Hewitt, conceptual Physics weight is the force of gravity on an object. Watch that movie. Try to do some of the demonstrations shown in the movie: (material : carts , 500g masses, 1kg masses, string, hammer and heavy book, sticks) - Use a cart. Push a cart with 500g inside. Then with 1000g inside - Hang 1 string at each end of a 1 kg mass. Try to break the top one and then the bottom one. - Hang 1 string to a large mass and lift it without breaking the string. And breaking the string. -Put your hand under a physics book and hit the physics book - Put your hand on a stick and hit the table with the stick. You hand keeps moving it hurts! - you go to the bathroom and at the same time you read a Physics book. Do you have to free both hands to get paper toilet ? or can you tear piece with one hand using inertia ? source: Paul Hewitt, Conceptual Physics
Then this one Try to do some of the demonstrations shown in the movie: (material : penny+glass+index card, newspaper + ruler, cart+ block, string + ball) - A penny on an index card. The index card is on a glass. Give a kick to the index card. - A newspaper flat on a
paper. Insert a flat ruler between the table and the newspaper. Push
down quickly on the ruler to lift the newspaper. - Fill a glass with water. Insert a paper between the glass and the paper. Try to pull the paper without spilling the water. -Put a block in a cart. Push the cart then stop it abruptly. What happens to the block/ - attach a soft ball to a string. Turn the ball in a vertical plan. Let the ball goes when it is at the top. watch this oneextra credits if you can do the same !
Why do you need to wear seat belt:
Try to answer the questions without the hints. Unless you are confused. 1) According to the intertia principle, once something starts moving it should keep going, at the same speed and direction forever. (unless acted upon by a force). You accelerate your car to reach 20mph and then let go with the accelerator. The car slows down. why ? It should keep going according to the principle of inertia ? (hint: unless acted upon by another force ..)
2) demonstration: Have you tried to drop a quarter on a card in a glass? Using the law of inertia?
3) demonstration: How could you hit
your hand with a hammer without breaking any bone ? Using a big Physics
book ?
Inertia will resist : (think of a car)
· Changing speed (going faster or slower)
· Changing direction (like stirring the wheels of a car), even if the speed stay constant
· Stopping (final speed =0) or getting started (initial speed =0)
4-1) Galileo was the one who explained the concept of inertia. He used incline planes. Let’s say you have 3 incline planes and you let go a ball at one edge. There is no friction between the ball and the plane. Can you predict the position of the ball next ? In the 3 following cases:
4 - 2) An elephant has a large inertia and can run very fast. What could you do to escape a charging elephant ?
Hint: a large inertia (mass ) resists change in direction.
5) The
principle of Inertia is not intuitive. Aristotle got it wrong and his
ideas were dominant until Galileo came along. How long is that ? WIKIPEDIA.ORG
6) Year Galileo died ? Year Newton was born ? WIKIPEDIA. ORG
7) If suddenly
the force of gravity of the Sun stopped acting on planets, in what king
of path would the planets go ? Make a sketch
8) The
principle of Inertia explains why we need to use the safety belts in a
car. Can you ? What about the air bag ?
9)
Can the principle explain head injury when struck by a car from behind?
10) Did you try the
magic trick with a table cloth and dishes ? Can you explain it now ?
(source: Paul Hewitt, Conceptual Physics)
Underline the correct answer or fill the gaps:
11)
An astronaut in outer space away from gravitational or frictional
forces throws a rock. The rock will : gradually slow to a
stop or
continue moving in a straight line at constant speed
12. The rock’s tendency to do so is called: __________________
13. Suppose you are standing in the
aisle of a bus that travel along a straight line road at 100 km/h, and
you hold a pencil still above your head. A)Then relative to the bus, the
velocity of the pencil is 0km/h and relative to the road, the pencil
has a horizontal velocity of:
Less than
100km/h
100km/h
more than 100km/h
B) You drop the pencil. Relative to the road, its horizontal velocity is : ____________
C) Relative to you, does the way the pencil fall depends on the velocity of the bus ? _______
The laws of Physics are he same is a referential at rest or moving at a constant speed.
16) stinger missiles are heat seeking missiles. They detect infra red and can be launched after planes. They reach them because the " feel " the heat. If you are an experienced pilot, how can you avoid these stingers ? (source: physics for future presudents, Richard MulleR)
17)How can you escape a charging rhinoceros ?
18) source: Paul Hewitt, Conceptual Physics Let s say you try to push a large mass like an Anvil. Is it harder to push it on Earth or in space, out of gravity reach ? (hint: inertia depends on the mass, not the weight)
19) EXCELLENT ANIMATION. 2 balls are falling, undergoing the same acceleration due to gravity along the vertical. one has an initial horizontal velocity, the other has no initial velocity. Imagine
you place an incline plane on a table. One ball rolls down the
plane, along the table then falls with an initial horizontal velocity
V1. At the same moment the ball leaves the table, a second ball is
dropped with no initial velocity. see image. They both reach the ground
at the same time, with the same final velocity along the vertical. Every second, their vertical velocity is increased by g t. Not that the the ball with an initial horizontal velocity follows a parabola. This is called a projectile motion./ as your instructor to show you the demonstration with the car and the jumping ball.
source: Paul Hewitt, conceptual Physics
source: Paul Hewitt, Conceptual Physics
