Download notes - MADD Physical Science

Motion
&
Forces
1.
2.
3.
Force (F)- push or pull.
Examples: pushing a box across the floor, hitting
a baseball, throwing a basketball, etc.
How Forces influence motion
1. Changes the velocity- speeds up, slow down or
changes the direction
2. Forces do not cause motion; they cause
acceleration
1.
Measured in Newtons (N)- amount of force it
takes to cause1kg of mass to accelerate at 1m/s2.
(1 N= 1 kg•m/s2).
2.
Representing Force-use an arrow
The arrow points in the direction of the force.
The length of the arrow represents the strength of
the force.
Examples: Give direction and which is strongest:



A
B
C
Net force = the combination of all
forces acting on an object.

Objects accelerate in direction of
the net force



Forces acting in the same direction
add together.
Forces acting in the opposite
direction subtract.
If Net force is zero, then there is no
acceleration)
1.
2.

Arrows pointing in same direction add
together.
Arrows pointing in opposite directions
subtract from each other.
Example:
3N
3N
1N
1N
= 4N to right
= 2N to right
1.
2.
3.
Balanced forces – Net Force = 0
(They cancel each other out)= no
acceleration but could be at
constant velocity.
Unbalanced forces – Net force ≠ 0
(they do not cancel) =objects will
accelerate
Remember: Forces in same direction
add, opposite directions subtract

Look at the Force diagrams below: Figure
up the net Force and in which direction the
object will accelerate.
DO NOT COPY -
CHECKING YOUR UNDERSTANDING
If the forces acting upon an object are balanced, then
the object
a. must not be moving.
b. must be moving with a constant velocity.
c. must not be accelerating.
d. none of these
The answer could be A and it could be B .
An object having balanced forces definitely cannot be accelerating.
This means that it could be at rest and staying at rest (one option) or could be
in motion at constant velocity (a second option).
Either way, it definitely is not accelerating
Friction- force that acts in an
opposite direction than the
moving object


Causes the object to slow down
To move an object, your force must be
larger than the force of friction
1.
2.
3.
4.
Static Friction- force that surface acts on an
object to keep it from moving. Ex: box sitting on
ground
Sliding friction- force that opposes the direction
of motion of an object as it slides over a surface:
Ex: slide
Rolling friction- force that opposes motion of a
rolling object. Ex: tires
Fluid friction- friction opposing object moving
through a fluid such as gas or water. Ex:
airplane, submarine
Gravity – attraction between two
objects due to their mass.
Gravity depends on:

the mass of the two objects

the distance between the objects


Greater mass = greater the attraction
of gravity
Greater distance b/t objects = gravity
decreases

Earth’s gravity overcomes all
the objects on Earth therefore,
causes objects to fall
(accelerate) towards the center
of the earth.
 All falling objects accelerate
at the same rate (9.8 m/s2) in
the absence of air resistance.
 Air resistances (air friction)acts against gravity and
slows objects down.
1.
2.
Free fall – motion of an object
when gravity is the only force
acting on it (no air resistance)
All objects near earth
accelerate at 9.8 m/s2 in the
absence of air resistance
The 1000-kg baby elephant obviously has more mass
(or inertia). This increased mass has an inverse affect
upon the elephant's acceleration. And thus, the direct
affect of greater force on the 1000-kg elephant is offset
by the inverse affect of the greater mass of the 1000kg elephant; and so each object accelerates at the
same rate - approximately 10 m/s/s. The ratio of
force to mass (Fnet/m) is the same for the elephant
and the mouse under situations involving free fall

What would happen if air
resistance was present.
when air resistance balances weight and the
object stops accelerating and reaches its
maximum constant velocity= 320 km/h (200
mi/h)
• Net force is 0= acceleration is 0 =
constant velocity.

Weight – measure of the force of gravity on
an object ‘s mass (changes as gravity
changes)
Weight = mass x gravity(9.8 m/s2)
w = mg
Unit: Newton
g = 9.8 m/s2 near earth
1 N = 0.225 lb; 1 lb = 4.448 N

Mass = does not change EVER
Weight = changes as gravity
changes= Will change when
you go to another planet.
1.
A 10 kg mass would weigh ____
1.
A 50 kg mass would weigh ____
490 N
1.
A 100 kg mass would weigh ___
980 N
1.
An astronaut that weighs 600 N on
Earth is standing on an asteroid
with a gravitational force one
hundredth that of Earth. What is
his weight on the asteroid?
98 Newtons
6 Newtons
DO NOT COPY CHECKING YOUR UNDERSTANDING
Which of the following statements are true of the quantity mass?
List all that apply.
a) The mass of an object is dependent upon the value of the
acceleration of gravity.
b) The standard metric unit of mass is the kilogram.
c) Mass depends on how much stuff is present in an object.
d) The mass of an object is variable and dependent upon its location.
e) An object would have more mass on Mount Everest than the same
object in the middle of Lake Michigan.
f) People in Weight Watcher's are really concerned about their mass
(they're mass watchers).
g) The mass of an object can be measured in pounds.
h) If all other variables are equal, then an object with a greater mass
would have a more difficult time accelerating.
i) The mass of an object is mathematically related to the weight of
the object.
ANSWER: B, C, F, H, I

Newton’s First Law- states an object
in rest will remain at rest, or an
object in motion will remain in
motion unless a force(unbalanced
force) acts upon it.
 Also known as the Law of Inertia
What will happen
when the car
hits the wall?
1.
2.
While riding a skateboard you fly forward off
the board when hitting a curb or rock or other
object which abruptly halts the motion of the
skateboard.
Headrests are placed in cars to prevent
whiplash injuries during rear-end collisions
Do Not Copy
Other Real life examples:
1. Blood rushes from your head to your feet while quickly
stopping when riding on a descending elevator.
2. The head of a hammer can be tightened onto the wooden
handle by banging the bottom of the handle against a
hard surface.
3. A brick is painlessly broken over the hand of a physics
teacher by slamming it with a hammer. (CAUTION: do not
attempt this at home!)
4. 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.
1.
2.
Inertia - the tendency of an object to
resist a change in its motion
Inertia depends Only on the mass of an
object
 More mass = more inertia
 Example: Which has more inertia
A. A car moving 50mi/h
B. A buss full of students
C. An tiger sitting on a big rock
DO NOT COPYCHECKING YOUR UNDERSTANDING
1. 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 at this speed
and in this direction?
Answer: 0 N
An object in motion will maintain its state of motion. The presence of
an unbalanced force changes the velocity of the object.
2. Mac and Tosh are arguing in the cafeteria. Mac says that if he
flings the Jell-O with a greater speed it will have a greater inertia.
Tosh argues that inertia does not depend upon speed, but rather
upon mass. Who do you agree with? Explain why.
Tosh is correct. Inertia is that quantity which depends solely upon
mass. The more mass, the more inertia.
3. Supposing you were in space in a weightless environment, would it
require a force to set an object in motion?
Absolutely yes!
Even in space objects have mass. And if they have mass, they have
inertia. That is, an object in space resists changes in its state of
motion. A force must be applied to set a stationary object in motion.
Newton's laws rule - everywhere!
1.
The unbalanced force acting
on an object equals the
object’s mass times its
acceleration
F
m a
F = ma
F
m a
a=acceleration (m/s2)
 m-mass (kg)
 F= force (N) newtons


1N = 1kg x m/s2
1.
Determine the accelerations which result
when a 12-N net force is applied to a 3-kg
object and then to a 6-kg object
A 3-kg object experiences an acceleration of 4 m/s/s.
A 6-kg object experiences an acceleration of 2 m/s/s.
2. An automobile with a mass of 1000 kilograms
accelerates when the traffic light turns green.
If the net force on the car is 4000 newtons,
what is the car’s acceleration?
4000N/1000kg=4m/s2
1.
2.
For every action
force, there is an
equal and
opposite
reaction force.
Forces occur in
pairs
4.
The action force and
the reaction force
occur to different
objects so the force is
not balanced.
(movement occurs)
DO NOT
COPY
1.
2.
You push on wall & wall pushes on you
Swimming = you push on water and
water pushes on you
3.
The hammer exerts a force on the nail
to the right. The nail exerts an equal but
opposite force on the hammer to the
left.
4.
The rocket exerts a downward force on
the exhaust gases.
5.
The gases exert an equal but opposite
upward force on the rocket.
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