Physical Science: Ch 12 Notes
In general, what will cause your car to move?
A. Forces
Force – a push or a pull that acts on an object
What can a force do?
A force can cause a resting object to move, or it can accelerate a
moving object (by changing its speed or direction)
Forces in the SAME direction ADD together and forces in
OPPOSITE directions SUBTRACT from one another.
Net force – the overall force acting on an object after all the forces
are combined
What happens when forces are all equal and balanced?
When the net force is zero there is NO CHANGE in the object’s
motion (like a tug-of-war between teams of equal strength)
When an UNBALANCED force acts on an object, the object
accelerates!
Physical Science: Ch 12 Notes
How are forces drawn and measured?
Arrows are used to represent the direction and strength of a force.
The arrow points the same direction as the force and the relative
length of the arrow represents the strength, or magnitude, of the
force.
Forces are measured in Newtons! One Newton = the force that
causes a 1-kilogram mass to accelerate at a rate of 1 m/s/s
B. Newton’s First Law
1st Law: The state of motion of an object does NOT change as long
as the net force acting on the object is zero.
Unless an unbalanced force acts:
1. An object at rest remains at rest
2. An object in motion remains in motion with the same
speed and direction
Inertia – the tendency of an object to resist a change in its motion
The greater the inertia of an object, the harder it is to change its motion.
So, what characteristics would give objects a lot of inertia?
Physical Science: Ch 12 Notes
What influences how quickly an object accelerates?
C. Newton’s Second Law
2nd Law: The acceleration of an object is equal to the net force acting on it
divided by the object’s mass.
a = F/m
F=ma m=F/a
Practice Problems:
How much force is needed to accelerate a 5,250 kilogram truck at a rate of
1.5 m/s/s?
How much force is needed to accelerate a 60 kilogram skater 5.3 m/s/s?
Julie’s mass is 60 kilograms and as she dives into a swimming pool her
acceleration is 8.14 m/s/s. What is her force as she hits the water?
With a force of 12.6 Newtons, Heath hit the 0.43 kilogram baseball. What
was the acceleration of the baseball?
Bob pushed his motorcycle with a force of 297.2 Newtons and it
accelerated at a rate of 3.1 m/s/s. If the friction force was 12.2 N, what
is the motorcycle’s mass? (hint: find the net force first, then use m=F/a)
Physical Science: Ch 12 Notes
How did your balloon-powered cars move? Why?
D. Newton’s Third Law
3rd Law: Whenever one object exerts a force on a second object, the
second object exerts an equal and opposite force on the first object.
These two forces are called Action and Reaction forces.
Action and Reaction forces do NOT cancel each other because they
do not act on the same object.
Example: Balloon-powered car
Example: Stubbing your toe
Example: Jellyfish
Not all Action and Reaction forces produce motion (ex: your hand
pushing against a wall)
Physical Science: Ch 12 Notes
Why did your cars stay on the floor as they moved?
E. Gravity
Gravity – a force that acts between any two masses
Gravity is an attractive force, that is, it pulls objects together. Every object
in the universe attracts every other object! Unlike friction, gravity can act
over large distances.
The greater the mass of the objects, the greater the gravitational force
Ex: The gravity force between you and your textbook vs. the force
between the earth and you
Ex: The gravity force between the sun and Earth
The closer two objects are, the greater their gravitational attraction
Ex: Our moon vs. ex-planet Pluto
Earth’s gravity acts downward toward the center of earth. An upward
force usually balances the downward force of gravity.
Gravity causes objects to accelerate downward, but air resistance
acts in the direction opposite to motion and reduces acceleration. Air
resistance increases with velocity and surface area.
Physical Science: Ch 12 Notes
Why is outer space called “weightless” but never “massless”???
F. Weight and Mass
Weight – the force of gravity acting on an object
An object’s weight is the product of the object’s mass (in kilograms) and
acceleration due to gravity acting on it …
because…WEIGHT IS A FORCE!
Weight = mass x acceleration due to gravity
W = mg
On earth: g = 9.8 m/s/s
So, weight is measured in NEWTONS and is different from mass!
Practice Problems: What is the weight of a 8 kilogram bowling ball?
What is the weight of a 8 kilogram bowling ball on earth?
What is the weight of a 8 kilogram bowling ball on the moon?
Physical Science: Ch 12 Notes
Why did some cars continue to travel even after their balloon deflated?
G. Momentum
Momentum – the product of an object’s mass and its velocity
Momentum = mass x velocity
units: kg x m/s
An object with large momentum is hard to stop!
The momentum for any object at rest is ZERO
Practice Problems:
1. Which has more momentum: a 0.032 kg golf ball traveling 45 m/s
or a 5.5 kg bowling ball traveling 4.5 m/s?
2. Which has more momentum: a 0.012 kg bullet traveling 30 m/s or
a 0.321 kg baseball rolling 3 m/s?
Law of Conservation of Momentum – if no outside force acts on a
system then the total momentum of the system does not change
Momentum can be transferred between objects but is not destroyed.
Physical Science: Ch 12 Notes
Why did Miss Hinkhouse have wheels as a required part of the balloonpowered cars?
H. Friction
Friction – a force that opposes the motion of objects that touch as
they move past each other
Friction acts at the surface where objects are in contact
Note that “in contact” includes solid objects that are directly
touching one another as well as objects moving through a
liquid or a gas
Sliding friction – a force that opposes the direction of motion of an
object as it slides over a surface
Rolling friction – the friction force that acts on rolling objects
Fluid friction – opposes the motion of an object through a fluid
(liquid or gas)
How do the effects of sliding, rolling, and fluid friction compare?