Download Motion and Forces

Motion and Forces
Physical Science
Unit 6
Motion
 Everyday life motion is so common, it
seems to appear very simple.
 However, understanding motion
requires some new and advanced
ideas.
 How do we know when an object is
moving?????
Motion
 Motion – change in position over a
certain time
 Distance,
time and direction
 Frame of reference
Energy
 Potential energy – stored energy
1. Gravitational – any object raised above
the surface of the Earth
 … such as the energy in a stretch or
compressed spring
 … such as stored in chemical bonds, food,
fuel, batteries
2.
Kinetic energy – energy of moving
objects or energy in motion
Speed and Velocity
 An object is moving if its position
changes but the background stays the
same.
 Horse
galloping but the trees remain
stationary
 Person walking but the road remains in
place
Speed and velocity cont.
 Stationary background = reference
frame
 This
can be used to measure the change in
position of the moving object
Speed
 Speed describes how fast an object
moves
 Which
moves faster?
• Flying eagle or galloping horse
• Speeding race car or flying jet
Speed cont.
 The speed depends on the distance
traveled and the time it took to travel
that distance.
 This is a scalar quantity in which only
the magnitude is being measured.
 Quantities
that can be described by a
single number are called scalars.
Speed
 To determine speed you must have
 Distance traveled by an object
 Time it takes to travel that distance
 SI unit for speed is meters per second
(m/s)
Speed cont.
 Speed equation:
 Speed = distance
time
Or
s=d
t
Units of speed
 Units:
1. Distance measured in meters,
kilometers, or centimeters
2. Time measured in seconds, minutes,
or hours
3. Therefore speed is in meters per
second (m/s) or kilometers per hour
km/hr
Types of speed
1. Instantaneous – speed at that exact
moment
2. Constant – does not change through
the time
3. Average – total distance over time
Practice problems
Wheelchair racer finishes a 132 m race in
18 s. What is the speed?
s = d = 132 m = 7.3 m/s
t
18 s
A person jogs 500 meters in 360 seconds.
Calculate the speed.
A person jogs 500 meters in 360 seconds.
Calculate the speed.
s = d = 500 m
t
360 s
= 1.39 m/s
A person walks 4 miles in 2 hours, then
stops for an hour for lunch. After lunch
they walk 8 miles in 3 hours. Calculate
the person’s average speed.
Distance Time
A person walks 4 miles
in 2 hours, then
stops for an hour for
lunch. After lunch
they walk 8 miles in
3 hours. Calculate
the person’s
average speed.
4 miles
2 hrs
0 miles
1 hr
8 miles
3 hrs
Total 12 miles 6 hrs
s = d/t
= 12 miles/6 hrs
= 2 mph
Velocity
 Velocity – quantity describing both
speed and direction.
 Speed
in a given direction or displacement
divided by time
 Needs a direction, third dimension, north,
east, south, west
 Vector quantity – uses both magnitude and
direction
Speed vs. velocity
Speed is a scalar
Velocity is a vector
Speed has no direction; scalars have
no direction
Velocity has direction; vectors have
direction
Speed or velocity?
1. Person walks 4 m/s
2. Person walks 2 m/s north
3. Car drives 60 mph toward Biloxi
4. Car drives 30 mph
5. Boat is pulled by a 53 newton force
Speed or velocity?
1. Person walks 4 m/s - - - speed
2. Person walks 2 m/s north - - velocity
3. Car drives 60 mph toward Biloxi - velocity
4. Car drives 30 mph - - - speed
5. Boat is pulled by a 53 newton force - - velocity
Acceleration
 Acceleration – rate of change in
velocity

Means of acceleration
1. Decrease speed = negative
acceleration or deceleration
2. Increase speed
3. Change the direction
Acceleration formula
a = V final – V initial = Δv
time
t
Acceleration examples
A plane starts at rest and ends up going
200 m/s in 10 secs. Calculate the
acceleration.
A = V final – V initial = 200 – 0 m/s = 20 m/s2
time
10 s
Positive answer = acceleration
A race car starts at 400 m/s and then
stops in 20 seconds. Calculate the
car’s acceleration.
A = V final – V initial = 0-400 m/s = - 20 m/s2
time
20 s
Negative answer = deceleration
Force
 Force – a push or a pull that one body
applies to another
1. A force can cause an objects motion
to change
2. When two or more forces combine…
they create a net force
Force cont.
3.
4.
Balanced forces are equal in
size and opposite in direction.
(They do not cause motion)
Unbalanced forces are unequal
in size and/or are in the same
direction. (They will cause an
object to move!)
Newton’s 1st Law of Motion
“An object moving at a
constant velocity keeps
moving at that velocity
unless a net force acts on it.”
Newton’s 1st Law of Motion
What does that mean?????
An object in motion stays in motion
unless acted on by an outside
force… same for an object at rest…
Newton’s 1st Law
 This is called inertia – an objects
resistance to change its motion.
1.
2.
3.
Inertia is a property of matter.
The amount of inertia that an objects has
depends on its mass. The more mass an
object has, the harder it is to change its
motion.
Newton’s First Law of Motion is also
called the Law of Inertia.
Example of Inertia
In an automobile crash a person not
wearing a seat belt will keep moving
forward at the car’s speed even after
the car has stopped.
Newton’s 2nd Law of Motion
“A net force acting on an
object will accelerate in the
direction of the force.”
Newton’s 2nd Law of Motion
In other words, force and motion are related:
1. The rate of acceleration is determined by
the size of the force and the mass of the
object.
2. An object will have greater acceleration if a
greater force is applied to it.
3. An object with less mass will accelerate
faster.
Example of Newton’s 2nd Law
A baseball and a bowling ball are both hit
with the same bat and the same force.
The baseball will have a greater
acceleration because it has less mass.
F=ma
All of this is represented mathematically in the
equation: F=ma
Where:
F = force (N)
 m = mass (kg)
 a = acceleration (m/s2)

“Newton” (N) is a unit of force.
It is equal to 1 kg ● m/s2
F=ma example
Zookeepers lift a stretcher with a sedated
lion. To total mass of the stretcher and
lion is 175 kg, and the upward
acceleration is 0.657 m/s2. What is the
force needed to move the lion?
F = ma = 175 kg x 0.657 m/s2
= 115 kg ● m/s2 = 115 N
Newton’s 3rd Law of Motion
“For every action there is an
equal and opposite reaction.”
Newton’s 3rd Law of Motion
Example:
When you throw a basketball with a force
of 3 newtons, the basketball exerts and
equal force back on you. You would
see it if you were wearing roller skates!
More information you need to know
about forces!
Friction: A force between two objects that are
touching each other.
The amount of friction depends on two things:
1. The kind of surface the objects have – how
smooth or rough
2. The force that is pressing the objects
together – the harder they are pressed
together the more friction there will be.
Air resistance
 Air resistance – an upward force that air
exerts on an object that is falling
through it. A force that works against
gravity’s downward force. The amount
of air resistance depends on the shape,
size, and speed of the falling object.
Bernoullis principle
 In a fluid:
High velocity = low pressure
Momentum
 Momentum – a property of a moving
object because of its mass and velocity.
It decides how much force is needed to
change an object’s motion.
Momentum cont.
Equation: p=mv
= momentum (kg ●m/s)
 m = mass (kg)
 v = velocity (m/s)
p
Example
A 75 kg speed skater moving forward at
16 m/s. Calculate the momentum.
p = mv
P = 75 kg ● 16 m/s
= 1200 kg●m/s
A 135 kg ostrich running north at 16.2
m/s. Calculate the momentum.
A 135 kg ostrich running north at 16.2
m/s. Calculate the momentum.
P = mv = 135 kg ● 16.2 m/s
= 2187 k●m/s
A 0.8 kg kitten running to the left at 6.5
m/s. Calculate the momentum.
A 0.8 kg kitten running to the left at 6.5
m/s. Calculate the momentum.
P = mv = 0.8 kg ● 6.5 m/s
= 5.2 kg●m/s
Momentum cont.
Simple terms:
The more mass an object has and the
faster it is moving, the harder it is to
change its motion. An object at rest has
no momentum!
Example: A toy truck is easier to stop than
a real truck because it has less mass,
therefore less momentum.
The Law of Conservation of Momentum
Momentum can be transferred
between objects: momentum is
neither created nor destroyed in
the transfer.
Gravity
Gravity:
1. Is the force that every object exerts on
every other object.
2. The amount of gravitational force
depends on 2 things:
a.
b.
The mass of the object
The distance between the objects.
Gravity cont.
3. Weight is a measure of the force of
gravity on an object.

W = ma, where “a” is acceleration due to
gravity (9.81 m/s2 on Earth)
4. All objects fall towards Earth the same
acceleration regardless of their mass:
9.81 m/s2. This is why if there were no
air resistance two falling objects would
hit the ground at the exact same time.
Free fall
 Free fall – the motion of a body when
only the force of gravity is acting on it.
 Free
fall acceleration results from gravity, it
is abbreviated as g.
 Near the Earth’s surface, g = 9.8 m/s2
Free Fall and Weight
 The force on an object due to gravity is
called its weight.
 On Earth, your weight is the amount of
gravitational force exerted on you by
Earth.
Free fall and Weight
 If you know the free-fall acceleration, g,
acting on a body then you can use
F=ma to calculate the body weight.
 Weight equals mass times free=fall
acceleration.
Weight = mass x free-fall acceleration
w = mg
Example
 A small apple weighs about 1 N.
 A 1.0 kg book has a weight of
1.0 kg x 9.8 m/s2 = 9.8 N
Terminal velocity
 Terminal velocity – the maximum
velocity reached by a falling object that
occurs when the resistance of the
medium is equal to the force due to
gravity.
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