Download Forces and Motion - Cortez High School

Physics: Forces and
Motion
Speed
 Speed: the distance traveled by
an object during a given time
 Average Speed = Distance
Time
S = D
T
Example
 A cheetah ran 4 km in 2 minutes. What was the
cheetah’s average speed?
 S=D
T
 S = 4 km
2 min
 S = 2 km/min
* Don’t
forget units! *
Finding Distance and Time
 What if you are given the speed but have to
find the distance or the time?
 Original formula:

S=D
T

D=S X T

T=D
S
Examples
 If a coyote can run at 43 mph, how far
can one go in 30 min?

Make sure your units match up!
D=S X T
D = 43 mph X 0.25 hrs
D = 10.75 mi
Examples cont.
 If a black mamba can slither at 20
mph, how long would it take to go 1
mile?

Make sure your units match up!
T=D
S
T = 1 mi
20 mph
T = 0.05 hr
What would your answer be in
minutes?
0.05 hours X 60 min = 3 min
Velocity
 Velocity: Speed that has a Direction

A car that is moving 40 mph West

A person that is walking 3 mph
Northeast

A ball that was thrown 2 mph “up”
Acceleration
 Acceleration: the rate at which
velocity (speed) changes
 Can be changes in speed
 Bus

moving down street
Can be changes in direction
 Staying
same speed around a
curve on a bike

Can be changes in both speed
and direction.
 Roller
coaster
Calculation Accleration
 Acceleration= change in velocity
total time
OR
a= (vfinal-vintial)
t
Force
 Force: a push or pull that acts on an
object


Causes resting objects to move
Causes moving objects to accelerate
 Measurement: Newton (N)
 Representation: Arrow
Balancing Forces
 Balanced Force: no movement
 Unbalanced Force: object(s) forced to
accelerate
Friction
 Friction: force slows down an object
as the object moves past

Without friction: walking on ice

Static Friction: force acting on
object at rest

Sliding Friction: opposes direction of
object in motion
Gravity
 Gravity: Force that acts downward
toward the center of the Earth


Does not require contact to act
Upward force usually balances the
force of gravity
 Falling Objects: Gravity causes
objects to accelerate downward

Air resistance is the balancing force
Terminal Velocity
 Terminal Velocity: velocity of object
when air resistance equals force of
gravity
Newton’s Laws of Motion
 Isaac Newton: Important in the study
of mass, force and motion
 Newton’s 1st Law of Motion: the
motion of an object does not change
as long as the net force is zero


Object won’t move unless something
moves it!!
Object won’t stop moving unless
something stops it
Newton’s Laws of Motion
 Newton’s 1st Law of Motion
a.k.a Law of Inertia:
 Soccer ball won’t move until someone
kicks it
 Soccer ball will keep moving after
kicked until something stops it

Newton’s Laws of Motion
 Newton’s 2nd Law of Motion



Force of an object is equal to the mass
times the acceleration of an object
F =ma
What is the force of an object with an
acceleration of 12 m/hr2 and a mass 1000
kg?
Newton’s Laws of Motion
 Newton’s 2nd Law of Motion


Also applies with a force that acts in the
opposite direction
In a collision, the seatbelt applies a
force that opposes a person’s forward
motion
Newton’s Laws of Motion
 Newton’s 3rd Law of Motion:
Whenever an object exerts a force on
a second object, the second object
exerts an equal and opposite force on
the first object


For every action there is an equal and
opposite reaction
Called ACTION AND REACTION forces
Newton’s Laws of Motion
 Action Reaction Forces


Hand pushing on wall: action force
Wall pushing on hand: reaction force
 Doesn’t mean an object won’t move


If the action reaction forces are on the
same object…no movement
If the action reaction forces are on
different objects….movement can occur
Momentum
 Momentum: Inertia in motion. Momentum is
determined by multiplying the mass and
velocity of an object.
 The larger the mass, the more momentum
 The larger the velocity, the more momentum
 Momentum= Mass x Velocity

p=mv
Momentum
 Two cars collide. Car A has a mass of
5000 kg and was traveling 35 km/hr
west.
 Car B has a mass of 1000 kg and
was traveling 35 km/hr east.
 After the collision, what direction did
both cars travel
Collisions - Elastic
 Elastic – a collision in which the total
kinetic energy of the objects before and
after collision is equal
 Two objects collide and bounce off one
another.
Elastic Collisions
Collisions - Inelastic
 Inelastic - a collision in which kinetic
energy is not conserved. (the kinetic
energy before and after the collision is
NOT equal)
 Two objects collide and stick together
and move with the same velocity
Inelastic Collisions
What kind of collision is
this?
What kind of collision is this?
What kind of collision is this?
What kind of collision is this?
What kind of collision is this?
Sources
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
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