Transcript Force & Motion

Warm-Up: Energy Review

Answer the questions below.
3.
1.
2.
Where does the car have the
most potential energy?
1.
2.
3.
4.
Between A and B
Between B and C
Between C and D
After D
Where does the car have more
kinetic energy than potential?
1.
2.
3.
4.
A
B
C
D
Force &
Motion
Motion

Speed = rate at which an object
moves
Motion

Speed = rate at which an object moves
speed equation: speed = distance ÷ time
(v = d ÷ t)
units of speed:
meters per second (m/s) or
kilometers per hour (km/hr)
Example: A jogger runs the first 1000 m of a race in
250 sec. What is the jogger’s speed?
What is the correct way to plug in the numbers?
1.
2.
3.
4.
v
v
v
v
=
=
=
=
250 sec ÷ 1000 m
1000 m ÷ 250 sec
1000 m x 250 sec
250 sec x 1000 m
Motion

Speed = rate at which an object moves
speed = distance ÷ time (v = d ÷ t)
Example:
A jogger runs the first 1000 m of a race in
250 sec. What is the jogger’s speed?
v = 1000 m ÷ 250 sec.
Example: A jogger runs the first 1000 m of a
race in 250 sec. What is the jogger’s speed?
1.
2.
3.
4.
0.25 m/sec
0.25 sec/m
4 m/sec
4 sec/m
Motion

Speed = rate at which an object moves
speed = distance ÷ time (v = d ÷ t)
Example:
A jogger runs the first 1000 m of a race in
250 sec. What is the jogger’s speed?
v = 1000 m ÷ 250 sec.
v = 4 m/sec.
Motion


Speed = rate at which an object moves
Velocity = speed with a specific
direction
Example: A running back moves towards the east
end zone at 10 m/sec. What is the running back’s
velocity?
1.
2.
3.
4.
10
10
10
10
m/sec
m/sec west
m/sec east
meters east
Motion

Speed = rate at which an object moves

Velocity = speed with a specific direction
Example:
A running back moves towards the east end zone at
10 m/sec.
– What is the running back’s velocity?
velocity = 10 m/sec. east
Motion

Acceleration = change in velocity
Motion

Acceleration = change in velocity
– Acceleration can mean:

Speeding up
Motion

Acceleration = change in velocity
– Acceleration can mean:

Speeding up

Slowing down
Motion

Acceleration = change in velocity
– Acceleration can mean:

Speeding up

Slowing down

Changing direction
– Think of a rollercoaster because it can
do all three!
Which car is not
accelerating?
1.
2.
3.
4.
A car driving up a hill and
down the other side.
A car turning a corner.
A car turning a corner at a
constant speed.
A car driving at a constant
speed along a straight
highway.
Which is NOT an example
of acceleration?
1.
2.
3.
4.
As you see your friend,
you jump up and run to
greet them.
Your dog runs in circles
chasing his tail.
You pedal your bike
along your street at 5
km/hr.
A car slows down as it
comes to a red light.
Motion

Acceleration = change in velocity
acceleration equation:
acceleration = velocity ÷ time (a = v ÷ t)
units of acceleration:
m/s per second (m/s2)
or
km/hr per hour (km/hr2)
Example: A car accelerates from 0 to 72 km/hr in 8 sec.
What is the car’s acceleration?
What is the correct way to plug in the numbers?
1.
2.
3.
4.
a
a
a
a
=
=
=
=
72 km ÷ 8 sec
0 km/hr ÷ 8 sec
72 km/hr ÷ 8 sec
8 sec ÷ 72 km/hr
Motion

Acceleration = change in velocity
acceleration = velocity ÷ time (a = v ÷ t)
Example:
A car accelerates from 0 to 72 km/hr in 8
sec. What is the car’s acceleration?
a = 72 km/hr ÷ 8 sec.
Example: A car accelerates from 0 to 72 km/hr
in 8 sec. What is the car’s acceleration?
1.
2.
3.
4.
9
9
9
9
km/hr
km/hr/sec
km/hr/hr
km
Motion

Acceleration = change in velocity
acceleration = velocity ÷ time (a = v ÷ t)
Example:
A car accelerates from 0 to 72 km/hr in 8
sec. What is the car’s acceleration?
a = 72 km/hr ÷ 8 sec.
a = 9 km/hr/s
Force

Force = a push or a pull
Force

Force = a push or a pull
– forces have a strength and direction
– Forces are measured in units called
Newtons (N)
1N
Small force pushing
to the right
20 N
Big force
pushing to the left
Force

Balanced forces = equal forces
working on 1 object in opposite
directions
Total force on rope = 0 N
100 N
100 N
Rope’s motion does not change (it is in equilibrium)
Force

Unbalanced forces = unequal
forces cause an object to accelerate
Total force on rope = 20 N to the right
100 N
120 N
Rope’s moves to the right (it is in disequilibrium)
Which box is experiencing
unbalanced forces?
1.
2.
3.
4.
Box
Box
Box
Box
A
B
C
D
5N
A
5N
B
10 N
10 N
10 N
10 N
C
5N
D
10 N
10 N
5N
What will Box D do?
1.
2.
3.
4.
Accelerate to the left
Accelerate to the right
Stay still
Explode
D
5N
10 N
5N
Resistance Forces

Resistance forces = forces that go
against the motion of an object
Which force is acting on the ball
rolling across the counter?
1.
2.
3.
4.
Downward
force of gravity
Backward
force of friction
Upward force
of the counter
All of these forces are
acting on the ball
Resistance Forces

Resistance forces = forces that go
against the motion of an object
– Friction = force exerted when 2 surfaces
rub against each other
Resistance Forces

Resistance forces = forces that go
against the motion of an object
– Friction = force exerted when 2 surfaces
rub against each other

Sliding = solid surfaces slide over each other
motion
friction
Resistance Forces

Resistance forces = forces that go
against the motion of an object
– Friction = force exerted when 2 surfaces
rub against each other
Sliding
 Rolling = object rolls over a solid surface

M
F
Resistance Forces

Resistance forces = forces that go
against the motion of an object
– Friction = force exerted when 2 surfaces
rub against each other
Sliding
 Rolling
 Fluid = object moving
through a fluid (liquid or gas)

Resistance Forces

Resistance forces = forces that go
against the motion of an object
– Friction
Sliding
 Rolling
 Fluid

– Force of friction depends on:
types of surfaces
 how hard the surfaces push together

When might you want to
decrease friction?
1.
2.
3.
4.
When driving on the
highway
When walking across
a wet floor
When snowboarding
in the Olympics
When climbing a
steep hill
How could you increase
friction?
1.
2.
3.
4.
Polishing metal
surfaces
Adding oil to the
gears on a bike
Wearing slick
shoes
Putting sand on
an icy driveway
Resistance Forces

Resistance forces = forces that go
against the motion of an object
– Gravity = force that pulls objects
toward each other
Resistance Forces

Resistance forces = forces that go
against the motion of an object
– Gravity = force that pulls objects toward
each other
– Force of gravity
depends on:
objects’ mass
 distance between
objects

Resistance Forces

Resistance forces = forces that go
against the motion of an object
– Air resistance = a type of fluid friction
that affects falling objects
Air
resistance
gravity
Resistance Forces

Resistance forces = forces that go
against the motion of an object
– Air resistance = a type of fluid friction
that affects falling objects
– Force of air resistance depends on:
Object’s surface area
 Object’s speed

Air
resistance
gravity
When dropped, which object
would experience the MOST
air resistance?
1.
2.
3.
4.
A flat sheet of
paper
A wadded up
sheet of paper
A paperclip
A tennis ball