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J. McLeod
What is a force?
Force (N) = a push or a pull
Forces can cause
a resting object to move
a change in velocity
a change in direction
Net Force = total force acting on an object.
Balanced Forces = No movement
When the forces on an object are balanced, the net
force is zero.
Unbalanced Forces = Movement in the
direction of the larger force
When an unbalanced force acts on an object, the
object accelerates.
Frictional Force (friction)
A force that opposes the motion of objects.
Magnified
view of a
smooth
surface.
There are four main types of friction:
Static friction: force acting on stationary
objects.
One must overcome this force to move an object
Sliding friction: ice skating
Rolling friction: bowling
Fluid friction (air or liquid): air or water
resistance
Gravity
An attractive force between any two masses.
Force of gravity increases as objects get closer
to one another and the more mass and object
has.
Who experiences more gravity - the flag or the
astronaut?
Which exerts more gravity - the Earth or the
moon?
less
distance
more
mass
Is the following statement true or false
An astronaut has less mass on the moon since the
moon exerts a weaker gravitational force.
False! Mass does not depend on gravity,
weight does. The astronaut has less weight on
the moon.
What is the difference between weight and mass??
Weight is dependent upon gravity.
Because gravity is a force and weight is
dependent upon gravity, then weight is also a
force.
Fg = mg
Fg =
m=
g=
Look on your reference tables
and write what each symbol
stands for and the value for g
in the blank provided.
m
Fg
÷
g
1. What is the force on a 1-kg ball that is falling
freely due to the pull of gravity?
2. A barbell weighs 850 N on Earth. What is its mass
in kg?
3. What is the weight of a 65 kg object?
4. What is the mass of an object that weighs 500 N?
5. If you drop a 20 kg object, what is its weight?
6. What is the mass of an object that weighs 120 N?
7. What is the force needed to accelerate a 4.8kg
object?
Falling without air resistance
In the absence of air resistance, all
falling objects have the same
acceleration! 9.8m/s2
Will the elephant or feather hit the
ground first???
Air Resistance
If an object falls for a long period, the upward
force of air resistance becomes equal to the
downward force of gravity.
When force of air resistance = force of gravity,
this is called terminal velocity.
Fair = Fgravity
Falling with Air Resistance
Heavier objects fall faster because
they accelerate to higher speeds
before reaching terminal velocity
Fgrav = Fair
Now… will the elephant or feather hit the
ground first???
1. With what force will a ball hit the
ground if it has a mass of 350kg?
3430 N
2. What is the mass of a falling rock if it
hits the ground with a force of 147
Newtons?
15 kg
“The Law of Inertia”
The tendency of an object to resist a change in
motion.
If an object is stationary, it will remain stationary
(zero velocity).
If an object is moving at constant velocity, it will
keep moving at a constant velocity.
All objects have inertia.
The more mass an object has, the more inertia
it has (and the harder it is to change its motion).
Which is harder to change it’s motion….
F150 or a mini-cooper?
These pumpkins
will not move
unless an
unbalanced force
acts on them.
Because of inertia, objects (including you) resist
changes in their motion. When the car going
60mph is stopped by the brick wall, your body
keeps moving at 60 mph.
Don’t let this be you. Wear seat belts!!!!
When you stop suddenly,
like in a collision, (think
football hit) the skull
stops, but the brain keeps
going – crashing into the
skull.
Causing a bruise to the
brain (a concussion).
The net force of an object is equal to
mass x acceleration.
Mass (kg)
Acceleration (m/s2)
Force = Newtons (N) = kg·m/s2
m
F
÷
a
Newton’s 2nd Law proves that different masses
accelerate to the earth at the same rate, but
with different forces.
Very massive objects that
accelerate at a low rate,
like a glacier, have great
force.
Objects with low masses
that accelerate at a high
rate, like a bullet, have a
great force.
Example 1:
How much force is needed to accelerate a
1400kg car 2 m/s2?
F= ?
m = 1400 kg
a = 2m/s2
F = ma
= (1400kg x 2m/s2)
= 2800kgm/s2
Example 2:
A 4.0 kg shot-put is thrown with 30 N of force.
What is its acceleration?
F = 30 N
m = 4.0 kg
a = ? 7.5 m/s2
Example 3:
What force would be required to accelerate a
40 kg mass by 4 m/s2?
F = ? 160 N
m = 40 kg
a = 4 m/s2
4. What is the acceleration of a 1,500 kg truck if acted upon by a
net force of 7,500 N? 5m/s2
5. A 60.0 kg runner accelerates at 2.2 m/s2. What is the net force
on the runner? 132 N
6. Find the mass of a flying disc that has a net force of 1.05 N and
accelerates at 3.5 m/s2. 0.3kg
7. While playing baseball, Jim swung at a 150 kg baseball and
accelerated it at a rate of 30,000 m/s2. What was the net force
applied to the baseball? 4,500,000 N
8. What is the net force required to accelerate a 0.030 kg arrow at
1720 m/s2? 51.6 N
9. What is the mass of Butch, the football quarterback, if a net
force of 1440 N causes him to accelerate at 20.0 m/s2? 72 kg
10. Claudia stubs her toe on the coffee table experiencing a net55.56m/s2
force of 100 N. What is the acceleration of her 1.80 kg foot?
Force (N)
30
20
10
0
5
10 15 20 25
acceleration (m/s2)
30
What is the mass of the object being
accelerated?
For every action there is an equal & opposite
reaction.
Sliding friction
Rolling friction
Fluid friction
What happens if you are standing on a
skateboard or a slippery floor and push against a
wall?
You slide in the opposite direction (away from the
wall), because you pushed on the wall but the wall
pushed back on you with equal and opposite force.
Why does it hurt so much when you stub your
toe?
When your toe exerts a force on a rock, the rock
exerts an equal force back on your toe. The harder
you hit your toe against it, the more force the rock
exerts back on your toe (and the more your toe
hurts).
Action: tire pushes on road
Reaction: road pushes on tire
Action: rocket pushes on gas
Reaction: gas pushes on rocket
Momentum = mass x velocity
Mass = kg
Velocity = m/s
Momentum = kg·m/s
A 1 kg object with a speed of 100m/s & a 100kg
object with a speed of 1m/s have the same
momentum.
m
p
÷
v
Example 1:
Find the momentum of a bumper car if it has a
total mass of 280 kg and a velocity of 3.2 m/s.
p=?
m = 280 kg
v = 3.2 m/s
Example 2:
The momentum of a second bumper car is 675
kg·m/s. What is its velocity if its total mass is
300 kg?
p = 675 kgm/s
m = 300 kg
v=?
3. What is the momentum of a 700 kg bird flying at a
velocity of 2.5 m/s
4. What is the momentum of a 1000 kg car traveling at 5
m/s
5. What is the momentum of a 40 kg shopping cart
rolling along at 0.5 m/s
6. What is the momentum of a 300 kg roller coaster car
traveling at 625 m/s
7. A marble rolling at a velocity 10m/sec has a
momentum of a 5,000 kgm/sec. What is its mass?
8. An object whose mass is 3 kg is fired from a cannon,
giving it a forward momentum of 1050 kg-m/sec.
What is its velocity?
1.What acceleration will result when a 12-N
net force applied to a 3-kg object? A 6-kg
object?
2.A net force of 16 N causes a mass to
accelerate at a rate of 5 m/s2. Determine
the mass.
3.How much force is needed to accelerate a
66-kg skier 1 m/s2?
4.What is the force on a 1000-kg elevator
that is falling freely at 9.8 m/s2.?
1. Of which type of friction is the following an
example? A dolphin swimming through the
ocean.
2. What is the value for acceleration due to
gravity?
1. 2 examples of a physical change
2. 2 examples of a chemical change
3. On the periodic table where are the metals
located?
4. On the periodic table where are the
nonmetals located?
5. On the periodic table where are the
following located? alkali metals, alkaline
earth metals, transition metals, halogens,
noble gases
1. How much does a 1,000kg weight weigh? Use
your reference sheets to determine which
formula you should use.
2. If a cylinder rod moves at 3m/s and has a mass
of 50kg, what is its momentum?
3. What type of friction exists in liquid water? Is
this friction higher in water or syrup?
1. What is the weight of someone who has a mass
of 300kg?
2. What is the momentum if a 34kg object is
moving at a velocity of 3.5m/s?
3. What is the acceleration of object that has a
mass of 8kg and is thrown with a force of
55kgm/s2?
4. What is the velocity of an object that has an
acceleration of 5m/s2 in a time of 23.4seconds?
1. Which type of friction is exerted between the tires of
a car and the road?
2. What is the distance covered by a car traveling at a
velocity of 50mph north for 5 hours?
3. A truck was traveling at 55km/hr and slows down to
37 km/hr in 0.2hr. What is the truck’s acceleration?
4. Identify which law applies to the following situation.
While running down the hall, you miss the corner, and
hit the wall and get a concussion.
5. What is the mass of a rock that has an acceleration of
2.3 m/s2 after a force of 2300 Newtons has hit it?
1. What force is necessary to accelerate a 6 kg
bowling ball at 2.1 m/s2?
2. A force of 250 N is applied to a 1 kg softball
when struck with a bat. What is the
acceleration?