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Forces
Chapter 10
10.1 The Nature of force
 Force
-a push or pull
-described by its strength and by the direction in which
it acts
-an arrow shows you direction and the arrows length
represents its strength
ex.
20N ,
7N
 Net Force
-combination of all forces acting on an object
- it determines whether an object moves and in what
direction it moves
Unbalanced Forces
-forces that produce a nonzero net force
-changes an objects motion
Balanced Forces
-equal forces acting on one object in opposite
directions
- Net Force = 0
- The Nature of Force
Balanced Forces
 Balanced forces acting on an object do not
change the object’s motion.
- The Nature of Force
Unbalanced Forces
 Unbalanced forces acting on an object result in a net
force and cause a change in the object’s motion.
- The Nature of Force
Combining Forces
 The combination of all forces acting on an
object is called the net force.
10.2 Friction & Gravity
 Friction
-friction acts in a direction opposite to the direction of the motion
-2 Factors that affect the force of friction
- how hard the surfaces push together
-types of surfaces involved
-4 Types of Friction
-Static Friction
-Sliding Friction
-Rolling Friction
-Fluid Friction
Gravity
-Law of Universal Gravitation – forces of gravity acts between all
objects in the universe.
-Factors affecting Gravity
-Mass
-Distance
Gravitation
Gravity
- Friction and Gravity
 Two factors affect the gravitational
attraction between objects: mass and
distance.
- Friction and Gravity
Gravity
 The force of gravity on a person or object
at the surface of a planet is known as
weight.
 Gravity and Motion
-The force of gravity is an unbalanced force,
which causes an object to accelerate
-Air resistance is a fluid friction, that is an
upward force on falling objects.
-Air resistance increases with velocity, as a
falling object speeds up the force of air
resistance increases.
-When velocity and the force of air resistance
are equal the force is balanced.
- Friction and Gravity
Free Fall
 Interpreting
Graphs:
 What variable is on
the horizontal axis?
The vertical axis?
Time is on the
horizontal axis, and
speed is on the vertical
axis.
- Friction and Gravity
Free Fall
 Calculating:
 Calculate the slope of
the graph. What does
the slope tell you about
the object’s motion?
The slope is 9.8. The
speed increases by 9.8
m/s each second.
- Friction and Gravity
Free Fall
 Predicting:
 What will the speed of
the object be at 6
seconds?
58.8 m/s
- Friction and Gravity
Free Fall
 Drawing
Conclusions:
 Suppose another
object of the same
size but with a greater
mass was dropped
instead. How would
the speed values
change?
The speed values would
not change.
- Friction and Gravity
Air Resistance
 Falling objects with a greater surface area
experience more air resistance.
10.3 Newton’s 1st & 2nd Laws
 Newton’s 1st Law of Motion
-An object will remain at rest, and an object
moving at a constant velocity will continue
moving at constant velocity, unless it is
acted upon by an unbalanced force.
- Inertia depends on mass
Newton’s 2nd Law of Motion
-Acceleration depends on the object’s
mass and on the net force acting on the
object.
Acceleration = Net Force / Mass
Mass x Acceleration = Net Force
Newton’s 1st Law of Motion
Newton’s 2nd Law of Motion
- Newton’s First and Second Laws
Calculating Force
 A speedboat pulls a 55-kg water-skier. The force
causes the skier to accelerate at 2.0 m/s2.
Calculate the net force that causes this
acceleration.
 What information have you been given?

Mass of the water-skier (m) = 55 kg

Acceleration of the water-skier (a) = 2.0 m/s2
Calculating Force
A speedboat pulls a 55-kg water-skier. The force
causes the skier to accelerate at 2.0 m/s2. Calculate the
net force that causes this acceleration.
Plan and Solve
What quantity are you trying to calculate?
 The net force (Fnet) = __
What formula contains the given quantities and the
unknown quantity?
 a = Fnet/m or Fnet = m X a
Perform the calculation.
 Fnet = m X a = 55 kg X 2.0 m/s2
 F = 110 kg • m/s2
 F = 110 N
Calculating Force
 A speedboat pulls a 55-kg water-skier. The
force causes the skier to accelerate at 2.0
m/s2. Calculate the net force that causes this
acceleration.
 Look Back and Check
 Does your answer make sense?
 A net force of 110 N is required to accelerate
the water-skier. This may not seem like
enough force, but it does not include the force
of the speedboat's pull that overcomes
friction.
- Newton’s First and Second Laws
Calculating Force
 Practice Problem
 What is the net force on a 1,000-kg
object accelerating at 3 m/s2?
 3,000 N (1,000 kg X 3 m/s2)
- Newton’s First and Second Laws
Calculating Force
 Practice Problem
 What net force is needed to
accelerate a 25-kg cart
 at 14 m/s2?
 350 N (25 kg X 14 m/s2)
10.4 Newton’s 3rd Law
 Newton’s 3rd Law of Motion
-For every action there is an opposite but
equal reaction.
- Action Force = Reaction Force
Momentum
-Quantity of motion
- It is described by its direction and quantity
Momentum= Mass x Velocity (kg·m/s)
Conservation of Momentum
- The total momentum of any group of objects
remains the same, or is conserved, unless
outside forces act on the objects.
Newton’s 3rd Law of Motion
- Newton’s Third Law
Conservation of Momentum
 In the absence of friction, momentum is
conserved when two train cars collide.
- Newton’s Third Law
Calculating Momentum
 Which has more momentum: a 3.0-kg
sledgehammer swung at 1.5 m/s or a 4.0-kg
sledgehammer swung at 0.9 m/s?
 What information have you been given?
 Mass of smaller sledgehammer = 3.0 kg
 Velocity of smaller sledgehammer = 1.5 m/s
 Mass of larger sledgehammer = 4.0 kg
 Velocity of larger sledgehammer = 0.9 m/s
- Newton’s Third Law
Calculating Momentum
Which has more momentum: a 3.0-kg
sledgehammer swung at 1.5 m/s or a 4.0-kg
sledgehammer swung at 0.9 m/s?
What quantities are you trying to calculate?
 Momentum = Mass X Velocity
 Smaller sledgehammer = 3.0 km X 1.5 m/s =
4.5 kg•m/s
 Smaller sledgehammer = 4.0 km X 0.9 m/s =
3.6 kg•m/s
- Newton’s Third Law
Calculating Momentum
 Which has more momentum: a 3.0-kg
sledgehammer swung at 1.5 m/s or a 4.0-kg
sledgehammer swung at 0.9 m/s?
 Look Back and Check
 Does your answer make sense?
 The 3.0-kg hammer has more momentum
than the 4.0-kg one. This answer makes
sense because the 3.0-kg hammer is swung
at a greater velocity.
Calculating Momentum
 Practice Problem
 A golf ball travels at 16 m/s, while a
baseball moves at 7 m/s. The mass of
the golf ball is 0.045 kg and the mass
of the baseball is 0.14 kg. Which has
the greater momentum?
 Golf ball: 0.045 kg X 16 m/s = 0.72
kg•m/s
 Baseball: 0.14 kg X 7 m/s = 0.98
kg•m/s
 The baseball has greater momentum.
- Newton’s Third Law
Calculating Momentum
 Practice Problem
 What is the momentum of a bird with
a mass of 0.018 kg flying at 15 m/s?
 0.27 kg•m/s (0.018 kg X 15 m/s = 0.27
kg•m/s)