FORCE
Chapter 10 Text

Force
• A push or a pull in a certain direction
• SI Unit = Newton (N)

Combining Forces
• The combination of all forces acting on an object is the Net
Force.

• When forces act in the
SAME DIRECTION =
ADDITION

• When forces act in the
OPPOSITE DIRECTION
= SUBTRACTION

• Balanced Forces result in
NO MOVEMENT

= 0

• Unbalanced forces
• cause a change in the object’s motion.

=

=

Friction and Gravity
• Friction – the force that two surfaces exert on each other when they rub

Types of Friction
• 1. Static Friction
• 2. Sliding Friction
• 3. Rolling Friction
• 4. Fluid Friction

Static Friction
• Friction that acts on objects that are NOT moving

Sliding Friction
• Occurs when two surfaces slide over each other

Rolling Friction
• Occurs when an object rolls across a surface

Fluid Friction
• Occurs when solid objects moves through fluids such as water, oil, or air

• Friction depends on two factors:
• 1. How hard the surfaces push together

• 2. The type of surfaces involved

Gravity
• A force that pulls objects toward each other

2 Factors Affecting Gravity
• 1. Distance
• 2. Mass

If Distance Increases the force of gravity
Decreases

Mass
• The amount of matter in an object

If mass increases, the force of gravity increases

Mass vs. Weight
Mass is Constant because of the amount of matter does not change

• The force of gravity on a person or object at the surface of a planet is known as weight .

• Weight varies with the strength of the gravitational force, mass does not .

Gravity and Motion
• Free fall – when the only force acting on a falling object is gravity

• Acceleration due to gravity is
• 9.8 m/s/s

• All objects in free fall accelerate at the SAME rate regardless of their masses

Why don’t objects fall at the same rate?

Air Resistance
• Falling objects with a greater surface area experience more air resistance.

Terminal Velocity
• Reached when the force of air resistance equals the weight of the object

Newton’s Laws of
Motion

Newton’s 1 st Law
• an object at rest will remain at rest, an object in motion will stay in motion UNLESS acted upon by another force .

Inertia
• Resistance to a change in motion
• Newton’s 1 st Law

• Inertia depends on mass, the greater the mass the greater the inertia

Newton’s 2 nd Law
• Force = mass x acc.

• Acceleration =Net Force
Mass

End Unit for Acceleration
• kg•m/s/s
• Kg•m/s 2
• Newton

• Textbook samples

• 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.

– What will the speed of the object be at 6 seconds?
58.8 m/s

• A speedboat pulls a 55kg water-skier. The force causes the skier to accelerate at 2.0 m/s 2 .
Calculate the force that causes this acceleration.

• F net
= m X
X 2.0 m/s 2 a = 55 kg
• F = 110 kg • m/s 2
• F = 110 N

What is the net force on a 1,000-kg object accelerating at 3 m/s 2 ?

Force = m x acc.
• (1,000 kg X 3 m/s 2 )
• 3,000 N

• What net force is needed to accelerate a
25-kg cart at 14 m/s 2 ?

• Force = mass x acc.
• (25 kg X 14 m/s 2 )
• 350 N

Newton’s 3 rd Law
• For every action there is an equal and opposite reaction

Momentum
Momentum = Mass x Velocity
End Unit = 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

• 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?

Momentum = Mass X
Velocity
Smaller sledgehammer = 3.0 km X 1.5 m/s =
4.5 kg • m/s
Larger sledgehammer = 4.0 km X 0.9 m/s =
3.6 kg • m/s

• 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.

• What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s?

• (0.018 kg X 15 m/s =
• 0.27 kg•m/s

Energy
Chapter 13

Energy
• The ability to do work

• 2 Kinds of Energy
• 1. Kinetic Energy
• 2. Potential Energy

Kinetic Energy
• Energy of
MOTION

• Kinetic energy increases as mass and velocity increases

Potential Energy
• Stored Energy

Gravitational potential energy increases as weight and height increase.