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. WeightMass 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 1st 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 1st Law

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

Newton’s 2nd Law

•Force = mass x acc.

•Acceleration =Net ForceMass

End Unit for Acceleration

•kg•m/s/s•Kg•m/s2

•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 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the force that causes this acceleration.

•Fnet = m X a = 55 kg X 2.0 m/s2

• F = 110 kg • m/s2

• F = 110 N

What is the net force on a 1,000-kg object

accelerating at 3 m/s2?

Force = m x acc.

•(1,000 kg X 3 m/s2)•3,000 N

•What net force is needed to accelerate a 25-kg cart at 14 m/s2?

•Force = mass x acc. •(25 kg X 14 m/s2)•350 N

Newton’s 3rd 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/sLarger 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.