Chapter 3: Forces

Section 1 (part 1) - Introduction to Forces and Newton’s 2nd Law Of Motion

Warm-up: How did we previously define the term “force”? Describe the relationship between forces and motion.

Learning Goals: Distinguish between the different types of forces. Define and apply Newton’s 2nd Law of Motion.

Types of Forces There are actually many types of forces including: Applied force Frictional force Gravitational force Normal force Thrust Drag Lift

Applied Force Objects are often touching A push or a pull on anything Contact force Example: A person picking up a box applies a force the the box in order to pick it up.

Frictional Force Works opposite of an object’s motion Takes away energy Works to slow down or stop objects Contact force Example: As cart moves along the road, friction slows the cart down.

Gravitational Force Pulls toward the center of the earth Non-contact force Relatively constant on Earth, regardless of location and size of the object Example: A stone thrown in the air will be pulled back down to the Earth’s surface.

Normal Force Opposes gravity Contact force Usually upward Does not cause acceleration Usually equal and opposite to the applied force Example: The book sitting on the table has a normal force holding it up.

Thrust Rockets, planes, boats Something pushes backwards causing forward motion Gas, water, air pushed by engines, propellers, or explosions Example: An airplanes thrust is provided by its engines.

Drag Air resistance Type of friction Opposes the motion of an object Moving through water can also cause drag on a boat Example: As the airplane moves through the air the force of drag pushes back on the airplane.

Lift Opposes gravity Lift is usually “upward” Causes airplanes, hot air balloons to go up but not forward Example: The force of lift pushes the airplane up and thrust pushes it forward.

Newton’s Laws of Motion Sir Isaac Newton published his three laws of motion in his book Principia in 1687. Laws describe the effects of forces on the motion of objects.

Newton’s 1st Law of Motion (aka Inertia) An object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts upon it. An object at rest will stay at rest unless an unbalanced force acts upon it.

Newton’s 2nd Law of Motion Newton’s 2nd Law of Motion: the acceleration of an object is in the same direction as the net force on the object. Also states that acceleration is equal to the net force divided by mass

Newton’s 2nd Law of Motion An object will have greater acceleration if a greater force is applied to it. Tossing vs. Throwing

Newton’s 2nd Law of Motion The mass of the object also affects acceleration. A softball’s mass is about 0.20 kg while a baseball’s mass is about 0.14 kg. If you throw both with the same force, the baseball has greater acceleration because it has less mass.

Using Newton’s 2nd Law F = ma Force = mass / acceleration F = force in Newtons (N) m = mass in kilograms (kg) a = acceleration in meters per second squared (m/s2)

Formula Sheet F = ma m = F/a a = F/m

Example Problem #1 Engineers must determine the net force needed for a rocket to achieve an acceleration of 70 m/s2. If the mass of the rocket is 45,000 kg, how much net force must the rocket develop?

Example Problem #2 What is the mass of a truck if it produces a force of 14,000 N while accelerating at a rate of 5 m/s2 ?

Example Problem #3 If the mass of a helicopter is 4,500 kg, and the net force on it is 18,000 N, what is the helicopter’s acceleration?