Introduction to Forces and Newton’s 1 st and 2 nd Laws of Motion

1. Introduction to Forces and Newton’s 1st and 2ndLaws of Motion

2. Review • How did we previously define the term “force”? • Describe the relationship between forces and motion.

3. Types of Forces There are actually seven types of forces: • Applied force • Frictional force • Gravitational force • Normal force • Thrust • Drag 7. Lift

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

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

12. Newton’s 1st Law of Motion • An object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it. • An object at rest will stay at rest unless an unbalanced force acts on it. • How do these billiard balls exhibit the 1st law?

13. Newton’s 1st Law of Motion • This law is also called “the law of inertia”. • Inertia – tendency of an object to resist any change in motion • The greater the mass, the greater the inertia. • Think about a ping pong ball and a bowling ball. Why can’t you use a bowling ball to play ping pong?

14. Auto Crashes – The Law of Inertia at Work • Passengers not wearing a seat belt keep moving forward at the car’s speed even after the car stops. • Seat belts and air bags save lives because they exert a force on the passenger causing the passengers to slow down.

15. Newton’s 2nd Law of Motion • Force and motion are connected. • An object will have greater acceleration if a greater force is applied to it. Tossing vs. Throwing

16. 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.

17. Using Newton’s 2nd Law • From this information Newton derived the a mathematical equation to summarize the relationship between acceleration, mass, and force. F = ma F = force in Newtons (N) m = mass in kilograms (kg) a = acceleration in meters per second squared (m/s2)

18. Example Problem #1 • Engineers at the Johnson Space Center 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?

19. 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?