C H A P T E R   4Forces and Newton's Laws

of Motion

4.5 Newton's Third Law of Motion

4.5 Newton's Third Law of Motion

4.5 Newton's Third Law of Motion

Whenever one body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body.

 Examples of Newton's 3rd Law

Example 4

Suppose that the mass of the spacecraft in Figure 4.7 is mS = 11 000 kg and that the mass of the astronaut is mA = 92 kg. In addition, assume that the astronaut exerts a force of P = +36 N on the spacecraft. Find the accelerations of the spacecraft and the astronaut.

Example 4

Astronauts use a tether to stay connected to the space capsule.

Suppose that the mass of the spacecraft in Figure 4.7 is mS = 11 000 kg and that the mass of the astronaut is mA = 92 kg. In addition, assume that the astronaut exerts a force of P = +36 N on the spacecraft. Find the accelerations of the spacecraft and the astronaut.

Application of Newton's Third Law

Some rental trailers include an automatic brake-actuating mechanism.

4.6 Types of Forces:In nature there are two general types of forces, fundamental and non-fundamental.

Fundamental forces:•Gravitational force•Strong nuclear force•Weak nuclear force-----|•Electromagnetic force--!—Electroweak force

Non-fundamental forces: Pushing, Pulling, Kicking, Grabbing, etc….

These are related to the electromagnetic force. They arise from the interactions between the electrically charged particles that comprise atoms and molecules.

Fundamental Forces

Fundamental Force

Example Particles Affected

Relative Strength

Strong nuclear Nucleus Nuclear 1

Electromagnetic +, - Charges Charged 10-2

Weak nuclear Radioactivity Nuclear 10-15

Gravitational Your weight All 10-38

Unification of Fundamental Forces

Newton’s Law of Universal Gravitation

Every body in the universe attracts every other body with a force that is directly proportional to the product of the masses of the bodies and inversely proportional to the square of the distance between the bodies.

Newton’s Law of Universal Gravitation

Every body in the universe attracts every other body with a force that is directly proportional to the product of the masses of the bodies and inversely proportional to the square of the distance between the bodies.

Universal Gravitational Constant

).(1067.6; 112

21 SIGr

mmGF

Universal Gravitational Constant

).(1067.6; 112

21 SIGr

mmGF

The proportionality constant, G is called the universal gravitational constant. Its value in the SI system of units is,G = 6.67 10-11N.m2/Kg2.

Universal Gravitational Constant

).(1067.6; 112

21 SIGr

mmGF

The proportionality constant, G is called the universal gravitational constant. Its value in the SI system of units is,G = 6.67 10-11N.m2/Kg2.

The law of gravitation is universal and very fundamental. It can be used to understand the motions of planets and moons, determine the surface gravity of planets, and the orbital motion of artificial satellites around the Earth.

Acceleration Due to Gravity

Acceleration Due to Gravity

Acceleration Due to Gravity

Acceleration Due to Gravity

Calculate g for planet Earth at sea level.

Weight

The weight of an object is the gravitational force that the planet exerts on the object. The weight always acts downward, toward the center of the planet.

SI Unit of Weight: : newton (N)

gmW Weight = Mass x Gravity

The Hubble Space Telescope

Example 6

The mass of the Hubble Space Telescope is 11 600 kg. Determine the weight of the telescope (a) when it was resting on the earth and (b) as it is in its orbit 598 km above the earth's surface.

4.8 The Normal Force

The normal force FN is one component of the force that a

surface exerts on an object with which it is in contact, namely, the component that is perpendicular to the surface.

Normal Force Is Not Always Equal to the Weight

Elevator Ride

What happens to your weight during an elevator ride?

Apparent Weight

Apparent Weight