Newton’s Laws

Weight•The gravitational force (Fg) exerted

on an object by Earth is a vector quantity, directed toward the center of Earth.

•The magnitude of this force (Fg) is a scalar quantity called weight.

•Weight changes with the location of an object in the universe.

Calculate the weight of a 53 kg object.

If an object weighs 98.1 N, what is the mass of that object?

A 70-kg person travels to a planet where he weighs 1,750 N. What is the value of g on that planet?

Answer: 25 N/kg

Force

•A force is an action exerted on an object which may change the object’s state of rest or motion.

•Forces can cause accelerations.

•The SI unit of force is the newton, N.

•Forces can act through contact or at a distance.

Newton’s First Law

An object at rest will remain at rest and an object in motion will continue in motion with a constant velocity unless acted upon by a force.

Force Diagrams•The effect of a force depends on both

magnitude and direction. Thus, force is a vector quantity.

•Diagrams that show force vectors as arrows are called force diagrams.

•Force diagrams that show only the forces acting on a single object are called free-body diagrams.

Equilibrium• Equilibrium is the state in which the net

force on an object is zero.

• Objects that are either at rest or moving with constant velocity are said to be in equilibrium.

• Newton’s first law describes objects in equilibrium.

Tip: To determine whether a body is in equilibrium, find the net force. If the net force is zero, the body is in equilibrium. If there is a net force, a second force equal and opposite to this net force will put the body in equilibrium.

Find the net force:

Find the net force:

Determine the force on the ball.

Determine the mass of the ball.

5 kg

5 kg

Determine the force of the cable(s) on the ball in case A and case B.

Case A 1 cable Case B 2 Cables

Determine force of each cable on the 7 kg ball.

7 kg

4 kg

Inertia• Inertia is the tendency of an object to resist

being moved or, if the object is moving, to resist a change in speed or direction.

• Newton’s first law is often referred to as the law of inertia because it states that in the absence of a net force, a body will preserve its state of motion.

• Mass is a measure of inertia.

Newton’s 3rd Law

Newton’s Third Law• If two objects interact, the magnitude of

the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction.

• In other words, for every action, there is an equal and opposite reaction.

• Because the forces coexist, either force can be called the action or the reaction.

Hammer Striking a Nail• What are the action/reaction pairs for a hammer

striking a nail into wood? • Force of hammer on nail = force of nail on hammer • Force of wood on nail = force of nail on wood

• Which of the action/reaction forces above act on the nail?• Force of hammer on nail (downward)• Force of wood on nail (upward)

Does the nail move? If so, how?

Fhammer-on-nail > Fwood-on-nail

so the nail accelerates downward

Action and Reaction Forces•Action-reaction pairs do not imply

that the net force on either object is zero.

•The action-reaction forces are equal and opposite, but either object may still have a net force on it.

Consider driving a nail into wood with a hammer. The force that the nail exerts on the hammer is equal and opposite to the force that the hammer exerts on the nail. But there is a net force acting on the nail, which drives the nail into the wood.

Action-Reaction: A Book on a Desk

Reaction Force

•The book pushes

down on the desk.

Action Force

• The desk pushes up on the book.

Action-Reaction: A Falling Book

Action

• Earth pulls down on the

book (force of gravity).

• What is the result of the

action force (if this is the

only force on the book)?

• Unbalanced force produces

an acceleration of -9.81 m/s2

.

Reaction

• The book pulls up on Earth.

• What is the result of the

reaction force?

• Unbalanced force produces a

very small upward

acceleration (because the

mass of Earth is so large).

Momentum1. 70,000 kg x m/s2. 35,000 kg x m/s3. 2 m/s4. 0.50 kg 5. 40,000 kg x m/s6. 29 m/s7. The 4 kg more requires more force to stop.8. 1,225 kg9. 4.2 kg x m/s10.15 m/s11.0.01 kg x m/s

Momentum Conservation1. 6.7 kg2. 400 m/s3. 3.38 kg x m/s4. -0.50 m/s5. 100 kg6. A) 480 kg x m/s B) 6.0 m/s7. A) 60,000 kg x m/s8. A) 8.4x1011 kg x m/s9. A) 360 kg x m/s10. Since the gun and bullet are stationary before being fired, the momentum of

the system is zero. The “Kick” is the momentum of the gun that is equal to that of the bullet.

11. It means that momentum is transferred without loss.

Momentum

•Momentum measures a moving object’s resistance to changes in its motion. Momentum includes the effects of speed and direction as well as mass.

Newton’s Second Law The acceleration of an object is directly

proportional to the net force acting on the object and inversely proportional to the object’s mass.

F = manet force = mass acceleration

F represents the vector sum of all external forces acting on the object, or the net force.

Normal Force•The normal force acts on a surface in a

direction perpendicular to the surface.

•The normal force is not always opposite in direction to the force due to gravity.

– In the absence of other forces, the normal force is equal and opposite to the component of gravitational force that is perpendicular to the contact surface.

– In this example, Fn = mg cos .

Fundamental Forces•There are four fundamental forces:

▫Electromagnetic force▫Gravitational force▫Strong nuclear force▫Weak nuclear force

•The four fundamental forces are all field forces.