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CAMPUSIB PHYSICS

PRE-IB PHYSICS

TSOKOS LESSON 2-5NEWTON’S SECOND AND THIRD LAWS

Reading Activity Questions?

Assessment Statements

2.2.8. State Newton’s second law of motion.

2.2.9. Solve problems involving Newton’s second law.

2.2.14. State Newton’s third law of motion.

2.2.15. Discuss examples of Newton’s third law.

Objectives Recognize situations of equilibrium, i.e.

situations where the net force and hence the acceleration are zero.

Draw the forces on the body of interest and apply Newton’s second law on that body, .

Recognize that the net force on a body is in the same direction as the acceleration of that body.

Identify pairs of forces that come from Newton’s Third Law.

Video: Newton’s Second Law of Motion

Newton’s Second Law

The net force on a body is proportional to that body’s acceleration and is in the same direction as the acceleration

where m is mass, the constant of proportionality

amF

Newton’s Second Law

The unit of force is the Newton (N) 1N = 1kg•m/s2 ma = (kg)(m/s2)

Weight is a force caused by gravitational attraction

amF

gamgamF

Newton’s Second Law

When you jump from the emergency exit after the fire alarm gets hit by a volleyball, how do you minimize the force on your body?

amF

Newton’s Second Law

When you jump from the emergency exit after the fire alarm gets hit by a volleyball, how do you minimize the force on your body?

You bend your knees to decrease the acceleration and execute a parachute landing fall to translate vertical acceleration into rotational acceleration.

amF

Newton’s Second Law

How do car manufacturers try to minimize the forces absorbed by passengers during a collision?

amF

Newton’s Second Law

How do car manufacturers try to minimize the forces absorbed by passengers during a collision?

Air bags and crumple zones.

amF

Newton’s Second LawSample Problem A 200-kg hot air

balloon is held to the ground by two wires that make a 60-degree angle to the ground. When the wires are released, the balloon accelerates upward at 3 m/s2. What is the tension in each cable?

amF

Newton’s Second LawSample Problem A 200-kg hot air

balloon is held to the ground by two wires that make a 60-degree angle to the ground. When the wires are released, the balloon accelerates upward at 3 m/s2. What is the tension in each cable?

amF

FL

FgFTy

Newton’s Second LawSample Problem A 200-kg hot air

balloon is held to the ground by two wires that make a 60-degree angle to the ground. When the wires are released, the balloon accelerates upward at 3 m/s2. What is the tension in each cable?

mgmaF

maFFamF

mgF

FFF

FFFamF

L

gL

g

gLTy

TygL

0

Newton’s Second LawSample Problem A 200-kg hot air

balloon is held to the ground by two wires that make a 60-degree angle to the ground. When the wires are released, the balloon accelerates upward at 3 m/s2. What is the tension in each cable?

NmaF

mgmgmaFmgmaF

mgF

FFF

Ty

Ty

L

g

gLTy

600

Newton’s Second LawSample Problem A 200-kg hot air

balloon is held to the ground by two wires that make a 60-degree angle to the ground. When the wires are released, the balloon accelerates upward at 3 m/s2. What is the tension in each cable?

NFSinNF

FF

Sin

NF

T

T

T

Ty

Ty

346602

6002

60

600

Terminal Velocity

When a body moves through a fluid (gas or liquid) it experiences an opposing force, similar to friction, called drag

This force is dependent on velocity For lower velocities, drag is proportional

to velocity For high velocities, drag is proportional

to velocity squared

Terminal Velocity In horizontal

movement through a fluid, when drag equals the propulsion force, velocity is constant.

In a falling body, terminal velocity occurs when the force of aerodynamic drag equals the force of gravity. k

mgv

kvmg

vFFkvF

mgF

T

T

TDg

D

g

@

Inclined Plane Problem

A 150kg mass is placed on an plane inclined at a 17° angle. The coefficient of static friction is 0.30 and the coefficient of dynamic friction is 0.25. What happens?

Fuzzy Dice Problem

A pair of fuzzy dice hang from the rearview mirror of a car. The car accelerates from a stoplight at 2m/s2. What happens?

Video: Newton’s Third Law of Motion

Newton’s Third Law of Motion If Body A exerts a force F on Body B,

then Body B exerts an equal but opposite force F on Body A.

Newton’s Third Law of MotionExamples Pushing against a wall while wearing

rollerskates. Stepping off a boat onto a dock. A helicopter hovering. A book sitting on a table.

Summary Review Can you recognize situations of

equilibrium, i.e. situations where the net force and hence the acceleration are zero?

Can you draw the forces on the body of interest and apply Newton’s second law on that body?

Can you recognize that the net force on a body is in the same direction as the acceleration of that body?

Can you identify pairs of forces that come from Newton’s Third Law?

Assessment Statements

2.2.8. State Newton’s second law of motion.

2.2.9. Solve problems involving Newton’s second law.

2.2.14. State Newton’s third law of motion.

2.2.15. Discuss examples of Newton’s third law.

QUESTIONS?

#1-25Homework