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Newton’s Laws Chapter 5 Physics 521

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Page 1: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Newton’s LawsChapter 5Physics 521

Page 2: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Introduction

•Our understanding of force and motion has progressed with time.

•Sir Isaac Newton published his laws in 1686 and we still use them today to predict force and motion interaction for macroscopic objects.

Page 3: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Force and motion are divided into 2 branches:•Classical or Newtonian Mechanics which

treats energy and matter as separate things and uses Newton’s laws to predict the results of objects interacting.▫It provided connections between acceleration and

forces acting on objects.▫Deals with larger objects.

•Quantum Mechanics attempts to explain motion and energy of atoms and subatomic particles.

Page 4: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Newton’s 1st Law – Law of Inertia

•An object with no net force acting upon it remains at rest (if it was at rest) or remains in motion (if it was in motion), and it moves with a constant velocity in a straight line.

•Objects tend to keep doing what they are already doing.

•Ex. If you leave a book on a desk, you expect it to be there when you return – no net force.▫A hockey puck on an air table will move in a

straight line indefinitely because of its own inertia.

Page 5: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Recall …

• Inertia –an object’s resistance to a change in its motion.

•Net force – the sum of all forces acting on an object. It is the net force that causes an object to accelerate.

Page 6: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Definitions

•Equilibrium – the state when there is no net force acting on an object.

• Inertial frame of reference – object is at rest or has a constant velocity; Newton’s laws apply.▫Ex. You are inside a windowless, perfectly smooth

riding tractor trailer, that is moving at a constant velocity. You have no way of knowing if you are moving or not. Whatever happens to you inside that trailer follow the laws of motion.

Page 7: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Non-inertial frame of reference – object is accelerating; Newton’s laws do not apply.▫Ex. You are traveling in a car that suddenly and

quickly decelerates. You feel as if you are being thrown forwards, when in reality your body just wants to keep its inertia (forward motion) until a force (seatbelt) causes a change in the inertia.

▫No force has caused you to move forward.

Page 8: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Do Conceptual Problems on Page 158 together.•Do Section Review on Page 159 #s 1-3

Page 9: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Newton’s 2nd Law•The acceleration of an object is directly

proportional to the net force on it and inversely proportional to its mass.

•The direction of the acceleration is the same as that of the net force.

•Therefore: or •Example: if you push a desk there is a net force

because your applied force is greater than the force of friction and the desk accelerates.

Page 10: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Newton’s 2nd law states that the force that causes a mass (m) to accelerate (a) is the net force (Fnet) acting on the mass.

•Fnet is the vector sum of all the forces involved whether it be the applied force (FA) and frictional force (Ff) or the force of gravity (Fg) and the normal force (FN).

Page 11: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•You must be careful and watch the signs.

•Horizontal motion:

•Vertical motion:

•We will combine dynamics and kinematics equations to fully analyze motion.

Page 12: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Do Model Problem on Page 162-163•Do Practice Problems on Page 163 #s 1-3

•Do Model Problems on Pages 165-167•Do Practice Problems Pn Page 168 #s 4-8

•Do Practice Problems on Pages 170-171 #s 9, 10, & 13

•Do Section Review on Page 176 #s 1-4, 6, & 7

•Do Chapter 5 Review Problems P 206-209 #s 2, 6, 23, 27, 29 – 35, & 45.

Page 13: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Dynamics Test 1

Page 14: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Newton’s 3rd Law

•The Law of Action-Reaction•When an object exerts a force on a second

object, the second object exerts an equal force on the first but in the opposite direction.

•Ex.▫You hit a wall, it “hits” you just as hard▫You shoot a gun, the bullet goes forward and the

gun ‘kicks back’.▫You jump forward from a rowboat onto a dock and

the boat goes backward.

Page 15: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

▫A falling boulder is accelerated toward Earth, but Earth is also pulled upward by the boulder with an equal force. We don’t notice the Earth’s acceleration, because if you recall from Newton’s 2nd law, acceleration is inversely proportional to mass. Since the Earth is so massive, its acceleration is very, very small.

Page 16: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Forces always act in pairs. •Action-Reaction forces act on different objects

so they do not cancel each other out and give a net force of zero.

•Ex.▫If you kick a football, the action force is on the

ball, and it is basically the only force on the ball so it accelerates. The reaction force is on your foot and this causes your foot to decelerate.

▫Think about kicking a kettle bell or medicine ball instead, does your foot now feel the deceleration?

Page 17: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Apparent Weight

•Weight or Fg is due to gravity pulling down on a mass.

•Apparent weight is when the force of gravity has changed (temporarily) in such situations as traveling in an elevator.

•When the direction of acceleration is positive (ascending or stopping while going down), then your apparent weight is greater than your true weight.

•You “feel” heavier because the floor is pushing on you with a greater force than when elevator is stationary or moving at a constant velocity.

Page 18: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•When the direction of acceleration is negative, (descending or slowing down while going up) then your apparent weight is less than your true weight.

•You “feel” lighter as the floor is falling out from beneath you.

Page 19: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Model Problem Page 184•Do Practice Problems #s 21-23 on Page 186

Page 20: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Free Falling Objects

•Recall that in the absence of air (friction), all objects fall with the same acceleration.

•When the the only force acting on the object is gravity, the object is said to be in free fall and its acceleration is equal to (-9.81 m/s2).

•However, in air, there is an additional force acting on moving objects.

•This friction-like force is known as air resistance or the drag force.

Page 21: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Air resistance or drag force is the result of the moving object colliding with air molecules as it falls.

•The size of the drag force depends on the size and shape of the object, the density of the air, and the speed of the motion.

Page 22: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Dropping an Object•Just after you drop an object, the force of gravity, Fg

is larger than Fdrag so the object accelerates downward.

•As the velocity of object increases, so does the Fdrag.

•At some point, the Fdrag equals Fg so Fnet = 0, velocity becomes constant (no a)... this is known as the terminal velocity of the object.

•The value of terminal velocity depends on the shape and orientation of the object, the density if the air and the speed of motion.

•See Table 5.1

Page 23: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Momentum and Newton’s Laws

•We have stated that inertia was the resistance to any change in the position of an object, whether the object was at rest or in motion.

•Now, we will look at the inertia of moving objects which is known as momentum.

•Momentum ( ) is an effect created by the mass of an object traveling at a certain velocity.▫The product of the mass and velocity of a body.▫Vector quanitity with the same direction as the

velocity of the object.

Page 24: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Momentum is not a force. •Therefore, its units are not Newtons, instead

they are .

•A single object with constant velocity and mass also has constant momentum, we say momentum is conserved.

•Do Model Problem on Page 197•Do Practice Problems # 29 on Page 197

Page 25: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Impulse

• Impulse ( ) is the product of a force and the time over which the force acts.

▫A vector quantity with the same direction as the force.

▫Units are N × s

•Do Model Problem on Page 199-200•Do Practice Problems #s 30-32 on Page 200

Page 26: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

Impulse- Momentum Theorem•The impulse-momentum theorem states that the

impulse is equal to the change in momentum.• It is another way of writing Newton’s 2nd Law.

Where - final momentum ( ) – inital momentum ( )

Or So

Page 27: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•When you are playing sports you are trying to change the momentum as much as possible.

•To do this, you need to have a large impulse.•You can get a large impulse by applying a large

force for a short time or a smaller force for a long time.

•Ex. The longer you leave the bat in contact with the baseball, your foot on the football, or the racket on the tennis ball, the more you can increase the momentum.

•This idea is known as ‘follow-through’, but in physics we call it impulse.

Page 28: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Do Model Problem on Page 201-202•Do Practice Problems #s 33-35 on Page 203

•Do Section Review on Page 205 #s 1-4, & 5

Page 29: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Read about “Impulse and Auto Safety” on Pages 203-205.

Page 30: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Chapter 5 Review Pages 206-209•#s 34, 37 - 46

Page 31: Newton’s Laws Chapter 5 Physics 521. Introduction Our understanding of force and motion has progressed with time. Sir Isaac Newton published his laws

•Force Worksheet•Problems – Red Book•Assignment•Chapter 5 Review•Dynamics Test 2