Newton's Laws of Motion

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# Newton's Laws of Motion - PowerPoint PPT Presentation

Newton's Laws of Motion. Dr. Robert MacKay. Clark College, Physics. Introduction . Newtons 3 laws of motion 1. Law of inertia 2. Net Force = mass x acceleration ( F = M A ) 3. Action Reaction Newton’s Universal Law of Gravity. Isaac Newton 1642-1727.

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### Newton's Laws of Motion

Dr. Robert MacKay

Clark College, Physics

Introduction
• Newtons 3 laws of motion
• 1. Law of inertia
• 2. Net Force = mass x acceleration
• ( F = M A )
• 3. Action Reaction
• Newton’s Universal Law of Gravity
Isaac Newton 1642-1727

Isaac Newton 1689

Knighted by Queen Anne 1705

Isaac Newton 1702

Isaac Newton 1726

Other topics
• Why do objects accelerate?
• Why do objects not accelerate?
• Forces in Balance (Equilibrium)
• Forces out of Balance
• Friction
• Air resistance
• Terminal Velocity
Law of inertia (1st Law)
• Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.
• acceleration = 0.0 unless the objected is acted on by an unbalanced force
Law of inertia (1st Law)
• Inertia (The intrinsic tendency of an object to resist changes in motion)
• Mass is a measure of an object’s inertia
• Mass is also a measure of the amount of an object’s matter content. (i.e. protons, neutrons, and electrons)
• Weight is the force upon an object due to gravity
Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A
Newton’s Law of Action Reaction (3rd Law)
• You can not touch without being touched

For every action force there is

and equal and oppositely directed reaction force

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

M=2.0 kg

F=?

F=M A

= 2.0 kg x 6.0 m/s2

=12.0 Newtons

= 12.0 N

A= 6.0 m/s2

An object experiences a net force and exhibits an acceleration in response. Which of the following statements is always true?

(a) The object moves in the direction of the force.

(b) The acceleration is in the same direction as the velocity.

(c) The acceleration is in the same direction as the force.

(d) The velocity of the object increases.

An object experiences a net force and exhibits an acceleration in response. Which of the following statements is always true?

(a) The object moves in the direction of the force.

(b) The acceleration is in the same direction as the velocity.

X(c) The acceleration is in the same direction as the force.

(d) The velocity of the object increases.

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

M=2.0 kg

F=6.0 N

A=F / M

= ? m/s2

A= ?

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

M=2.0 kg

F=6.0 N

A=F / M

= 3.0 m/s2

A= 3.0 m/s2

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

M=?

F= 10.0 N

M= F/A

= ? kg

A= 20.0 m/s2

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

M=?

F= 10.0 N

M= F/A

= 0.50 kg

A= 20.0 m/s2

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

M= 8.0 kg

F= ? N

F=M A

= 80.0 N

A= 10.0 m/s2

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

A= 10.0 m/s2

M= 8.0 kg

F=M A

= ? N

F= ? N

F= weight

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

A= 10.0 m/s2

M= 8.0 kg

F=M A

= 80 N

F= 80 N

F= weight

Weight

m= 6.0 kg

• W = m g
• g = 10 m/s2

weight is the force due to the

gravitational attraction between

a body and its planet

W= ?

Weight

m= 6.0 kg

• W = m g
• g = 10 m/s2

weight is the force due to the

gravitational attraction between

a body and its planet

W= 60N

Question 1: A force of 45 N pushes horizontally on a 15 kg

crate resting on a level frictionless surface. (Actually the crate

has real good tiny wheels) What is the acceleration of the crate?

3.0 m/s/s

30.0 m/s/s

60.0 m/s/s

0. 33 m/s/s

Question 1: A force of 45 N pushes horizontally on a 15 kg

crate resting on a level frictionless surface. (Actually the crate

has real good tiny wheels) What is the acceleration of the crate?

X 3.0 m/s/s

30.0 m/s/s

60.0 m/s/s

0. 33 m/s/s

Question 2: A 15.0 kg crate is in contact with a 30.0 kg

crate on a level frictionless surface as shown. If the 15.0 kg

mass is pushed with a force of 45.0 N what is the acceleration

of the two masses?

A. 1.0 m/s/s

B. 1.5 m/s/s

C. 2.0 m/s/s

D. 3.0 m/s/s

Question 2: A 15.0 kg crate is in contact with a 30.0 kg

crate on a level frictionless surface as shown. If the 15.0 kg

mass is pushed with a force of 45.0 N what is the acceleration

of the two masses?

A. 1.0 m/s/s

B. 1.5 m/s/s

C. 2.0 m/s/s

D. 3.0 m/s/s

• crate on a level frictionless surface as shown. If the 15.0 kg
• mass is pushed with a force of 45.0 N what is the force that
• the 15.0 kg mass exerts on the 30.0 kg mass?
• 15 N
• 20 N
• 25 N
• 30 N
• crate on a level frictionless surface as shown. If the 15.0 kg
• mass is pushed with a force of 45.0 N what is the force that
• the 15.0 kg mass exerts on the 30.0 kg mass?
• 15 N
• 20 N
• 25 N
• 30 N

The contact force is what gives the 30 kg crate its acceleration of 1 m/s/s. Thus this contact force must be equal to 30kg(1 m/s/s)=30 N

Weight
• W = m g

8.0 kg

W= ?

Weight
• W = m g

8.0 kg

W= 80 N

A baseball of mass m is thrown upward with some initial speed. A gravitational force is exerted on the ball

(a) at all points in its motion

(b) at all points in its motion except at the highest point

(c) at no points in its motion

A baseball of mass m is thrown upward with some initial speed. A gravitational force is exerted on the ball

(a) at all points in its motion

(b) at all points in its motion except at the highest point

(c) at no points in its motion

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

F= 150.0 N

M= 5.0 kg

D =120.0 N

Net Force = ?

A= ? m/s2

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

F= 150.0 N

M= 5.0 kg

D =120.0 N

1. Net Force = 30.0 N

2. A= 6.0 m/s2

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

F= 150.0 N

M= 5.0 kg

2. D=??

• Net Force =??

A= 20.0 m/s2

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

F= 150.0 N

M= 5.0 kg

2. D=50 N

• Net Force = 100N
• Using MA

A= 20.0 m/s2

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

F= 150.0 N

M= 5.0 kg

D=?

Net Force = ?

A= 0.0 m/s2

Newton’s 2nd Law
• Net Force = Mass x Acceleration
• F = M A

F= 150.0 N

M= 5.0 kg

D=150.0 N

Net Force = 0.0

A= 0.0 m/s2

Which of the following statements is most correct?

(a) It is possible for an object to have motion in the absence of forces on the object.

(b) It is possible to have forces on an object in the absence of motion of the object.

(c) Neither (a) nor (b) is correct.

(d) Both (a) and (b) are correct.

Which of the following statements is most correct?

(a) It is possible for an object to have motion in the absence of forces on the object.

(b) It is possible to have forces on an object in the absence of motion of the object.

(c) Neither (a) nor (b) is correct.

(d) Both (a) and (b) are correct.

An object experiences no acceleration.

Which of the following cannot be true for the object?

(a) A single force acts on the object.

(b) No forces act on the object.

(c) Forces act on the object, but the forces cancel.

An object experiences no acceleration.

Which of the following cannot be true for the object?

(a) A single force acts on the object.

(b) No forces act on the object.

(c) Forces act on the object, but the forces cancel.

QUICK QUIZ 5.3

(end of section 5.5)

On Earth, where gravity is present, an experiment is performed on a puck on an air hockey table, with negligible friction. A constant horizontal force is applied to the puck and its acceleration is measured. The experiment is performed on the same puck in the far reaches of outer space where both friction and gravity are negligible. The same constant force is applied to the puck and its acceleration is measured. The puck’s acceleration in outer space will be a) greater than its acceleration on Earth, b) less than its acceleration on Earth, c) exactly the same as its acceleration on Earth, d) infinite since neither friction nor gravity are holding it back?

QUICK QUIZ 5.3

(end of section 5.5)

On Earth, where gravity is present, an experiment is performed on a puck on an air hockey table, with negligible friction. A constant horizontal force is applied to the puck and its acceleration is measured. The experiment is performed on the same puck in the far reaches of outer space where both friction and gravity are negligible. The same constant force is applied to the puck and its acceleration is measured. The puck’s acceleration in outer space will be a) greater than its acceleration on Earth, b) less than its acceleration on Earth, c) exactly the same as its acceleration on Earth, d) infinite since neither friction nor gravity are holding it back?

Newton’s 2nd Law
• Friction depends
• on surfaces in contact (roughness)
• contact force pushing surfaces together

Friction

?

F= 130.0 N

M= 5.0

A= 0.0 m/s2

Net Force = ?

Fnet=P-fK

P

fs,max=msN

fK=mKN

f

fs,max

fK=mKN

kinetic friction

(sliding friction)

Static friction

F

Applied Force=Static frictional force F=fs

Air ResistanceForce
• Depends on:
• velocity
• Air density
• Shape and aerodynamics of object
Terminal Velocity
• When air resistance force balances an objects weight

Air Drag

Acceleration= 0.0

===>

Terminal velocity

w

Terminal Velocity

Acceleration = 0.0 ===>Terminal velocity

Air Drag

Air Drag

80 kg

10 kg

w

w

which has the greatest force of air resistance?

Not Terminal Velocity

Acceleration = ?

Air Drag = 240N

80 kg

W = ?

which has the greatest force of air resistance?

Not Terminal Velocity

Acceleration = ?

Air Drag = 240N

80 kg

+x

W = 800 N

Not Terminal Velocity

Acceleration = ?

Air Drag = 240N

SF = m a

+800N - 240N= 80kg a

+560N=80kg a

a=7.0 m/s2 Down

80 kg

+x

W = 800 N

Which encounters the greater force of air resistance—
• A falling elephant, or
• 2. A falling feather?
1. A falling elephant, or 2. A falling feather?

Which encounters the greater force of air resistance—

Two smooth balls of exactly the same size, one made of wood and the other of iron, are dropped from a high building to the ground below. The ball to encounter the greater force of air resistance on the way down is

1. the wooden ball.

2. the iron ball.

3. Neither. The force is the same.

Two smooth balls of exactly the same size, one made of wood and the other of iron, are dropped from a high building to the ground below. The ball to encounter the greater force of air resistance on the way down is

Just after jumping off a bridge, her acceleration is

1. 10 m/s/s down.

2. zero

3. increasing

Just after jumping off a bridge, her acceleration is

1. 10 m/s/s down.

2. zero

3. increasing

1. increases.

2. decreases.

3. remains the same.

1. increases.2. decreases.3. remains the same.

As she falls faster and faster through the air, her acceleration

When she reaches terminal velocity her acceleration is

1. zero.

2. decreasing

3. Equal to gravity.

When she reaches terminal velocity her acceleration is

1. zero.

2. decreasing

3. Equal to gravity.

Who’s falling faster?

• Heavy
• Light
• Both falling at same speed

Who’s falling faster?

• Heavy
• Light
• Both falling at same speed