Exam 1: Chapters 1-4

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Chapter 5 The Laws of Motion

5-1a Force – a push or pull.

CT1: The force of the elevator on Norbert and Zot is

A. up.

B. down.

CT2: The force of the non-sticky elevator surface on Norbert and Zot is

A. up.

B. down.

Assume the elevator is near the Earth’s surface and that it is right-side up in the cartoon.

CT3: The acceleration of Norbert and Zot is

A. up.

B. down.

C. zero

Assume the elevator is near the Earth’s surface and that it is right-side up in the cartoon.

Fig. 5.1, p.113

5-1 B. Mass

The mass of an object is the amount of matter (stuff) in that object. It is the same for a given object everywhere in the Universe. Mass is also called inertia because it resists being moved.

5-2 Newton’s First Law: A body remains in uniform motion (or at rest) unless acted upon by a net external force.

acceleration due to Earth’s rotation 0.03 m/s2

acceleration due to Earth’s orbit 0.006 m/s2

acceleration due to Sun’s orbit 2 x 10-10 m/s2

We will assume that the Earth’s surface is an inertial frame and not make errors greater than 0.03/10 = 0.3%.

An inertia frame of reference is a coordinate system (or frame) in which Newton’s Law’s hold.

A.

B.

C.

D.

E.

CT4

A.

B.

C.

D.

E.

CT5

5-3 Newton’s Second Law: The net external force is equal to the mass times the acceleration.

F = maForce and acceleration are vectors

Fx = max

Fy = may remember Galileo!

Fz = maz

Normally we will do problems in a plane with only x and y components.

Calibrating a “Force Meter”

Experiments show forces add as vectors.

Acceleration is Proportional to Force

Acceleration is Inversely Proportional to Mass

Applications of Newton’s Laws - Method Draw picture of the problem.

Choose body (bodies) to isolate.

Draw Free Body Diagrams (FBDs) for isolated bodies.

Choose and label coordinate axes.

Apply Newton’s 2nd Law: Fx = max and Fy = may

Solve for F, m or a.

Work out kinematics.

Check solution is reasonable.P5.6 (p.136)

Concept Question 6

P5.6b: If the parachutist comes to rest over a shorter distance, the force exerted by the ground will be

A. greater.

B. less.

C. the same.

5-4 Newton’s Third Law: If body A acts on body B, then body B acts back on body A with a force equal in magnitude but opposite in direction.

A BFABFBA

FAB and FBA are 3rd Law pairs.

A.

C.

B.

D.

E.

CT7

Histogram for PHYS151 Exam 1 Spring 2008Average = 65 Low = 27 High = 100

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Histogram for PHYS151 Total Spring 2008Average = 68 Low = 22 High = 102

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Concept Question 8

P5.18b: If the 5 N force is applied to the heavy box instead, the contact force between the boxes will be

A. greater.

B. less.

C. the same.

P5.18

(p.137)

Two Astronauts Pushing a Satellite with Forces that Differ in Magnitude and Direction

5-5 The Vector Nature of Forces

P5.22 (p.137)

P5.28 (p.138)

Orbit of Moon about the Sun

Exaggerated and incorrect Actual

5-6 Gravitational Force or Weight

Fg = mg (weight)

g = -9.81 j (m/s2 )

x

y

Apparent Weight is the force you actually feel pushing on you – often the floor.

P5.35 (p.138)

Concept Question 9

P5.35b: If the constant speed of descent is greater than 1.1 m/s, the force of air resistance is

A. greater.

B. less.

C. the same.

Concept Question 10

P5.33a: Your normal weight is 610 N and the scale in the elevator reads 720 N. The acceleration of the elevator

A. upward.

B. downward.

C. zero.

Concept Question 11

P5.33c: Your normal weight is 610 N and the scale in the elevator reads 720 N. The magnitude of the velocity of the elevator is

A. increasing.

B. decreasing.

C. zero.

D. impossible to determine without more

information.

5-7 Normal Forces

The Normal Force May Equal the Weight

The Normal Force May Differ from the Weight

An Object on an Inclined Surface

P5.42 (p.138)

Concept Question 12

P5.42b: If the angle of the incline is decreased, the normal force will

A. increase.

B. decrease.

C. stay the same.

A.

B.

CT13