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Exam 1: Chapters 1-4 60% Problems – one problem from WebAssign with different numbers

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- 60% Problems – one problem from WebAssign with different numbers
- Understand homework problems
- Review notes and text
- Try new problems
- 40% Multiple Choice Questions
- Review Power Points (on web page)
- Review Before Class Assignments
- Try Questions
- Graphs, Pictures and Drawings (Sketches)
- Don’t rely on memorized equations

5-1a Force – a push or pull.

- CT1: The force of the elevator on Norbert and Zot is
- up.
- down.

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

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

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

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

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

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

F = ma

Force 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.

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)

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

A

B

FBA

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.

FAB and FBA are 3rd Law pairs.

(p.137)

- 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
- greater.
- less.
- the same.

5-5 The Vector Nature of Forces

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

5-6 Gravitational Force or Weight

Fg = mg (weight)

g = -9.81 j (m/s2 )

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

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

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

- 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
- increasing.
- decreasing.
- zero.
- impossible to determine without more information.

An Object on an Inclined Surface

P5.42 (p.138)

- P5.42b: If the angle of the incline is decreased, the normal force will
- increase.
- decrease.
- stay the same.

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