Forces and Newton’s Laws of Motion

• Motion and Newton’s first lawTopics:

• What is a force?

• Identifying forces

• Newton’s second law 

• Free‐body diagrams

• N t ’ thi d l• Newton’s third law

Sample question:p qThese ice boats sail across the ice at great speeds. What gets the boats moving in the first place? What keeps them from going even faster?even faster?

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What Causes Motion?

In the absence of any forces acting on it, an object will continue moving forever. Motion needs no “cause.”

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Seat Belts: An Application of Newton’s First Law

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What Is a Force?

A forceA force...

... is a push or pull. ... acts on an object

... requires an agentobject. agent.

... is a vector. ... is a contact force or a long-range force.

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Force Vectors

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A Short Catalog of Forces: Weight w→

→→

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Spring Force Fsp

→

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Tension Force T

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Normal Force n

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Friction fk and fs→

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Drag and Thrust D Fthrust

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Identifying Forces – The free body diagram

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Example

A block is dragged uphill by a rope Identify allA block is dragged uphill by a rope. Identify all forces acting on the block.

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Example

Block A hangs from the ceiling by a rope Another block B hangsBlock A hangs from the ceiling by a rope. Another block B hangs from A. Identify the forces acting on A.

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Exercise

A ball hanging from the ceiling by a string is pulledA ball, hanging from the ceiling by a string, is pulled back and released. Identify the forces acting on it just after its release.

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Newton’s Second Law

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Example

An elevator lifted by a cable is going up at a steady speedAn elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator. • Is T greater than, equal to, or less than w? Or is there not

h i f ti t t ll?enough information to tell?

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Free-Body Diagrams

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Newton’s Third Law

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Identifying Forces for Interacting Objects

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Checking Understanding

10 year old Sarah stands on a skateboard Her older brother10-year-old Sarah stands on a skateboard. Her older brother Jack starts pushing her backward and she starts speeding up. The force of Jack on Sarah is

A. greater than the force of Sarah on Jack.B. equal to than the force of Sarah on Jack.C l th th f f S h J kC. less than the force of Sarah on Jack.

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10 year old Sarah stands on a skateboard Her older brother

Answer

10-year-old Sarah stands on a skateboard. Her older brother Jack starts pushing her backward and she starts speeding up. The force of Jack on Sarah is

B. equal to than the force of Sarah on Jack.

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Quiz

1 What is a “net force?”1. What is a net force?

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Quiz

2 List at least three of the steps used to identify the forces2. List at least three of the steps used to identify the forcesacting on an object.

A Identify “the system” and “the environment ”A. Identify “the system” and “the environment.”B. Draw a picture of the situation.C. Draw a closed curve around the system.D L t i t th b d f thi h thD. Locate every point on the boundary of this curve where the

environment touches the system.E. Name and label each contact force acting on the object.F N d l b l h l f ti th bj tF. Name and label each long-range force acting on the object.

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Quiz

3 Which of these is not a force discussed?3. Which of these is not a force discussed?A. The tension force. C. The orthogonal force.B. The normal force. D. The thrust force.

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Reading Quiz

4 An action/reaction pair of forces4. An action/reaction pair of forcesA. point in the same direction.B. act on the same object.C are always long range forcesC. are always long-range forces.D. act on two different objects.

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