• A push or pull that causes a change in

Forces

Define Force

• A push or pull that causes a change in

motion.

Inertia

• If there is no force, there is no change in motion.

Newton’s First Law

• An object at rest will stay at rest, and an object in motion will stay in motion unless acted upon by a force.

Inertia

• Inertia is defined as the tendency of an object to resist a change in motion

Newton’s First Law = inertia

• An object at rest will stay at rest, and an object in motion will stay in motion unless acted upon by a force.

Question??

• How did the loops demo demonstrate inertia?

Why should you wear seatbelts?

Inertia

When in motion, a car and the riders inside all move 60 mph. If a car suddenly stops, then you will be going 60 mph into the

dashboard/windshield.

Explain each of the following using a couple sentences

• Explain why the items stay in place when a magician pulls a tablecloth out from a table set with dishes and silverware.

• Explain why a hockey puck after being hit continues to coast along the frictionless ice until it hits the wall.

Weight and Mass

Weight vs. Mass

WEIGHT ≠ MASS

Weight vs. Mass

• Weight is the force of gravity on an object

• Your weight will change when the amount of gravity

changes.

• Units are Newtons

• Mass is the amount of matter in an object.

• Mass is not affected by the amount of gravity.

• Units are kilograms

Figuring out weight

Weight = acceleration due to gravity x mass

F

= a

x m

To be accurate, on earth use a_g = 9.8 m/s²

Example

• Suppose a textbook has a mass of 2.0 kg. Determine the weight of the textbook on earth.

• Ans:

W = g x m

W = 9.8 x 2 = 19.6 N

Measuring a Force

►Force is measured in Newtons.

►Symbol: N

►How would you measure a force?

►Measure compression of a spring

►Measure the movement of an object with known mass

Magnitudes

1 N = weight of an apple

720 N = average force of human bite

4.7 kN = max Olympic heavyweight punch 1.8 MN = thrust of space shuttle

1.98×10²⁰ N = attraction between moon and earth

Examples of Forces

Contact Forces Field Forces

• Forces by touching

• Examples

– Pushes, punches, kicks – Support forces

– Friction – Tension

• Forces at a distance

• Examples

– Gravity – Magnets

– Atomic forces

Representing Forces in Diagrams

Goal: Learn the rules for drawing forces in Free-Body-Diagrams (FBDs)

Drawing forces

• Forces are represented by vectors (arrows)

– Their direction tells you which way the force is pointing

– Their length tells you the strength of the

force

Which force is strongest?

Green represents strongest force because it is the longest.

Drawing Forces on Objects:

Free Body Diagrams

• Represent the object as a dot.

• Draw all forces on the object pointing away from the dot.

• Label EVERY force.

DO NOT DRAW FORCES PERFORMED BY THE OBJECT.

Some Special Force Names

►Normal Force: F_N

►A force from a surface acting perpendicular to the surface.

i.e. a support force

►Friction: F_fr

►A force from a surface acting parallel to the surface, in the direction against the intended motion

►Tension: F_T

►Pulling force along a rope/string/cord

FBD ex.

1. Hat on head.

2. Person pushing against a wall.

3. Box sitting at rest on a slope.

What is wrong with each of the following free-body-diagrams?

A sled being pushed on the ground

A horse, which is pulling a carriage

• A skydiver falling downwards with a parachute

F_g

F_parachute

A ball, just as it is being hit by a bat

1. A book sitting at rest on a table.

2. A monkey hanging from tree

3. A chair that has a person sitting in it.

4. A wagon being pulled to the left 5. A anvil after it has been dropped

6. A ball after it has been thrown by a pitcher

7. A car driving on the freeway at constant speed

8. A Tapia going down a slippery slide sloped down and to the right.

Force and Equilibrium

FBD recap

►Represent the object as a dot.

►Draw all forces on the object pointing away from the dot.

►Label EVERY force.

DO NOT DRAW FORCES PERFORMED BY THE OBJECT.

FBD examples

Adding Forces

• When forces are in the same direction, just add their magnitudes:

1 N

+

1 N

+

3 N

1 N

Vector Addition

• Vectors can be added, but not merely by adding their magnitude, but by the parallelogram rule.

• The sum of vectors is called the resultant.

Parallelogram Rule

Resultant

Opener Question:

• A rocket is traveling through the emptiness of space. Describe and justify what

happens to the motion of the rocket once it turns off its thrusters.

HW

• Finish WS

Net Force and Equilibrium

Pre-lecture Discussion ?’s

• If an object is moving, does it have to have a force on it?

• If an object is not moving, does it have no forces pushing on the object?

Net force

• Finding the sums of all the forces on an object yields the net force.

F

= ΣF

ΣF = F

+ F

+ F

+…

Equilibrium

• Equilibrium is the state when the motion does not change.

– In other words, equilibrium is when the velocity does not change.

Equilibrium

• When the net force of an object is equal to zero, an object is in equilibrium.

F

= 0

Discussion Instructions

• On a piece of paper to turn in, you will write your answers to the following questions on each slide.

Question 1

• Agree or Disagree with the following statement: “Applying a force to an object guarantees that an object will move.”

Create an example that supports your argument.

Question 2

• While standing on the scale sitting on your bathroom

floor, the scale reads 100 N. What is the net force on the scale?

Question 3

• Can an object be in equilibrium if there is only one force on the object?

Question 4

• A heavy box is sitting on the floor. If you sit down on the box, will the support force of the floor on the box change?

Explain using free body diagrams.

Question 6

• Can a moving object be in equilibrium? Demonstrate using examples and free body diagrams.

Question 7

• On earth, suppose you have a mass of 100 kg.

– What is your weight on earth?

– What is your mass in space?

– What is your weight in space?

Question 8

• Suppose you have a mass of 100 kg and you travel to Planet X where the gravity is twice as strong?

– What is your mass on Planet X?

– What is your weight on Planet X?

Question 9

• A bar that weighs 10 N is supported on both ends by two ropes. A 4 N squirrel jumps onto the bar, and sits there for a while. When everything comes to rest, one of the ropes feels a force of 6 N. Calculate the force on the other rope.

Question 5

• If an object is in equilibrium, will applying/changing a single force always break it out of equilibrium?

Equilibrium ?’s

1. Can an object be in equilibrium if there is only one force on the object?

2. If an object is in equilibrium, will applying another force break it out of equilibrium?

3. Can a moving object be in equilibrium?