Forces & Newton’s Laws of Motion

Chapter 4(no friction)

Forces

A push or a pull

Gravitational Electromagnetic Weak Strong

Sir Isaac Newton

Newton’s First Law An object in motion remains in motion

and an object at rest remains at rest unless acted on by an outside force.

Sometimes called the Law of Inertia Examples

Sudden braking Flooring it Seeing stars

Inertia A body’s tendency

to keep doing what it’s already doing

Measured by mass

Does not need gravitational force.

Newton’s Second Law

The acceleration of an object is directly proportional to the force on the object and inversely proportional to the mass.

maF

Units

Ns

mkgmaF

2

Newtons English equivalent is

pounds 4.45N=1lb

Examples

1. A 2988kg elephant accelerates on a skateboard at a rate of 2.7m/s2. If the elephant is being pushed with a 10,000N force, what is the force of friction?

2. A 58.7N force is applied to a go kart to get it started. If the car starts from rest and accelerates for 4.0s covering a distance of 7.8m, what is the mass of the kart?

Newton’s Third Law

When one object exerts a force on a second object, the second object exerts a force on the first of equal magnitude and opposite direction

Action-reaction pairs Switch the nouns

Weight

Weight and gravitational force are the same thing.

Since acceleration due to gravity (g) is known, we can define weight as:

Fg = mg

Example #3

How much does a 3 kg object weigh?

Example #4An astronaut with a mass of 75 kg travels to Mars.

A) What is his mass on Mars?

B) What is his weight on Mars where the acceleration due to gravity is 3.8 m/sec2?

C) What is the acceleration due to gravity on top of a mountain if he weighs 683 N?

Normal Force Force pushing up from a

surface When vertical

acceleration is zero and the only two vertical forces are gravity and normal force, normal force and gravity are equal.

Scales read normal force Elevators Weightlessness (

Newton’s Mountain)

Example #5

A 52 N sled is pulled across a cement sidewalk. A horizontal force of 36 N is exerted.

A) What is the acceleration of the sled?

B) What is the acceleration of the sled if the sled’s mass is 30 kg?

Tension

Upward force coming from a string or rope.

All vertical ropes supporting an object account for an equal amount of force unless specified otherwise.

Example #6

A 24 N fish hangs from a scale in an elevator. The elevator accelerates upward at 3 m/sec2. What does the scale read?

This time the car accelerates downward at 1.8 m/sec2.

Example #7

An 800 N student stands in an elevator. The elevator accelerates upward at 3 m/sec2. What is the tension in the cables if the car’s mass is 1000 kg?

This time the car accelerates downward at 1.8 m/sec2.

Atwood Machine – Example #8

Assumptions• a1= a2

• FT1= FT2

Pulley switches which direction is positive.

Consider the direction of motion of the box to be the positive direction.

m1

m2

•If mass 1 is 5kg and mass 2 is 7kg, what is the acceleration of the system?

Box 1

amFF gT 1

maF

Fg

FT

a

amgmFT 11

Box 2

amFF Tg 2

maF

Fg

FT

a amFgm T 22

Putting it together

amgmFT 11 amFgm T 22

amgmFT 11 TFamgm 22

amgmamgm 2211

gmgmamam 1212

Putting it together cont.

gmgmamm 1212 )(

gmgmamam 1212

2

21

12 63.1s

mmm

gmgma

Example #9

What is the acceleration of an Atwood machine if the two masses hanging on it is 3 kg and 4 kg?

What is the tension in the cable?