Apr 20, 2023 Physics 1 (Garcia) SJSU

Chapter 7 Energy

The “money” of physics

Apr 20, 2023 Physics 1 (Garcia) SJSU

Primitive Economics

Do your job Get paid

Apr 20, 2023 Physics 1 (Garcia) SJSU

Modern Economics

Using money simplifies economics and accounting.

Do your job

Get paid

Buy stuff

Apr 20, 2023 Physics 1 (Garcia) SJSU

Why Energy Helps

Motion, in general, is hard to calculate.

Using forces, momentum, acceleration, etc. gets complicated because they are all vectors (have magnitude & direction).

Energy is not a vector; it’s just a number.

Can predict motion by figuring out how much energy that motion will “cost.”

Apr 20, 2023 Physics 1 (Garcia) SJSU

Potential Energy (PE)

Gravitational potential energy of an object is,

(Potential Energy) = (Weight) x (Height)

Think of potential energy as stored energy or energy “in the bank.”

Metric unit of energy is Joules (same as for work).

Apr 20, 2023 Physics 1 (Garcia) SJSU

Sample Problem

What is the potential energy of a 6kg bowling ball at a height of 20 meters?

What is potential energy at zero height?

20 m

6 kg

Apr 20, 2023 Physics 1 (Garcia) SJSU

Kinetic Energy (KE)

Kinetic energy of an object is,

(Kinetic Energy) = ½ x (Mass) x (Speed)2

Kinetic energy is the energy of motion.A stationary object has zero kinetic energy.Kinetic energy is related to momentum but

concepts are not the same!

Apr 20, 2023 Physics 1 (Garcia) SJSU

Sample Problem

What is the kinetic energy of a 6kg bowling ball, falling from a height of 20 meters, just as it reaches the ground?

20 m

6 kg

20 m/s

Apr 20, 2023 Physics 1 (Garcia) SJSU

10 m

PE = 600 J, KE = 600 J

The Big Idea20

m

6 kgPE = 1200 J, KE = 0 J

PE = 0 J, KE = 1200 J

Energy is the “currency” of motion

Apr 20, 2023 Physics 1 (Garcia) SJSU

Conservation of Energy

Conservation of mechanical energy is

(Potential Energy) + (Kinetic Energy)

stays constant during motion.

Energy “bookkeeping” makes motion simple.

Apr 20, 2023 Physics 1 (Garcia) SJSU

Pendulum

Energy exchange from PE to KE and back.

MaximumHeight

MaximumHeight, again

MaximumSpeed

Can predict speed from heightsince PE+KE constant

Note: Use this concept in lab experiment entitled “Projectiles”

Apr 20, 2023 Physics 1 (Garcia) SJSU

Demo: Don’t Flinch

When pendulum returns, never hits you.

Apr 20, 2023 Physics 1 (Garcia) SJSU

Movie: Bowling Ball Pendulum

Apr 20, 2023 Physics 1 (Garcia) SJSU

Demo: Interrupted Pendulum

By conservation

of energy we

know that the

pendulum ball

will never rise

above its initial

height.

Apr 20, 2023 Physics 1 (Garcia) SJSU

Demo: Bunny Bash

www.bunnybash.org

Apr 20, 2023 Physics 1 (Garcia) SJSU

Demo: Bunny Bash

Would the ball strike at a higher speed by falling straight down instead of swinging as a pendulum?

No, by energy conservation the kinetic energy at impact only depends on the initial potential energy (on initial height).

Apr 20, 2023 Physics 1 (Garcia) SJSU

Demo: Bunny Bash

The following year, we simplified the process.

Apr 20, 2023 Physics 1 (Garcia) SJSU

Demo: Ball Races

Can predict ball speeds along the tracks.

Ball on track B goes the same speed as ball on track A whenever the two balls are at the same height

Apr 20, 2023 Physics 1 (Garcia) SJSU

Demo: Blaster Balls

How high does the ping-pong ball go?

Pingpongball

Golfball

After collision the speed ofping-pong ball

is 3x larger

Speed after collision is 3 times larger so kinetic energy is 9 times greater.Ping-pong ball rises to 9 times its original height (9x original potential energy).

Apr 20, 2023 Physics 1 (Garcia) SJSU

Work

Define work done on an object by a force as

(Work) = (Force) X (Distance traveled)

Force acting in direction of motion: Positive work.

Force acting in opposite direction: Negative work.

Force perpendicular to motion: Zero work

Apr 20, 2023 Physics 1 (Garcia) SJSU

Check YourselfSlaves pull a heavy load.

Work done by slaves is positive, negative, or zero?

Work done by friction force?

Work done by the ground?LOAD

SupportPull

Friction

Apr 20, 2023 Physics 1 (Garcia) SJSU

Work & EnergyWhen forces do work on an object, the work

done equals the change in energy.

(BIG FORCE) X (short distance)

(small force) X (LONG DISTANCE)

Two persons do the same work in different ways.

Apr 20, 2023 Physics 1 (Garcia) SJSU

Jumping

How high you jump depends on the force and on the distance over which you apply that force

This is because the work done, (Force)x(distance), equals the energy gained.

Can only push while in contact with the ground so squatting helps by increasing distance.

Apr 20, 2023 Physics 1 (Garcia) SJSU

Check Yourself

Compared with going 30 mph, a car going 60 mph has ____ times the kinetic energy.

Four times the KE means ____ times the work required to stop the car.

Four times the work means ____ times the distance (same friction force on brakes).

Apr 20, 2023 Physics 1 (Garcia) SJSU

Stopping & Braking Distance

35’ 55’90’

70’ 220’290’

Reaction Distance

Braking Distance

Stopping Distance

30 mph

60 mph

45 mph

At twice the speed, braking distance is four times longer