Force, Work, and Power

Net Force Practice Problems1. A 1200kg truck that is traveling at 50 m/s2 has a

head-on collision with a 400kg car that is traveling at 30 m/s2. What is the net force of the collision and which direction will the crash go?

48,0002. Josh and I are pushing a T.V. cart down the hallway…

Josh pushes with a force of 25N and I push with a force of 15N. What is the net force?

40N of force3. I am playing tug of war with my dog… she is pulling

on the rope with a force of 13N to the right and I am pulling with a force of 23N to the left. What is the net force and which direction is the rope going?

10N

Newton’s First LAWLAW 1 (Law of Inertia): An object at rest will remain at rest and an object in motion remains in motion unless acted upon an unbalanced force. Inertia: The property of an object to resist changes in motion unless acted upon by an unbalanced force. Unbalanced force: A net force that is not equal to zero. Ex. When the car suddenly stops and your body snaps forward.

Newton’s Second LAWLAW 2: The unbalanced force acting on an object equals the object’s mass times its’ acceleration. To accelerate the mass of an object you must apply a force!

F= m x a Ex. Any time you move an object

Newton’s Third LAWLAW 3:For every action force, there is an equal and opposite reaction force.

Ex. Hitting a homerun off of a pitcher in baseball.

Examples of Newton’s Laws

If you are pushing an empty grocery cart with 20N of force… will the grocery cart have the same acceleration if you continue to push it at the same force of 20N when it is full. Which Law and why?A: 2nd law. NO…the acceleration will be less once the cart is full b/c you have added mass, and a = F/m

Examples of Newton’s Laws

What would happen if you were standing on a skateboard and threw a really heavy ball?Which law and why?A 3rd law. : You will go backwards on the skateboard b/c the ball is pushing you with an equal and opposite force the other way. For every action there is an equal and opposite reaction.

Examples of Newton’s Laws

If you are riding on the bus and your books are on the seat next to you… if the bus driver stomps on the brakes…where do your books go???Which law and why?A: 1st law. onto the floor… An object in motion stays in motion until acted on by an unbalanced force.

WORKThe Transfer of Energy

How does the previous investigation help us to understand how forces transfer energy?

The Transfer of Energy is called WORK

Work measures the effects of a force acting over a distance.

Work = F*d so…Work= energy transferredThe units are N * m = Joules (J)1 Joule= 1kg x m2/s2

(F) * (d) = Work(1kg x m/s2 x m)= 1kg x

m2/s2

Work and Power Warm Up1. Who does more work - a man

who lifts a large box from the ground up into the back of a truck, or a man who puts the same box on a trolley and wheels it up a ramp into the truck?A the man who lifts itB the man with the trolleyC they do the same work

Work and Power Warm Up

1. Who does more work - a man who lifts a large box from the ground up into the back of a truck, or a man who puts the same box on a trolley and wheels it up a ramp into the truck?A the man who lifts itB the man with the trolleyC they do the same work

Work and Power Warm Up

2. Energy and work use the same unit of the Joule because:A that is the unit chosen by the scientist named Pascal who studied heatB energy is required to do workC both measure the speed at which power is used

Work and Power Warm Up

2. Energy and work use the same unit of the Joule because:A that is the unit chosen by the scientist named Pascal who studied heatB energy is required to do workC both measure the speed at which power is used

Work Practice Problems Pg. 285*Challenge*

5.) d = .5m F = m*g (g = 9.8m/s2)

m = 1200kg F = ? N = (1200kg)*(9.8m/s2) = Force= 11,760N F = 11,760N W = F*d d = .5m W = ?J = (11,760N)*(.5m) =

5880JSpectrum Text Book pg 697-698 (81 and

83)

WORKQ: If you try to move a lab table with

12,000N of force and it doesn’t move…How much work have you done?

A: NONE!!! Because…W = 0J b/c the distance was 0m and W = 12,000N* 0m = 0J

POWERWhat’s Power?Power = how much work is done in a given

amount of time. Does running require more work than walking the

same distance?No! So what is the difference between running

and walking? A: The time it takes!!!So we need to account for the time it takes to do

work… the equation that measures that is P = W (Units for Power are Joules = Watts) t sec

Think about light bulbs!

POWERin DUFAS

1. While running track, Drew’s legs do 5780J of work in 183sec. What is his power output?

31.58 Watts2. The chain that is pulling a rollercoaster up

the first hill does 24652J of work over a 79sec time interval. What is the power output of the chain?

312.05 Watts3. It takes Ms. Webb 20s to apply 23N of

force to lift a box 5m. What was her power output?

5.75 Watts

Methods of Energy TRANSFER

Energy TRANSFER:

Energy transfer - is how the same type of energy is passed from one object to another object.

There are only THREE ways that energy can be transferred between objects…

Energy TRANSFER:

Method 1: Conduction

The transfer of energy through matter by direct contact of particles.

Examples: a pot on a stove, a metal spoon in a bowl of hot soup, wires carrying electricity, friction/air resistance transforms energy to heat.

Energy TRANSFER:

Method 2: Radiation

The transfer of energy in the form of a wave. (mechanical or electromagnetic waves)

Examples: How Earth gets it’s energy from the sun, a microwave oven cooking food, sound.

Energy TRANSFER:Method 3: Convection

The transfer of energy by the movement of gases or liquids. (fluid movement only!!!)

Hot fluids will rise while cold fluids will sink… this creates a swirling motion.

Examples: Hurricanes, pasta or rice swirling in boiling water, a hot air balloon.