Newton’s Second Law of Motion: Force and Acceleration

ICPWeek of 2/9/15

Bell Work

Get out your IP (your completed Net Force worksheet and Newton’s First Law activity if they have not yet been turned in) and have it ready to turn in. Next, answer the following questions on a scrap sheet of paper.

1. What is your average speed if you run 90 m in 15 seconds?

2. A ball sits on the floor in a bus. When the driver hits the brakes, the ball slides forward. The explanation for this is thata. Objects can sometimes begin moving without an applied force.

b. The ball was moving along with the bus yet did not experience the braking force that the bus did, so the bell kept going.

c. Friction on the ball pushed it forward.

3. What does Newton’s First Law state?

Acceleration Acceleration is the rate of

change of velocity Acceleration=

Acceleration includes speeding up AND slowing down!

Acceleration Checkpoint

Suppose that in 1 s, we steadily increase our velocity from 30 km/h to 35 km/h. In the next second, we go from 35 km/h to 40 km/h, and so on. What is our acceleration?

When a car maintains its speed while making a turn, is it accelerating?

What does the unit for acceleration look like?

Force Causes Acceleration

Acceleration is directly proportional to the net force

What happens when a force is applied in the opposite direction?

What happens what a force acts at right angles?

If you push on a shopping cart, it will accelerate. If you apply four times the net force, how much greater will the acceleration be?

Mass and Inertia The greater an object’s

mass, the greater its inertia Think about kicking a

can of rocks

Mass: the amount of matter in an object. Also a measure of its inertia (or “laziness”)

Weight: the force due to gravity that acts on an object’s mass

Volume: measure of space

Checkpoint Does a 2-kg iron block have twice as much inertia

as a 1-kg iron block? Twice as much mass? Twice as much volume? Twice as much weight when weighed in the same location?

Does a 2-kg iron block have twice as much inertia as a 1-kg bunch of bananas? Twice as much mass? Twice as much volume? Twice as much weight when weighted in the same location?

How does the mass of a bar of gold vary with location?

One Kilogram Weighs Ten Newtons

The standard unit of mass is the kilogram

The standard unit of force is the newton

A 1-kg bag of any material has a weight of 10 N in standard units

How many Newtons does a 4-kg block of gold weigh?

Mass and Acceleration

Objects with more mass are more difficult to accelerate

Mass and acceleration are inversely proportional That is, as one increases,

the other decreases

Twice as much mass results in half the acceleration

Suppose you apply the same amount of force to two carts, one with a mass of 4 kg and the other with a mass of 8 kg. Which cart will

accelerate more?

How much greater will the acceleration be?

Newton’s Second Law

The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportion to the mass of the object.

In short: Acceleration = Alternatively, Force=mass*acceleration

Checkpoint: Newton’s Law

Consider a 1000-kg car pulled by a cable with 2000 N of force. What will be the acceleration of the car? Hint: N is the same unit as

kg*m/s

An object weighing 10 kg is accelerating at 200 m/s2. What force is being exerted on the object?

Friction

Friction always acts in a direction to oppose motion Air drag: friction acting

upward as an object falls through the air

The amount of friction between two surfaces depends on the kinds of materials and how much they are pressed together

Check Point: Friction Two forces act on a bowl resting on a table:

the bowl’s weight and the support force from the table. Does a force of friction also act on the bowl?

Suppose a high-flying jumbo jet flies at a constant velocity when the thrust of its engines is a constant 80,000 N. What is the acceleration of the jet? What is the force of air drag acting on the jet?

https://www.youtube.com/watch?v=WzvhuQ5RWJE

Free Fall and Acceleration

Free fall: when air resistance doesn’t affect the motion of a falling object

Acceleration in the same for all freely falling objects Always gains speed at a rate of 10 m/s each second (OR 10

m/s2)

How? Why? The ratio of weight to mass is the same for these or any objects

We know that a feather drops more slowly than a coin when both are dropped in air—that’s because of air drag. But in a vacuum…

Checkpoint

A 5-kg bag of sand has a weight of 50 N. When dropped, its acceleration is:

A 10-kg bag of sand has a weight of 100 N. When dropped, its acceleration is:

Calculate the free-fall acceleration of a 20-kg bag of sand.

Air Drag

Air drag depends on speed and surface area

Results in reduced acceleration Think about a skydiver with a

parachute

Terminal speed: when acceleration is equal to 0 Occurs when air drag equals

weight

Air Drag: Checkpoint

Consider two parachutists, a heavy person and a light person, who jump from the same altitude with parachutes of the same size. Which person reaches terminal speed first?

Which person has the greatest terminal speed?

Which person gets to the ground first?

If there were no air drag, like on the Moon, how would your answers to these questions differ?

Gliding: Biological Approach

In nature, when do we see things taking advantage of air drag and surface area?

Why would this be advantageous?

Escape

Less energy climbing from tree to tree

Less injury upon a fall

https://www.youtube.com/watch?v=1-FHzf4xnWw

Exit Ticket

1. In what way does mass correspond to inertia?

2. What two things does friction depend upon?

3. One pound is the same as 4.45 N. Show that 1 N equals 0.23 lb.

4. A 40-kg block of cement is pulled sideways with a net force of 200 N. What is its acceleration?

To Study For Wednesday/Thursday Quiz

Vocabulary (see next slide)

Write out Newton’s First (words) and Newton’s Second (formula)

Calculate net force

Calculate average speed`

Vocabulary Acceleration: the rate at which velocity changes with time; the

change may be in magnitude or direction or both

Air drag: frictional resistance due to motion through air

Free fall: motion under the influence of gravitational pull only

Friction: the resistive force that opposes motion

Inversely: when two values change in opposite directions, so that if one increases and the other decreases by the same amount, they are said to be inversely proportional to each other

Kilogram: the fundamental SI unit of mass

Mass: the amount of matter in an object; the more mass an object has, the more it resists changes in motion

Inertia: the property of objects that resists changes in motion

Volume: the quantity of space an object occupies

Weight: the force due to gravity on an object