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Second Law of Motion

Section 3.3

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What does it mean to accelerate?

What is required to make an objet accelerate?

Why is it hard to stop when you run downhill?

Review

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1. Place a quarter and a flat eraser at one end of a book that is lying flat.

2. Slowly raise the end of the book, creating an incline for the objects to slide down.

3. What object slides first?4. Why?5. What is this an example of?

Friction Race

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When forces are unbalanced, motion is created.

When two opposing forces are unbalanced, the combine, or net, force is determined by subtracting the smaller force from the larger one.

1. Suppose you are pushing a couch across the room, what forces are acting on the couch?

2. Are the forces balanced or unbalanced?3. What is the net force?

Unbalanced Forces

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Force, mass, and acceleration are all related.

Motion of an object changes, or accelerates, when a force acts on it.

If you know the mass and acceleration of an object, you can determine the force.

Newton’s Second Law of Motion: The net force on an object equals its mass

times it acceleration. F=ma

Look at the Picture of the two skateboarders on pg. 64, assuming both girls apply equal force, why will Maria accelerate faster?

Accelerated Motion

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A direct relationship exists between force (F), mass (m), and acceleration (a).

Both mass and force can have an effect on acceleration.

Look at Figure 3.10 on pg. 65: The boy applies the same amount of force

to both carts however, their accelerations are different. Why?

For both carts to have the same acceleration, what mass should the cart on the left?

What if instead he applied more force to the cart on the right?

Force, Mass, and Acceleration

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There are two graphs that can help us understand this relationship:

Force vs. Acceleration Graphs Mass vs. Acceleration Graphs

Looking at the Graph on pg. 65, we see two lines that represent two sleds being pulled by sled-dogs.

Given that there are multiple dogs pulling one sled, how might each dog affect the acceleration of the sled?

How does the acceleration of the 200 kg sled compare to the acceleration of the 100 kg sled?

How does mass affect the acceleration of each sled?

Graphing Accelerationhttp://session.masteringphysics.com/problemAsset/1073469/7/knight_Figure_04_05.jpg

Have you ever had to push a stalled car? What happens if you pushed it yourself? What happens if you have others helping

you push it? What causes the car to move? Remember:

Force = mass x acceleration Acceleration = force/mass

Using Newton’s Second Law

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1. Newton’s second law shows the relationship between what values?

2. What does Newton’s second law state?3. How does the weight of an object affect

its acceleration?4. How would increasing the force applied

to an object affect the acceleration?a. What if more mass was added to this

object? What would happen to the force?

What have we learned?

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If you had a bowling ball and a marble moving at the same velocity, which would be easier to stop?

Both objects contain momentum Momentum- inertia of motion

Momentum of a moving object is related to its mass and velocity.

momentum = mass x velocity If an object has a large momentum, it

has a large mass, large velocity, or both.

Momentum

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Momentum is constant unless something changes.

What would have to change for momentum to change?

Momentum can transfer from one object to another.

Transfer of Momentum

Momentum

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Think back to our spinning an eraser tied to a string…

What happened? What was affecting the eraser?

A force is acting on the eraser to keep the tension moving toward the center

Centripetal Force- center-directed force that causes an object to follow a circular path.

Force in Circular Motion

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Look at Figure 3.12 on pg. 68:1. How does the centripetal force on sharp

curves change if there is an increase in the car’s acceleration?

2. What happens to the centripetal force on sharp curves if there is a decrease in the car’s acceleration?

3. If the tire applies less friction than is necessary on the curve, how would the car move?

4. Why do racing cars have wide tires?

Centripetal Force

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1. How does a change in the force applied to an object affect its acceleration?

2. If an eagle and a bumble bee are both traveling at 16 km/hr, which has more momentum? Explain.

3. A 28-kg meteor hits the surface of the moon at 130 km/s. What is the meteor’s momentum?

Questions?

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