chapter 3 – force, mass & acceleration some review… a force is a push or pull. the si unit...

chapter 3 –

force, mass &

acceleration

Some review…

A force is a push or pull.

The SI unit for force is the Newton.

What IS a Force?

Newton’s 1st law of motion (Law of Inertia) states that the motion of an object changes only if an unbalanced force acts on the object.

Mass, Force, & Acceleration

Newton's Second Law of Motion:

F = ma

This means that the net force (F) acting on an object equals the product of the mass (m) times the acceleration (a) of the object.

The direction of the force is the same as that of the acceleration.

kg·ms·s kg

m .s·s

In other words, The more force that hits an object, the faster

it moves.

force

acceleration

directly related

Calculating Force

mass: m=0.70 kg

acceleration:

a=3500m/s/s

The maximum acceleration of a fist in a karate blow has been measured at 3500 m/s/s. The mass of the fist is 0.70 kg. If the fist hits a wooden block, what force does the wood place on the fist?

Unknown: force (F) What info is given in the question?

= (0.70 kg)(3500m/s/s)= 2450 kg·m/s/s =

F = m a

2450 N

What’s different about throwing a baseball and throwing a bowling ball?

You can throw them with the same force, but will get much different results!

Force and Mass 3.13.1

Newton’s Second LawNewton’s Second Law

acceleration

mas

sinversely relatedbig mass = small acceleration

small mass = big acceleration

F = ma

Suppose you give a skateboard a push with your hand.

An Important Force 3.13.1

Newton’s Second LawNewton’s Second Law

According to Newton’s first law of motion, it will keep moving forever.

Does the skateboard keep moving with constant speed after it leaves your hand?

No!! It accelerates—negatively!!! According to Newton’s second law (F=ma), if

the skateboard is accelerating, there must be a net force acting on it.

The force that slows and brings things to a stop is friction.

Friction 3.13.1

Newton’s Second LawNewton’s Second Law

The amount of friction between two surfaces depends on two factorsthe kinds of surfaces and the force pressing the surfaces together.

We’re going to learn about 3 kinds of friction—static, sliding, and rolling.

• Suppose you have filled a cardboard box with books and want to move it.

Static Friction 3.13.1

Newton’s Second LawNewton’s Second Law

• It’s too heavy to lift, so you start pushing on it, but it doesn’t budge.

Static Friction 3.13.1Newton’s Second LawNewton’s Second Law

Static friction is the frictional force that prevents two surfaces from sliding past each other.

• You ask a friend to help you move the box.

Sliding Friction 3.13.1

Newton’s Second LawNewton’s Second Law

• Pushing together, the box moves. You have exerted enough force to overcome the static friction.

• If you stop pushing, the box quickly comes to a stop.

Sliding Friction 3.13.1

Newton’s Second LawNewton’s Second Law

• Sliding friction is the force that opposes the motion of two surfaces sliding past each other.

Rolling Friction 3.13.1

Newton’s Second LawNewton’s Second Law

• Rolling friction is the frictional force between a rolling object and the surface it rolls on.

As a wheel rolls over a surface, the wheel digs into the surface, causing both the wheel and the surface to be deformed.

Air resistance acts in the opposite direction to the motion of an object through air.

Air Resistance

3.13.1

Newton’s Second LawNewton’s Second Law

The amount of air resistance on an object depends on the speed, size, and shape of the object.

Air Resistance 3.13.1

Newton’s Second LawNewton’s Second Law

If you drop two identical plastic bags from the exact same height, but crumple one and open one, the crumpled one will fall faster. It falls faster because it is smaller, therefore it has less air resistance.

Mass has nothing to do with it!!

• As an object falls, the downward force of gravity causes the object to accelerate.

Terminal Velocity 3.13.1

Newton’s Second LawNewton’s Second Law

• However, as an object falls faster, the upward force of air resistance increases.

• This causes the net force to decrease as the object falls.

The terminal velocity is the highest speed a falling object will reach.

Terminal Velocity 3.13.1

Newton’s Second LawNewton’s Second Law

Force of air resistance

Force of gravity

3.13.1Section CheckSection Check

Question 1

A.) accelerationB.) momentumC.) speedD.) velocity

Newton’s second law of motion states that _________ of an object is in the same direction as the net force on the object.

A!Everything accelerates

in the direction of the force that hits it.

3.13.1Section CheckSection Check

Question 2

A.) jouleB.) muscleC.) newtonD.) watt

The unit of force is __________.

C!1 newton = 1 kg · m/s2

3.13.1Section CheckSection Check

Answer

Friction results from the sticking together of two surfaces that are in contact.

Question 3

What causes friction?

What is gravity? • Gravity is an attractive force between any

two objects that depends on the masses of the objects and the distance between them.

3.23.2GravityGravity

No matter how far apart two objects are, the gravitational force between them never completely goes to zero.

3.23.2GravityGravity

Earth’s Gravitational Acceleration

• When all forces except gravity acting on a falling object can be ignored, the object is said to be in freefall.

• Close to Earth’s surface, the acceleration of a falling object in freefall is 9.81 m/s2

.

• This acceleration is given the symbol g and is sometimes called the acceleration of gravity.

• A space shuttle in orbit is in free fall, but it is falling around Earth, rather than straight downward.

3.23.2

GravityGravity

Floating in Space

• Objects in the shuttle seem to be floating because they are all falling with the same acceleration.

The gravitational force exerted on an object is called the object’s weight.

Weight

Weight and mass are not the same.

Weight is a force and mass is a measure of the amount of matter an object contains.

Weight increases as mass increases.

F = ma weight = mg

How much does a 100-kg object weigh?

3.23.2GravityGravity

Calculating Weight

weight = mg

weight = (100 kg)(9.81 m/s2)

weight = 981 kg•m/s2

= 981 N

Projectile motionIf you throw an object, it doesn’t travel in a straight line. If you throw a ball, it will curve down.

Definition: A projectile is an object acting under only one force GRAVITY

The acceleration of an object moving in a circular path is toward the center of a circle. The force that causes acceleration is also towards the center of the circle. This is called centripetal force .

Centripetal force this allows a car or a bike to round a sharp turn.

3.23.2

Question 1

Gravity depends on the masses of the objectsand the distance between them.

Gravity is an attractive force between any two objects and depends on __________.

Answer

Hint: there are 2 things that the strength of

gravity depends on!!!

Newton’s Third Law

Newton’s third law of motion describes action-reaction pairs this way. When one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength and opposite in direction.

3.33.3The Third Law of MotionThe Third Law of Motion

For every action, there is an equal and opposite reaction.

Action and Reaction • When you jump on a trampoline, for

example, you exert a downward force on the trampoline.

3.33.3The Third Law of MotionThe Third Law of Motion

• Simultaneously, the trampoline exerts an equal force upward, sending you high into the air.

The swimmer “acts” on the water. The “reaction” of the water pushes the swimmer forward.

3.33.3The Third Law of MotionThe Third Law of Motion

Name the action and reaction in this picture:

Action/Reaction Pairs in Rockets

In a rocket engine, burning fuel produces hot gases. The rocket engine creates momentum by exerting a force on these gases, causing them to escape out the back of the rocket.

By Newton’s third law, the gases exert a force on the rocket and push it forward.

3.33.3The Third Law of MotionThe Third Law of Motion

task

• Come up with at least 10 examples of action reaction pairs.

Momentum

A moving object has a property called momentum that is related to how much force is needed to change its motion.

The momentum of an object is the product of its mass and velocity.

3.33.3The Third Law of MotionThe Third Law of Motion

momentum = mass x velocity

Momentum• Momentum is given the symbol p and can

be calculated with the following equation:

• The unit for momentum is kg · m/s. Momentum has a direction because velocity has a direction.

3.33.3The Third Law of MotionThe Third Law of Motion

Conservation of Momentum

All matter in motion has momentum.

3.33.3The Third Law of MotionThe Third Law of Motion

• Momentum, however, can be transferred from one object to another.

• In a collision, the total momentum of the bodies does not change. (This is just another way of stating the third law of motion)

Momentum & Colliding Objects When two objects

collide, momentum can be used to help understand what happens.

When two pool balls collide (one at rest and one in motion) the one in motion loses momentum and the resting ball gains exactly the amount the other one lost.

When Objects Collide 3.33.3

The Third Law of MotionThe Third Law of Motion

• If two pucks are speeding toward each other with the same speed, and if they collide, the total momentum is zero.

3.33.3Section CheckSection Check

Question 1According to Newton’s third law of motion, For every action, there is an equal and opposite reaction. Sooooo, if you push it…

3.33.3Section CheckSection Check

Question 2

A.) mass, accelerationB.) mass, velocityC.) mass, weightD.) net force, velocity

The momentum of an object is the product of its __________ and __________.

B!momentum = mass x velocity

p = mv

3.33.3Section CheckSection Check

Question 3

A.) They’re going the same speed.B.) the ping pong ballC.) the cue ballD.) impossible to know

If a ping pong ball and a cue ball have the same momentum, which is travelling faster?

B!momentum = mass x velocity

p = mv

3.33.3Section CheckSection Check

Question 4

When two objects collide, what happens to their momentum?

Answer

Their total momentum doesn’t change, but momentum can be transferred from one object to another.

(the law of conservation of momentum)

3.33.3Section CheckSection Check