CH 3 ForcesCH 3 Forces

Sec 1 Newton’s 2nd Law Net force acting on an object

causes the object to accelerate in the direction of the net force

Amount of “a” depends on the mass of the object and the size of the net force

F = m x a Force is measured in

Newtons (N), m=kg, a=m/s2

So…. 1 N = 1 kg x 1 m/s2

Let’s Practice!

How much force is needed to accelerate an 80 kg rider and her 320 kg motorcycle at 6 m/s2?

It takes a force of 3000 N to accelerate an empty 1000 kg car at 3m/s2. If a 160 kg wrestler is inside the car, how much force will be needed to produce the same “a”?

A 75 kg skater pushes off from a wall with a force of 300 N. What is the skater’s “a”?

Types of Friction

Friction (f) is the force that opposes motion between 2 surfaces that are touching each other

Amount depends on surface types and amount of force pressing surfaces together

3 different types: Static-prevents 2 surfaces

from sliding past each other Sliding-opposes the motion

of 2 surfaces sliding past each other

Rolling-frictional force between a rolling object and the surface it is rolling on

Air Resistance

Force that opposes gravity on a falling object

Depends on 4 things: Speed Shape Density Size Air resistance is why a

feather falls slower than an object of the same mass but has a different shape

Terminal Velocity

Highest velocity that will be reached by a falling object

Air resistance balances the pull of gravity

According to the Law of Inertia, when the forces acting on an object are balanced, the motion of the object will not change and acceleration will stop

Sec 2 Gravity

Gravity (g)– force that every object in the universe exerts on every other object in the universe

Amount depends on the 2 objects masses and the distance between them

That is why a pencil falls to the floor (Earth) and not toward you when you drop it

Falling objects

Ignoring all forces except gravity on falling objects, the object is said to be in “free fall” (more later…)

Then all objects would fall with the same acceleration and hit the ground at the same time when dropped from same height (Let’s try it!)

That acceleration is 9.8 m/s2

Weight

Even if you are standing still, Earth exerts a gravitational force on you

This is your weight (W) in Newtons W = m x g m= your mass in kg g= acceleration of gravity (m/s2) which is 9.8 m/s2

W = m x 9.8 m/s2

Try converting your weight in kgs (divide lbs by 2.2) to N

Weight and Mass

They are NOT the same measurement

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

Compare your weight in N on Earth to the other planets (pg 78)

Which one would you weigh the most? The least?

Astronauts in space only appear to be weightless—they seem to be floating because they are inside the shuttle and they are all falling with the same acceleration

Projectile Motion

Anything thrown or shot through the air is called a projectile

They follow a curved path due to Earth’s gravitational pull and its own inertia

When the quarterback throws the ball it has horizontal motion (parallel to the Earth’s surface) due to inertia

Gravity pulls the ball to Earth, creating an increasing vertical motion

These 2 motions are independent of each other

Circular Motion

Acceleration toward the center of a curved or circular path is called centripetal acceleration

Force that causes a moving object to move in a curved or circular path is centripetal force

Gravity, the ultimate centripetal force: Earth’s gravity exerts this force on the Moon which causes it to move in a circular orbit around the Earth

Sec 3 The 3rd Law of Motion Forces always act in pairs—

action/reaction pairs Newton’s 3rd Law—to every

action there is an equal and opposite reaction

Action/reaction forces always act on different objects, so forces might be equal, but they are NOT balanced

Action—rockets burn fuel and produce hot gases that push against the inside of the rocket and escape out the back

Reaction—causes the rocket to move in the opposite direction

Let’s try it: Balloon Racers!

Momentum Property a moving object

has due to its mass and velocity

Designated by “p” p = m x v kg·m/s = kg x m/sPractice! A horse with a mass of

450.0 kg finished a race with a velocity of 20.0 m/s. What was his “p”?

What was the mass of another horse that was running at a velocity of 18.0 m/s if his “p” was 7500 kg·m/s?

Law of Conservation of Momentum– the total amount of p of a group of objects does not change unless outside forces act on the objects.

Law of Conservation of Momentum

The total amount of p of a group of objects does not change unless outside forces act on the objects

BUT…p can be transferred from one object to another