What do the words ‘net force’ mean? and net force mean the sum of all the forces applied to an object.

Newton’s second law of motion.

State of equilibrium?

Is there a net fore acting on the object when it is at the top of its trajectory? V Yes! Yes! Yes!

The object is not in a state of equilibrium at any point of its trajectory. Its velocity is always changing and therefore it is always accelerating.

All objects in free-fall near the earth’s surface, accelerate down at 10m/s2, if we disregard air friction.

Using the definition of acceleration:

A simple Jedi trick:

An object in free-fall is always accelerating down. Therefore there must be an unbalanced force acting on the object, in the down direction.

In a rear-end collision-without a head-rest. F The back of the seat pushes her body forward. Her head stays where it is until her neck pulls it forward.

In a rear-end collision with a head-rest, the head-rest pushes her head forward as the seat pushes her body forward. F F

Mechanical equilibrium indicates that the net force acting on a system is zero. Weight = ? 250 N What must the staging weigh in order to have a net force of zero?

Because the staging is not accelerating, we know that the sum of all the forces applied to the platform must be zero. 200 N 200 N Weight = ? 250 N The staging’s weight must be 150 Newtons

Another mechanical equilibrium question F=? 400 N 300 N 250 N 300 N The sum of the down forces is 850 Newtons The sum of the up forces must also be 850 Newtons The right rope must pull up with a force of 450 N

Satellite motion 8 km/s Falls 5 m in 1 s. 8 km/s The earth curves down 5 meters for every 8 km of surface. Does this bullet get any closer to the surface?

Satellite motion V V V A F V The satellite accelerates toward the center of the earth. There must be an unbalanced force acting on the satellite toward the center of the earth.

Weightless The astronaut has the same velocity and acceleration as the capsule and therefore is weightless.

‘Wind’ friction slows low altitude satellites down so they don’t quite get out over the edge of the earth. They slowly get closer and closer to the earth and eventually crash.

Geosynchronous satellites orbit 22,300 miles above the earth. They orbit the earth once every 24 hours.

A trip to the moon Once escape velocity has been achieved, the engines can be shut off.

The moon’s surface is weathered by micro-meteorite impact.

Throwing a ball to a friend will never be the same. Vvertical Vhorizontal The horizontal velocity remains constant. The vertical velocity is constantly changing.

Let’s say that the vertical velocity is initially 30 m/s up and 1 sec Let’s say that the vertical velocity is initially 30 m/s up and 1 sec. later it is 20 m/s up. The change in velocity is in the down direction.

At the top of its trajectory, the ball’s vertical velocity is zero. A moment later, the vertical velocity is down and we see that the change in velocity is still in the down direction.

Acceleration is in the direction of the change in velocity. Since the change in velocity is down, the ball must be accelerating down.

All objects in free-fall near the earth’s surface, accelerate down at 10 m/s2 disregarding air friction.

How will the landing times of the coins compare? Initial velocity horizontally Dropped Hint: They both have an initial velocity of zero in the down direction.

They will cover equal vertical distances in equal times. So they will land at the same time.

Newton’s second law of motion The acceleration of an object is directly proportional to the net force applied to an object and inversely proportional to the objects mass.

Mass is a measure of an object’s inertia Inertia is an objects resistance to a change in its velocity.

Calculate your mass by dividing your weight by 2.2 We will use kilograms as units of mass

It is easy to calculate your metric weight Weight = 750 Newtons Newtons are metric units of force