Download Force Review – Use the papers in your binder if you can`t think of the

Force Review – Use the papers in your binder if you can’t think of the answers.
1. A force is a push or a _________. You have learned that all forces are associated with
acceleration. Write down the three ways that objects can accelerate.
1.______________________ 2. _____________________ 3. _____________________
Hopefully you remembered the three ways. And you should also know now that
acceleration is not just a change in speed (speed up or slow down), but also a change in
direction because the definition of acceleration really means a change in velocity.
2. Write the definition of velocity: Velocity is ______________ and _______________
3. Since velocity means speed and direction, give a written example of a storm
approaching at a certain speed (m/s) and going west, east, north or south.
I see a storm approaching me from the ______________ at ________________
So if we change either the speed or the direction of an object it is accelerating. To help
you remember this, recall what it feels like if you speed up or slow down suddenly in a
car (floor it!, slam on the brakes!) and also if you are going really fast in the car and you
suddenly turn (swerve to avoid hitting a giant pothole in the road!). That’s right, you feel
a force!
4. You have also learned the equation for force and the unit of force (remember it’s
named after a famous scientist who studied gravity). Write the equation for force and
include all the correct SI units (mass, acceleration, force).
Force = ________________ x __________________ = _______________________
Here’s another way to help you remember that force is associated with mass and
acceleration. If you drop a massive rock on your toe, it hurts because it hits your foot
with a force (the force of gravity working on the rock!). When do you feel that force of
the rock? You feel it when that rocks mass suddenly decelerates (slowing down or
negative acceleration) when it hits your foot and comes to a stop.
5. Draw a simple picture of a rock hitting your foot below (ouch!) and include the
symbols for the mass of the rock (kg) and the unit of acceleration (m/s2, in this case,
deceleration…) as it hits your foot with a force in Newtons (N)! No color required this
time unless you want to…! Then turn the page over, there’s more on the back!
Here’s one more interesting thing to think about. Imagine if your foot could suddenly
accelerate in the same direction of the rock so that the rock could not actually hit your
foot. In other words the rock is flying down at your foot, but your foot (with you
attached!) suddenly are flying downward with the rock hovering just above your toe. As
you are flying downward with the rock just above your foot flying along with
you…would it have any force? Circle yes or no below.
Yes!
No!
Hopefully you realize that the rock has no force as you are falling with it. It is only when
you are not moving, but the rock is, and when the rock suddenly decelerates on your foot
with its mass, only then do you feel the force (double ouch!) of the rocks mass on your
foot.
Einstein imagined falling towards Earth is an elevator car if the steel cable holding the
elevator car suddenly snapped. At first your would feel the force of acceleration, but
eventually, if you could fall far enough with the car, you would feel nothing. This
condition is known as freefall when only the force of gravity is working on objects.
Skydivers who jump with parachutes from airplanes live for this moment of
weightlessness right before they pull the cord on their parachute and use air resistance
to slow them down so they don’t crash with a fatal force into the ground. Obviously
when that same freefalling elevator hits the ground with you in it….well, you can
imagine the consequence when there is a sudden deceleration of your mass and the mass
of the elevator car!
Newton thought about objects of mass moving in space just like the rock right before it
hits your foot. Although the rock has no force until it decelerates on your foot, it does
have something he called momentum, or the quantity of motion (amount of motion).
He thought of the momentum of the rock as being if you could freeze the rock in time and
know its exact speed and then multiply its mass to get momentum, or its quantity of
motion. It would be like taking a picture of a moving object – it would have only speed at
that instance.
Try writing the formula for momentum in SI units. It is mass (kg) times speed (m/s).
Momentum = __________ x _____________
What do you think the units of momentum are? Write your guess below!
6. Finally try these simple calculations. Show your work and include the proper units!!
a. What is the momentum of a moving rock of mass 20 kg with speed 2 m/s?
b. What is the momentum of a falling tree of mass 100,000 kg with speed 10 m/s
c. What is the force of a rock of mass 25 kg decelerating at 10 m/s2?