Download Unit 5 Notes: Forces

Name ______________________________________ Class ____________
Unit 5 Notes: Forces
A push or pull is called a ___________________. Forces have ____________________________
and ________________________________, so they are represented by _____________________.
The ______________ ______________________ is the total amount of force on an object.
Every force is caused by an _______________________. When drawing force diagrams,
include all agents.
Draw the following force diagrams:
1. a book held in your hand
2. a book pushed across the desk by your hand
3. a book pulled across the desk by a string
4. a ball just after the string that was holding it broke
______________________________________ explained the way in which forces influence
motion.
Newton’s 1st Law of Motion:
Also known as the ______________________________________________
Inertia:
If the net force on an object = 0 N, then the object is in _______________________________.
If the net force on an object does not equal 0 N, then the object will __________________.
Newton’s 2nd Law of Motion:
Anytime there is a change in velocity (acceleration), it is caused by an
______________________________ ____________________________.
One unit of force causes a 1 kg mass to accelerate at 1 m/s2. This force unit is called a
__________________________.
2. Two horizontal forces, 225 N and 165 N are exerted in the same direction on a
crate. Find the net horizontal force on the crate. If the crate’s mass is 12 kg, what
is its acceleration?
3. If the same two forces are exerted in opposite directions, what is the net
horizontal force on the crate? What is its acceleration?
4. The 225 N force is exerted on the crate toward the north and the 165 N force is
exerted toward the east. Find the magnitude and direction of the net force.
What is the crate’s acceleration?
5. Your hand exerts a 6.5 N upward force on a bag of sugar. If the force of gravity
on the sugar is 4.5 N, what is the net force on the sugar? Describe its
acceleration.
Newton’s 3rd Law of Motion:
Some common forces:
________________________ believed that heavier objects fall faster than lighter objects.
________________________ was able to prove that, neglecting air friction, all objects have
the same acceleration due to gravity.
_________________ is the amount of matter in an object. It does NOT change with
location.
___________________ is the amount of __________________ ______________________ pulling
on an object. This changes when gravity changes.
To calculate an object’s weight:
6. Calculate the force you exert as you stand on the floor (1 lb = 0.454 kg).
7. On Earth, a scale shows that you weigh 585 N. What is your mass? What would
the scale read on the moon where g = 1.60 m/s2?
When riding an elevator (or roller coaster), a scale inside will read your normal weight as
long as the elevator is in ________________________________.
If the elevator accelerates upward, the scale reads a ________________________ force.
(The floor presses harder into your feet.)
If the elevator accelerates downward, the scale reads a ________________________ force.
(The floor is falling out from under your feet.)
This is called _______________________ _________________________.
If the elevator cable breaks, the scale accelerates at the same rate as your body,
__________________, so its reading would be ____________________. You would feel
_______________________________ because the floor would not be pushing up on you.
8. Your mass is 75 kg. You stand on a bathroom scale in an elevator. What force
would the scale exert when:
a. The elevator moves up at constant speed
b. It slows at 2.0 m/s2 while moving upward
c. it speed up at 2.0 m/s2 while moving upward
d. it moves downward at a constant speed
In the real world __________________________________ is everywhere. All surfaces are
________________________ at the microscopic level.
____________________________________________________: the force exerted on one surface
by another when there is no relative motion between the surfaces.
___________________________________________________: the force exerted on one surface
by another when they slide past each other.
Friction is always __________________________________ to the surfaces and
_____________________________ to the normal force.
To calculate the force of friction:
9. A boy exerts a 36 N horizontal force as he pulls a 52 N sled across a cement
sidewalk at constant speed. What is the coefficient of friction between the
sidewalk and the metal sled runners? (Ignore air resistance)
10. Suppose the sled runs on packed snow. The coefficient of friction is now only
0.12. If a person weighing 650 N sits o n the sled, what force is needed to pull the
sled across the snow at constant speed?
11. You push a 25 kg wooden box across a wooden floor at a constant speed. How
much force do you exert on the box if the coefficient of friction is 0.20?
12. How much force must you use to get the box to accelerate at 2 m/s2?
13. You use 40 N of force to push a 15 kg grocery cart at constant speed in the store.
What is the coefficient of friction between the wheels and the floor?
When an object moves through any __________________________, including air, the fluid
exerts ___________________________________ on the object. This is called the
___________________________ ____________________________. This force increases as the
speed of the object increases. (The force is also affected by the size and shape of the
object and the density of the fluid.)
_______________________________________ : the constant velocity reached when