Transcript force - HomeworkNOW.com

Forces # 35
Net force = 20 – 15
= 5 N (to the right)
Net force = 10 – 0 =
10 N (upward)
20 N
15 N
10 N
Forces # 36
The previous object will accelerate to the
right and upward (both directions).
Forces # 37
Because the net force will tell you if the
object’s motion is constant or how it is
changing.
Forces # 38
The Newton (N) is the SI unit for force.
Forces # 39
Absolutely! Most 8th graders miss this one!
Examples
 A car going at a constant speed of 30 mph
in a straight line
 Terminal velocity
Newton’s Laws # 40
Inertia – resistance to change in motion
Weight – force due to gravity (w = m * g)
Momentum – mass in motion
Newton’s Laws # 41
More mass = more inertia
Less mass = less inertia
Newton’s Laws # 42
If an object has more inertia, you will need a
larger force to change its motion.
Newton’s Laws # 43
It takes more force to get a bowling ball
moving because the bowling ball has more
inertia than a golf ball.
Newton’s Laws # 44-a
First Law – Law of inertia
 Deals with balanced forces
 Objects want to keep doing what they are
doing
 I.E. a car accident; water races, etc.
Newton’s Laws # 44-b
2nd Law – F = ma
 Deals with unbalanced forces
 Relates the change in motion to the mass
and force
 I.E. an object falling; pushing a cart, riding
a bike/skateboard, etc.
Newton’s Laws # 44-c
3rd Law – Forces act in pairs
 Deals with action / reaction forces
 Explains how objects are able to move
 I.E. swimming; walking, airplanes, rockets,
etc.
Newton’s Laws # 45
F=m*a
F = 25 * 5
F = 125 N
Newton’s Laws # 46
F=m*a
6=m*2
6/2=m
m = 3 kg
Newton’s Laws # 47
Force = same; mass is decreased
Acceleration will increase.
Newton’s Laws # 48
Force = same; acceleration is decreased
Mass will increase.
Newton’s Laws # 49
Momentum is NOT lost; the momentum
before a collision = momentum after a
collision.
Newton’s Laws # 50
More mass = more momentum
More velocity = more momentum
Gravity # 51
Gravity – a force of attraction between 2
objects due to their masses and distance.
Gravity # 52
Law of Universal Gravitation – ALL objects in
the universe attract each other.
Gravity # 53
Gravity exists EVERYWHERE.
Gravity # 54
Mass and distance affect gravity.
Gravity # 55
If mass increases, gravity increases!
More mass = more gravity
Gravity # 56
If distance increases, gravity decreases.
More distance = Less gravity
Gravity # 57
Mass is amount of matter that an object has
Weight is a FORCE due to an object’s mass
and gravitational acceleration.
Mass does NOT change; weight changes once
you leave earth.
Gravity # 58
You can never truly be weightless, because
some object (star, planet, moon, etc.) will be
exerting a force on you!
Gravity # 59
Mass is measured with a balance
Weight (force) is measured with a scale
Gravity # 60
 In the absence of air resistance, the softball
and bowling ball will hit the ground at the
same time.
 Gravity affects all objects equally
(regardless of their mass) – this means that
if you drop a feather and hammer, they hit
the ground at the same time in the absence
of air resistance!
Work and Machines # 61
Work – a force causes an object to move in the
direction that the force is applied.
Work is measured in Joules
Work and Machines # 62
For work to be done, you must apply a force
to an object and the object must move in the
direction that the force is applied.
Work and Machines # 63
a) Lifting an object (yes)
b) Carrying a heavy object (no)
c) Pushing a house (no)
d) Holding 200 pounds over your head (no)
e) Playing a sport (yes)
f) Reading (n0)
g) Pushing a cart down the hall (yes)
Work and Machines # 64
W=f*d
W = 500 * 10
W = 5,000 J
Work and Machines # 65
Power – rate at which work is done
Power is measured in watts
Work and Machines # 66
If a device is more powerful, then it does
more work in the same amount of time.
Work and Machines # 67
If a device is more powerful, then it does the
same amount of work in less time.
Work and Machines # 68
Why is a battery operated screwdriver more
powerful than a hand operated screwdriver?
a) Because it does more work more quickly
b) Because it has a battery
c) Because it has a motor
d) Both B and C
Work and Machines # 69
P=w/t
P = 50 / 10
P=5w
Work and Machines # 70
To do less work, you must either reduce the
amount of force applied to the object or
reduce the distance that the object travels.
Work and Machines # 71
To do more work, you must either increase
the amount of force applied to the object or
increase the distance that the object travels.
Work and Machines # 72
In a simple machine, if the input force is
decreased, then the distance that this force is
applied must be increased. This is know as
the “force-distance trade-off.”