Do Now for 5/10/13
Take out E81 Write up and open
your books to page E-37
HW: None
E81 The Net Force Challenge
• Today’s Target: I will be able to tell if the forces
on an object are balanced or unbalanced.
• Review the concept of Net Force
• Introduce activity
• Gather Data
• Clean up
• Begin discussing data
• Analysis?
E81 The Net Force Challenge
• In many situations, more than one force is
acting on an object.
– Ex. Driving a car
Engine
Power
Wind
resistance
The combination
of all the forces
acting on an
object is called
the Net Force or
total force.
F=ma represents
the Net Force.
Friction
E81 The Net Force Challenge
A force diagram shows an object or a system as a simple square and is
labeled only with the forces applied to it.
Shows all the
forces applied to
the object.
Always need to show both the size AND the
direction of the forces.
The same situation,
but only shows the
NET FORCE.
E81 The Net Force Challenge
• Read the Background and Challenge on page
E-37.
• If there is a situation with unbalanced forces,
there is a net force.
• Balanced force means there is a net force of
zero.
E81 The Net Force Challenge
• Can you describe the motion of the
following:
E81 The Net Force Challenge
• Use of the Force Meters
– Gently pull
– Each mark on the scale is equal to 0.1 Newtons
• Complete parts A,B, and C.
• Motion – Any change in position, which could
mean an object moves at a constant speed and
direction.
• Change in Motion – A change in speed or
direction due to an acceleration or deceleration.
E81 The Net Force Challenge
• 1. Describe an example and draw a force
diagram of a situation with: a. balanced forces
This block will have NO
change in its motion – If it
was at rest, it will stay at
rest. If it was in motion, it
will stay in motion at a
constant speed.
• B. Unbalanced forces
This block will
accelerate
towards the
right.
E81 The Net Force Challenge
• 2. Imagine that a parked car is hit from the left
with 30,000 N of force at the exact same time
it is hit from the right with 40,000 N of force.
a. Draw a force diagram showing the forces
acting on the parked car.
E81 The Net Force Challenge
• b. Draw another force diagram showing only
the net force on the parked car.
E81 The Net Force Challenge
• 3. The force diagram below shows an object
with zero net force, but there is one force
missing. What is the missing force? Draw the
diagram in your notebook, and complete it by
drawing and labeling the missing force.
– The missing force is friction, and it must be 10 N
to the right in order to balance the forces.
E81 The Net Force Challenge
• 4. Look back at your work from Part A. Could
the block in Part A have been moving?
• Explain.
– Yes, the block could have been moving and still
have had balanced forces, as was seen in Part C.
Although the block in this case would be moving,
it has no change in motion to cause an
acceleration or net force.
E81 The Net Force Challenge
• 5. For each situation a-e below, explain why
there is or is not a zero net force acting on the
car.
• a. A car is parked on a level parking space.
– The car is stopped, which means it has a constant
speed (0) and direction, so there must be zero net
force acting on it.
E81 The Net Force Challenge
• b. A traffic light turns green, and a car starts
to move.
– The car is accelerating, and so there is a nonzero
net force acting on it.
• c. A car drives steadily at 25 MPH.
– The car is traveling at a constant speed and so
there is zero net force acting on it.
E81 The Net Force Challenge
• d. A car is slowing down from 30 MPH to 10
MPH.
– The car is decelerating (from 30 MPH to 10 MPH)
so there is a nonzero net force acting on it.
• e. A car goes around a corner at 10 MPH.
– The car is changing direction, a form of
acceleration, so there is a net force acting on it.
E81 The Net Force Challenge – Key
Points. EXAMPLES NEEDED!!!
1. When an object experiences a nonzero net force (unbalanced forces),
some aspect of its acceleration— speed and/or direction of
motion—will always change.
2. Any object that is not traveling at a constant speed and direction
must be in a situation where there is a nonzero net force
(unbalanced forces) acting on it.
3. When an object experiences zero net force (balanced forces), its
speed and direction of motion are constant.
4. Any object traveling at a constant speed and direction, which could
be either an object at rest or a moving object, has a zero net force
(balanced forces).