Do Now for 5/6/13
Take out E80
HW: Test on Wednesday
Analysis question #2 is due tomorrow
E80 Newton’s Laws of Motion
• Today’s Target: I will be able to state the
three laws of motion that Newton
discovered.
• Review reading
• Answer Stopping to Think Questions
• Analysis Questions
• Analysis #2
E80 Newton’s Laws of Motion
• 1. Which has more inertia, a heavy ball or a
light ball rolling at the same speed in the same
direction?
• Think about which one is more resistant to a
change in motion.
– The heavy ball has more inertia because it would
take more force to change its motion than the
light one.
E80 Newton’s Laws of Motion
• 2. What would happen to a baseball if you
could throw it in outer space? Explain in terms
of inertia and friction.
– Without friction, a thrown baseball would
continue in a straight line at a constant speed
forever, or until it encountered another force that
changes its motion.
E80 Newton’s Laws of Motion
• 3. A car travels along a straight road at a
steady 40 MPH. Are the forces on the car
balanced or unbalanced?
• Explain.
– The forces are balanced. The frictional forces
within the car and from the road are balanced by
the equal but opposite force applied by the
engine. The net force is zero, and the car travels
with constant speed and direction.
E80 Newton’s Laws of Motion
• 4. Can a light object that is hit with a small
force accelerate as rapidly as a heavier object
hit with a big force? Why or why not?
– Yes, because the ratio of force to mass could be
the same for the two situations. For example, the
acceleration of 400 newtons/100 kg is the same as
4 newtons/1 kg.
E80 Newton’s Laws of Motion
• 5. If you hold a backpack in your hand, the
force of gravity pulls it downward. What force
keeps it from falling to the ground?
– The “reaction” force is your hand that applies an
equal and opposite force on the backpack.
However, it is impossible to say which of the two
forces is the “action” and which is the “reaction”
since they are applied simultaneously.
E80 Newton’s Laws of Motion
• 1. Spaceships that travel millions of miles into
outer space use very little fuel. How can they
go so far on so little fuel?
– Because there is no friction in outer space, an
object in motion remains in motion. Once a space
probe leaves behind the friction of Earth’s
atmosphere, it needs very little fuel to get it to its
final destination. (However, it uses fuel to slow
down if it is going to land on a planet or to change
direction.)
E80 Newton’s Laws of Motion
• 2. Use Newton’s Laws to explain why it is
easier to turn a truck when it is empty than
when it is carrying a heavy load.
E80 Newton’s Laws of Motion
• 3. An engine can exert a force of 1,000
newtons. How fast can this engine accelerate:
– a. a 1,000 kg car
•
•
•
•
F=mxa
1,000 N = 1,000 kg x a
a = 1,000 N/1,000 kg
a = 1 m/s2
E80 Newton’s Laws of Motion
• b. a 2,000 kg car
–F=mxa
– 1,000 N = 2,000 kg x a
– a = 1,000 N/2,000 kg
– a = 0.5 m/s2
E80 Newton’s Laws of Motion
• 4. Use Newton’s third law to explain why a blown
up, but untied balloon flies around the room
when you let it go.
– Newton’s third law states that forces come in equal
and opposite pairs. For every action force there is an
equal and opposite reaction force. The balloon exerts
a force on the air (the action force), causing the air to
rush out the opening, while at the same time the air
exerts an equal and opposite reaction force on the
balloon, causing the balloon to fly around the room.
E80 Newton’s Laws of Motion
• 5. Motor oil, axle grease, and other lubricants
are slippery. Why do you think people spend
the money to put these lubricants in their
cars?
– Lubricants reduce friction, and less friction means
that the engine and the wheels move more
smoothly (and stay cooler). This means the engine
needs to exert less force, which often results in it
lasting longer, using less fuel, and requiring fewer
repairs.
E80 Newton’s Laws of Motion – Key
Points
• 1. An object that is not being subject to a force will
continue to move at a constant speed in a straight line.
• 2. Friction is a force that causes changes in the speed
of an object’s motion.
• 3. The magnitude of the change in motion can be
calculated using the relationship
• F = ma, which is independent of the nature of the force.
• 4. Whenever one object exerts a force on a second
object, the second object exerts an equal and opposite
force on the first.
E80 Newton’s Laws of Motion
• Complete sheet 80.1