Transcript F net = T

One day ,Stephen Su was driving a
motorcycle on the street. Meanwhile,
David Sun was driving a trunk at a
fast speed.
Stephen Su did not stop in time as
soon as the traffic light turned red.
David sun could not control the van
and collided with Stephen.
The remaining question is ,even
though Stephen slammed on the
brake, the accident happened
anyway. What caused this tragedy??
tendency
resist
inertia
motion
absence
According to Newton's first
law, an object in motion
continues in motion with the
same speed and in the same
direction unless acted upon
by an unbalanced force.
Newton’s first law of motion牛顿第
一定律:
Every body continues in its state of
uniform motion or rest unless acted
up on by an external force
Application of the Newton’s first law
Safety belts
Any passengers in the car will also be
deccelerated to rest if they are strapped to the
car by seat belts.
Being strapped tightly to the car, the
passengers share the same state of motion as
the car. As the car accelerates, the
passengers accelerate with it; as the car
decelerates, the passengers decelerate with it;
and as the car maintains a constant speed,
the passengers maintain a constant speed as
well.
If two objects, a uniform disk and
a uniform sphere, have the same
moment of inertia about their
axes of rotation and the same
angular velocity, then the disk
has the larger rotational kinetic
energy.
(T) True
(F) False
If two ends of a rope are
pulled with forces of
equal magnitude and
opposite direction, the
tension at the center of
the rope must be zero
True
False
A disk and a hoop of the same mass M and
radius R roll without slipping across a
horizontal floor. Both the disk and the hoop
are moving with velocity v when the floor
starts to slope upward. Which statement is
correct? (B)
(a) The disc makes it to a greater height
than the hoop.
(b) The hoop makes it to a greater height
than the disk.
(c) Both the disk and the hoop make it to
the same height.
Newton's Second Law of motion pertains to the
behavior of objects for which all existing forces
are not balanced. The second law states that the
acceleration of an object is dependent upon two
variables - the net force acting upon the object
and the mass of the object. The acceleration of an
object depends directly upon the net force acting
upon the object, and inversely upon the mass of
the object.
Newton's Second Law tells us that rate of
increase (or decrease) in the speed of
something which is moving is proportional
to the force acting on it.
Imagine that you are riding a bicycle
along a perfectly smooth and level road
and you decide to stop pedaling. If there is
a strong wind pushing against you, you will
stop completely in a shorter time than you
would if the wind was light.
Now that we know Newton's Laws of Motion, how
do we apply them? How can they let us predict the
motion of an object if we know all the forces acting
upon it? How can they let us predict the forces on an
object if we know its motion?
or
FORCE = MASS times ACCELERATION
Kobe's car, which weighs 1,000 kg, is
out of gas. Kobe is trying to push the
car to a gas station, and he makes the
car go 0.05 m/s2. Using Newton's
Second Law, you can compute how
much force Kobe is applying to the
car.
Answer = 50 newtons
Example 1:
Consider a crate being pulled along a horizontal, frictionless
floor. A rope is tied around it and a man pulls on the rope with a
force of T. T is the tension in the rope. What happens to the
crate?
These forces
are shown on
the "free body
diagram"
above. We
have drawn in
all the forces
acting on the
object. The net
force is the
vector sum of
these forces.
Fnet = F = T + n + w
Fnet, x=F x=Tx+n x+ w x
Fnet,y =Fy=Ty+ ny+ w y
n=w
Example 2:
This particular example is stated in
terms of weighing a fish in an
accelerating elevator. It is also fun to
think of weighing yourself in an
accelerating elevator. When does an
elevator accelerate upwards? When does
an elevator accelerate downwards?
The forces acting on the fish are shown in the
free-body diagram. T is the tension supplied by
the scale. This is the value the scale reads. We
may call it the apparent weight of the fish. The
net force on the fish is
Fnet = T - mg
The net force is (always!) equal to the
mass times the acceleration. This fish is
moving along with the elevator. In this
diagram we have taken the acceleration
to be up so it is positive.
Fnet = T - m g = m a
T = m g+ m a
T = m (g + a)
While the elevator accelerates upward, the apparent
weight of the fish is greater than its true weight, mg.
The forces on the fish are again shown in the free-body diagram
Fnet = T - mg
The net force is (always!) equal to the mass times the
acceleration. This fish is moving along with the elevator.
Now the acceleration to be up so it is negative.
Fnet = T - m g = m ( - a )
T=mg-ma
T = m (g - a)
While the elevator accelerates downward, the apparent
weight of the fish is less than its true weight, mg.
1. Who was the scientist who gave us the Laws of Motion?
Answer: Sir Isaac Newton
2. How many Laws of Motion are there?
Answer: three
3. What is another name for the first law of motion?
Answer: Law of Inertia
4. Which law explains why we need to wear seat belts?
Answer: First Law of Motion
5. Which law says that force is equal to mass times
acceleration (F=MA)?
Answer: Second Law of Motion
6. Which law says that heavier objects require more
force than lighter objects to move or accelerate
them?
Answer: Second Law of Motion
7. Which law explains how rockets are launched into
space?
Answer: Third Law of Motion
8. Which law says that for every action there is an
equal and opposite reaction?
Answer: Third Law of Motion
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