Physics and Forces
Newton’s Laws of Motion
Newton's laws are only valid in inertial reference frames:
This excludes rotating and accelerating frames.
For instance, when your car accelerates, it is not an inertial reference frame.
That's why you suddenly seem to accelerate
backwards without any force acting on you. You’re not accelerating, you’re sitting still.
Newton’s second law is the relation
between acceleration and force.
Acceleration is proportional to force
Mass is measured in kilograms (kg).
Acceleration is measured in
meters/second
2(m/s
2)
Therefore, the unit of force, the Newton,
can be found from Newton’s second
law.
Weight
Close to the surface of the Earth, where the
gravitational force is nearly constant, the weight is:
What is the weight of a 25 kg object located
Mass vs. Weight
Mass is a measure of the inertia of an
object or the amount of matter it contains.
Weight is the force exerted on that object
by gravity.
If you go to the moon, whose
gravitational acceleration is about 1/6 g
,
An object at rest must have no net force on it.
If it is sitting on a table, the force of gravity is still there; what other force is there?
The force exerted perpendicular to a surface
is called the normal force. It is exactly as large as needed to balance the force from the object
If the normal force gets too big, something
breaks!
Resting Force
An 8 N vase is sitting on a table.
Weight and Normal Force
Since the normal force on a surface must
The symbol "Σ" means "the sum of".
Sometimes ΣF is written as Fnet, it means the same thing.
It means you have to add up all the forces acting on an object.
The arrow above "F" reminds you that force is a vector. We won't always write the arrow but remember it's there.
It means that when you add forces, you have to add them like vectors: forces have direction, and they can cancel
Two forces act on an object. One force is 40N to
the west and the other force is 40N to the east. What is the net force acting on the object?
Two forces act on an object. One force is 8.0 N to
Newton’s Third Law
Whenever one object exerts a force on a
Drawing Free Body Diagrams
1. Draw and label a dot to represent the first object.
2. Draw an arrow from the dot pointing in the
direction of one of the forces that is acting on that object. Label that arrow with the name of the force.
3. Repeat for every force that is acting on the
object. Try to draw each of the arrows to roughly the same scale, bigger forces getting bigger arrows.
4. Draw a separate arrow next to your free body diagram indicating the likely direction of the
Sample Problem
A blimp hovers in the sky.
Draw a free body diagram.
Sample Problem
A boy pulls a sled along (with constant
velocity) on a string.
Draw a free body diagram.
Sample Problem
A man accelerates a crate along a rough
surface.
Draw a free body diagram.
Kinetic Friction Force
On a microscopic scale, most surfaces are
rough. The exact details are not yet known, but the force can be modeled in a simple way.
For kinetic – sliding friction, we write:
μk is the symbol for kinetic friction, and is
Static Friction Force
Static friction is the frictional force between two
surfaces that are not moving along each other.
Static friction keeps objects from moving when a
force is first applied:
μ
s is the symbol for static friction, and is different for every pair of surfaces.
The static friction force is greater than or equal
Sample Question
Two people push the back of a car with an
applied force of 2000 N so it accelerates at 2 m/s2. What is the force of friction between the
Sample Question
Someone pushes a dresser with an applied
force of 500 N so it accelerates at 3 m/s2.
Tension
When a cord or rope pulls on an
object, it is said to be under tension, and the force it exerts is called a
tension force, FT.
A 25 kg lamp is hanging from a rope.
Putting It Together
An 1800 kg elevator moves up and down on a
cable. Calculate the tension force in the cable for the following cases:
a)the elevator moves at a constant speed upward.
b)the elevator accelerates upward at a rate of 2.4 m/s2.
c)the elevator accelerates downward at a rate of 2.4 m/s2.
Two blocks, with masses m
1 = 400 g and m2 =
600 g, are connected by a string and lie on a frictionless tabletop. A force F = 3.5 N is
applied to block m2.
a. Find the acceleration of each object.
b. Find the tension force in the string between
A 12 kg load hangs from one end of a rope that
passes over a small frictionless pulley. A 15 kg
counterweight is suspended from the other end of the rope. The system is released from rest.
a.Find the acceleration of each mass.
A 500 g block lies on a horizontal tabletop with
negligible friction. The block is connected by string to the second block with a mass of 300 g. The string
passes over a light frictionless pulley as shown above. The system is released from rest.
Find the acceleration of the system by simultaneously
solving the system of two equations.
Hooke’s Law
Robert Hooke observed the relationship between the
force necessary to compress a spring and how much the spring was compressed.
Fspring = -kx
k represents the spring constant and is measured in N/m.
x represents how much the spring is compressed.
Sample Problem
What is the spring constant of a hanging
