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Ch. 10
What is a force
• When you ride a bike, your foot pushes
against the pedal. The push makes the wheels
of the bike move.
• When you drop something, it is pulled to the
ground by gravity.
What is a force
• Is a push or a pull.
• a force is described by a push or a pull in
particular direction.
• Forces are measured in Newton.
• Named after Isaac Newton.
• You exert one Newton of force when lifting a
• What SI unit measure forces?
Can make objects move
Can make objects move faster
Can make objects move slower
Can make objects stop moving
Can make objects change direction
Can make objects change shape
Combining forces
• Often, more than a single force acts on an object
at one time.
• A combination of forces on an object is called the
net force.
• When forces that act in the same direction, the
net force can be found by adding the strengths of
the individual forces.
• When forces act in opposite directions, they also
combine to produce a net force. (subtract)
Combining Forces
Unbalanced Forces
• Whenever there is a net • Top is balanced
force acting on an
• Bottom is unbalanced
object, the forces are
• These forces can cause
an object to start
moving, stop moving, or
change direction.
Balanced Forces
• Equal forces are acting on one object in
opposite directions are called balanced forces.
Answer the following Questions
Find the Net Force?
10 N
10 N
32 N
Section 2
Friction and Gravity
• Friction is the force that two surfaces exert on
each other when they rub together.
• The strength of the force of friction depends
on two factors.
How hard they push together and they type of
surfaces involved.
A smooth surface is much easier to slide on.
• A force that pulls two objects towards each
• Two factors that affect gravity
1. Mass- the more mass the greater the
gravitational pull.
2. Distance- the farther apart two objects are
the less gravitational pull.
Section 3
Newton’s 1st law of motion
or law of inertia
• An object at rest will remain at rest, and an
object at a constant velocity will stay at a
constant velocity unless acted on by an
unbalanced force.
• Inertia is the tendency of an object to resist
Newton’s 2nd law
• Acceleration depends on the object’s mass on
the net force acting on the object.
• Acceleration=net force/ mass
• Net force = mass X acceleration
• A speedboat pulls a 55 kg water skier. The
force causes the skier to accelerate at 2.0
meters per second per second. Calculate the
net force that causes this acceleration.
Newton’s 3rd law
• States that if one object exerts a force on
another object, then the second object exerts
a force of equal strength in the opposite
direction on the first object.
• For every action there is an equal and
opposite reaction.
• Characteristic of a moving object that is
related to the mass and the velocity of the
• Momentum of a moving object can be
determined by multiplying the object’s mass
and velocity.
• Momentum = mass X velocity
• Which has more momentum: a 3.0 kg
sledgehammer swung at 1.5 m/s, or a 4.0 kg
sledgehammer swung at 0.9 m/s?
Conservation of Momentum
• Conservation refers to the conditions before
and after some event.
• This law states that, in the absence of outside
forces, the total momentum of objects that
interact does not change.
• It is the same before and after they interact.
• Look at page 358 in your book.