Download Ch 5 Study Guide

Chp. 5: Laws of Motion
Study Guide
Force
• A push or pull on an object.
• Represented in Newtons (N)
Balanced Force
• Forces acting on an object that combine and form a net force
of zero.
• Usually object doesn’t move because forces on each side are
equal to each other.
Examples of Balanced Forces
• A book sitting on a desk
• A car parked in a garage
• Sitting on a chair
Unbalanced Force
• Forces acting on an object that combine and form a net force
that is not zero.
• Object is moving because forces on each side are not equal to
each other.
Examples of Unbalanced Forces
• Kicking a football through the uprights
• Rocket taking off
• Rock rolling down a hill
Net Force
• The combination of all forces acting on an object.
• If two forces on an object are moving in the same direction
then the net force is the sum of the two forces.
• Ex. Two people are pushing a car with forces of 200N and
100N to the right, the net force is 300N (200N+100N=300N)
-In this case this is an unbalanced force because the net force is
not zero.
Net Force continued
• Ex. Two people are pulling on one side of rope with a force of
10N and two other people are pulling on the rope with a force
of 12N, the net force is 2 N to the right (12N-10N=2 N)
-In this case this is a unbalanced force because the net force is
zero, and also a direction is given (right) because more force is
being pulled to one side.
Net Force continued
• If two forces on an object are moving in opposite directions
then the net force is the difference between the two forces.
• Ex. Two people are pushing on each other’s hands with forces
of 200N and 200N against each other, the net force is 0 N
(200N-200N=0 N)
-In this case this is a balanced force because the net force is zero
Newton’s First Law of Motion
• An object at rest will remain at rest, and an object in motion
will remain in motion unless acted upon by an unbalanced
force.
• This is also called the law of inertia because inertia means that
an object will resist a change in motion.
• Ex. A car crashes into a wall and the car crash dummy’s body
will continue to move forward resisting the change in motion
of the car moving to all of a sudden stopping after hitting the
wall.
Newton’s First Law of Motion
Examples
• A soccer ball does not move until someone kicks.
• A skateboarder hits a rock with his board. The
board stops but the boy flies off.
• A tennis ball is rolled on the floor.
Newton’s Second Law of Motion
• The size of a force and the mass of an object affect its
acceleration. The greater the force, the greater the
acceleration; the greater the mass, the greater the force
needed to move the object. The relationship among the force,
mass and acceleration can be written in one equation: Force =
Mass x Acceleration.
Newton’s Second Law of Motion
Examples
• A rock with more mass will accelerate less unless more
force it put on it.
• You have a box of books and a box of tissue boxes. You
push the box of books with more force to get it to move.
• One boy throws a football farther than another boy
because he used more force.
Newton’s Third Law of Motion
• When one object exerts a force on a second
object, the second object exerts an equal force in
the opposite direction of the first object.
• For every action, there is an equal and opposite
reaction.
Newton’s Third Law of Motion
Examples
• The force of the cannon ball being pushed out of the
cannon makes the cannon move backwards while the
cannon ball moves forward
• If air is let out of a balloon quickly, air pushes down &
balloon goes up.
• A fireman turns on his hose & is knocked backwards.
Momentum
• The measure of hard it is to stop a moving
object.
• Formula: SI Unit = kg m/s
Momentum(p)=mass(kg) x velocity(m/s)
• Example: What is the momentum of a
200-kg motorcycle traveling at 10 m/s?
p=m x v
200 kg x 10 m/s = 200 kg m/s
Law of Conservation of Momentum
• The total momentum of a group of objects stays
the same unless outside forces act on the
objects.
• Ex. If two pool balls collide with each other their
momentum stays the same as the original
momentum but they move in a different
direction.