Section 3: Motion and Forces

Chapter 2: Motion

Warm-up:What could you do to change the velocity of a soccer ball?

Learning GoalsExplain how motion and force are related.

Describe what inertia is and how it is related to Newton’s first law of motion.

Identify the forces and motion that are present during a car crash.

ForceForce: A push or a pull

ForceSometimes it is obvious that a force

is being applied. Kicking a soccer ball

Sometimes it is not obvious Force of the atmosphere pushing

against you

ForceForces can cause the motion of an object to change.Example: hitting a baseball

Measure force in Newtons (N)

Net ForceForce doesn’t always change velocity.

Net ForceNet Force: the sum of the forces acting upon an object.

Net ForceBalanced Forces: forces on an object that are equal in size but opposite in direction. Net force = zero Object does not move

Net ForceUnbalanced Forces: unequal

forces acting on the same object. Object moves in the direction of the

larger force

10 N

10 N

10 N

4 N

4 N8 N 4 N4 N

Inertia and MassInertia: the tendency of an object to resist any change in its motion. If an object is moving, it will keep moving at the same speed and in the same direction unless an unbalanced force acts on it.

Inertia and Mass If an object is at rest, it tends to remain at rest unless an unbalanced force acts on it.

Inertia and MassWhich has greater inertia, a truck or a hot wheels car?

Inertia and MassA greater force is needed to move the truck than the hot wheels car.

The inertia of an object is related to its mass. The greater the mass of an object, the greater its inertia.

Inertia and MassNewton’s First Law of Motion: an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it.Sometimes called the law of inertia.

Inertia and Mass When a car traveling

about 50 km/h collides head-on with something solid, the car crumples, slows down, and stops within about 0.1 s.

Inertia and MassWhat happens to any passenger not wearing a safety belt?They continue to move forward at the same speed the car was traveling.

Check-in:What is the net force on a refrigerator if you push on the refrigerator and it doesn’t move?