# Section 3: Motion and Forces

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Section 3: Motion and Forces
Chapter 2: Motion Section 3: Motion and Forces

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

Learning Goals Explain 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.

Force Force: A push or a pull

Force Sometimes 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

Force Forces can cause the motion of an object to change.
Example: hitting a baseball Measure force in Newtons (N)

Net Force Force doesn’t always change velocity.

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

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

Net Force Unbalanced Forces: unequal forces acting on the same object.
Object moves in the direction of the larger force

4 N 10 N 10 N 10 N 4 N 8 N 4 N 4 N

Inertia and Mass Inertia: 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 Mass Which has greater inertia, a truck or a hot wheels car?

Inertia and Mass A 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 Mass Newton’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 Mass What 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?

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