# Chapter 2: Forces 2.1: Forces change motion.

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Chapter 2: Forces 2.1: Forces change motion

Vocabulary A push or a pull is called a ____________. force

Vocabulary A ___________________ is when one object pushes or pulls another object by touching it. The first object is applying this force to the second object. contact force The ground produces a contact force on the skier as she pushes against the ground.

Vocabulary Gravity masses
_______________ is the force of attraction between two masses. The strength of the gravitational force between two objects depends on their _______________. Gravity Gravity pulls the balls toward the ground. masses

Vocabulary _______________ is a force that resists motion between two surfaces that are pressed together. Friction There is friction between the wheels and the ground.

Types of Forces Gravity: Friction: Contact Force:
Pulls the skater toward the ground. Contact Force: The skater applies a contact force as he pushes against the ground. The ground applies a contact force that pushes the skater forward. Friction: There is friction between the ground and the blades.

Friction Which surface would produce more friction? How would that affect the motion?

YES – it has both size and direction!
Thinking Question Is force is a vector? YES – it has both size and direction!

How to read a force diagram…
Red arrows show size and direction of force. Blue boxes show mass

Vocabulary The overall force acting on an object when all the forces are combined is called the _____________________. net force

Balanced Forces zero no
If the net force on an object is _______________, the forces acting on it are balanced. 2. Balanced forces have the same effect as ________ forces at all. no

Unbalanced Forces Only an unbalanced force can change the ____________ of an object. motion

No – if the forces are balanced it will not move.
Thinking Question Does an object always move when a force acts on it? No – if the forces are balanced it will not move.

Quick Review of Balanced and Unbalanced Forces
__________________ forces cannot change an object’s speed or direction. An __________________ force is needed to change an object’s motion. Balanced Was I a balanced or unbalanced force? unbalanced Unbalanced – I caused motion!

Forces on Moving Objects…
To increase the speed of your bike, you may exert ___________ forward force by pedaling harder or changing gears. The net force moves the bike faster. To turn your bike, you apply an ____________ force by leaning to one side and turning the handlebars. To stop the bike, you use the extra force of ____________ that your brake pedals provide. more unbalanced friction

Vocabulary An object at rest stays at rest, and an object in motion stays in motion unless acted upon by another force is known as _______________________________. Newton’s first law

An object at rest remains at rest unless acted upon by an unbalanced force.
Unbalanced force (from the foot) Object at rest (ball)

Object in motion (ball)
An object in motion remains in motion with the same velocity, unless acted upon by an unbalanced force. Unbalanced force (from the hand) Object in motion (ball)

What unbalanced forces change the motion of a volleyball that is hit hard over the net? How will its velocity change? The contact force of a hand changes the ball’s direction, and may increase its velocity.

Vocabulary The resistance of an object to a change in the speed or the direction of its motion is known as _____________. inertia When you measure the _________ of an object you are also measuring its inertia! mass

How would inertia affect a person in the back seat of this car?
The passenger will continue to move forward and will hit the back of the front seat.

Watch the video on Newton’s First Law