What are the four main types of friction? 12.1 Key Concepts How do forces affect the motion of an object? What are the four main types of friction? How do gravity and air resistance affect a falling object? Why does a projectile follow a curved path?

The wind pushes against the man and his umbrella The wind pushes against the man and his umbrella. The push from the wind is a _____________ force

What Is a Force? A force is a _________ or a _________ that acts on an object. push pull A force can cause… a resting object to ______________ a moving object to __________________ move accelerate

What Is a Force? Measuring Force You may have measured forces using a _________ ___________ The stretch of the spring in the scale depends on the amount of ____________ (a type of force) acting on it. spring scale weight

What Is a Force? Units of Force Force is measured in newtons, abbreviated as ___ One newton (N) is the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/s2). 1 N = N 1 kg · m/s2

What Is a Force? Representing Force The ___________ of the arrows show relative amounts of force. In which direction does the weight (force) of the apples point? In which picture is the weight (force) greater? length downward picture B

What Is a Force? Representing Force What upward force is keeping the apples from plummeting to the ground? The spring

Combining Forces When the forces on an object are balanced, the net force is _____________, and there is no change in the object’s motion. zero When an unbalanced force acts on an object, the object _________________. accelerates

Combining Forces The net force is the ___________________ acting on an object after all the forces are combined. Forces in the same direction ___________ together. Forces in opposite directions _________________ from one another or ________________ each other out. overall force add subtract cancel

Forces can add together or subtract from one another. Combining Forces Forces can add together or subtract from one another. Adding forces Subtracting forces Equal and opposite forces

Combining Forces yes opposite zero (canceled out) Are the forces shown equal? Are they moving towards each other or in opposite directions? What is the net force? yes opposite zero (canceled out)

Friction Friction is a force that ________________ the motion of objects that touch as they move past each other. opposes There are four main types of friction: ______________ friction Static Sliding Rolling Fluid

Static Friction Static friction is the friction force that acts on objects that are ________________. Static friction always acts in the direction opposite to that of the applied force. not moving push Static friction

Sliding friction is ___________ than static friction. slides Sliding friction is a force that opposes the direction of motion of an object as it _______________ over a surface. Sliding friction is ___________ than static friction. slides less push movement Sliding friction

Rolling Friction rolling less Rolling friction is the friction force that acts on ________________ objects. The force of rolling friction is about 100 to 1000 times ______________ than the force of static or sliding friction. rolling less ______________________ in these wheels greatly reduce friction by replacing sliding friction with rolling friction. Ball bearings

Fluid Friction Fluid friction opposes the motion of an object through a _______________. Fluids are anything that _____________ Gases, liquids Fluid friction acting on an object moving through the air is known as _________ ___________________. fluid flows air resistance

Gravity is a force that acts between any _______ ________________. two Gravity causes objects to accelerate ________________, whereas air resistance acts in the direction __________________ to the motion and reduces acceleration. downward opposite Gravity is a force that acts between any _______ ________________. Gravity is an attractive force. Gravity can act over _______________ distances. two masses large

Gravity Earth exerts an attractive, downward force on this boulder. The supporting rock exerts an upward force on the boulder. The forces are balanced.

No…forces are not equal Gravity Falling Objects Terminal velocity is the ______________ velocity of a falling object when the force of air resistance ____________ the force of gravity. constant equals Has the flying squirrel reached terminal velocity? Check the size of the arrows! No…forces are not equal

Projectile Motion A thrown ball follows a ____________ path. Projectile motion is the motion of a falling object (projectile) after it is given an initial _______________ velocity. curved forward

Assessment Questions If an object is at rest, which of the following statements must be true? There are no forces acting on the object. There is no friction acting on the object. The forces acting on the object are unbalanced. The net force acting on the object is zero.

Which of the following is not a type of friction? Assessment Questions Which of the following is not a type of friction? static friction sliding friction fluid friction pull friction

In which direction does Earth’s gravitational force act? Assessment Questions In which direction does Earth’s gravitational force act? opposite the direction of motion downward toward the center of Earth upward away from the center of Earth in the direction of motion

Assessment Questions A ball thrown into the air follows a projectile course due to the initial velocity and the force of gravity. effect of air resistance. motion of Earth beneath it. mass of the ball.