Chapter 12

Forces and Motion

Section 1

Forces

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?

• In what direction does Earth’s gravity act?

• Why does a projectile follow a curved path?

What Is a Force?

• A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

• A push or a pull

Measuring Force

• using a spring scale

The downward force arrows represent the weight (a type of force) on the scales. The dial indicator gives a visual measure of the weight.

You can use an arrow to represent the direction and strength of a force.

The direction of the arrow represents the direction of the force.

The length of the arrow represents the strength, or magnitude, ofthe force.

Units of Force

• Force is measured in newtons (N)

• One newton is the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/s2).

Combining Forces

• Forces in the same direction add together.

• Forces in opposite directions subtract from one another.

Adding forces

+ =

Subtracting forces

- =

Balanced Forces

• When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion.

3 In this tug of war, thetwo groups pull with equal forces in opposite directions. The forces combine by subtracting from each other

+ = 0

Unbalanced Forces

• When an unbalanced force acts on an object, the object accelerates.

• Often, the forces on an object are unbalanced.

• An unbalanced force is a force that results when the net force acting on an object is not equal to zero.Adding forces

+ =

Subtracting forces

- =

Friction

• Friction, a force that opposes the motion of objects that touch as they move past each other

• There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction.

Static Friction

• The friction force that acts on objects that are not moving.

• Static friction always acts in the direction opposite to that of the applied force.

• You experience static friction every time you take a step. As you push off with each step, static friction between the ground and your shoe keeps your shoe from sliding.

Staticfriction Push

Static friction acts opposite the direction of the force you apply to move theplant.

Sliding Friction

• A force that opposes the direction of motion of an object as it slides over a surface.

• Because sliding friction is less than static friction, less force is needed to keep an object moving than to start it moving.

Slidingfriction

Potted treeaccelerates.

When you push with more force, thepotted tree begins to slide.

Push

Rolling Friction

• The friction force that acts on rolling objects.

• For a given set of materials, the force of rolling friction is about 100 to 1000 times less than the force of static or sliding friction. Ball bearings

in these wheels greatlyreduce friction byreplacing sliding frictionwith rolling friction.

Fluid Friction

• Force that opposes the motion of an object through a fluid.

• Water and a mixture of gases such as air are known as fluids.

• You feel fluid friction when stirring thick cake batter.

• Fluid friction acting on an object moving through the air is known as air resistance.

• At higher speeds, air resistance can become a significant force.

Gravity• A force that acts between any two masses.• Gravity is an attractive force, that is, it pulls

objects together.• gravity does not require objects to be in contact

for it to act on them• Earth’s gravity acts downward toward the

center of Earth.• an upward force usually balances the downward

force of gravity

Gravity

Supporting force

Falling Objects

• Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.

• Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity.

Gravity

Air resistance

This flyingsquirrel takesadvantage of airresistance to slow itsfall and increase thedistance covered inthe jump.

Projectile Motion

• When you throw a ball forward, you’ll notice that it actually follows a curved path.

• projectile motion, the motion of a falling object (projectile) after it is given an initial forward velocity.

• Air resistance and gravity are the only forces acting on a projectile.

• The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.

Although their massesare different, the blue and greenballs fall at the same rate.

The yellow ball is a projectile, following a curved path. The two balls fall with thesame acceleration and strike the ground at the same time.

Reviewing Concepts

• 1. How is the motion of an object affected when a force acts on it?

• 2. List the four types of friction.• 3. How does air resistance affect the

acceleration of a falling object?• 4. Earth’s gravitational force acts in what

direction?• 5. Describe why a projectile follows a

curved path.