Motion, Acceleration, and Forces

Physical Science2012-2013

MotionSection 3-1

• Motion is a change in position of an object

Distance vs. Displacement

Distance

• The total length of the path an object follows:

Displacement

• The distance and direction of an objects final position from its initial position

• Displacement includes both a length/size and direction and is called a vector

What is the actual displacement?

Speed

• The distance an object travels per unit time.• We use metric (SI) units and it is meters per

second (m/s)• Usually the speed of an object changes as it

moves from one place to another

Instantaneous Speed

• The speed of an object at a single point in time

• Speedometer

Average Speed

• The total distance traveled divided by the total travel time

___ V = d/t

____ V = average speed d=distance t=time

Practice Problems

• What is the average speed of a car that travels a distance of 750m in 25s ?

• What is the average speed of a motorcycle that travels a distance of 300m in 10s ?

• What is the average speed of an airplane traveling a distance of 1000m in 10s ?

Velocity

• The speed of an object and the direction of the motion

• Objects have different velocities if they are moving in different directions or a different speeds

Graphing VelocityTime vs. Distance Graph

AccelerationSection 3-2

• Acceleration is the change in velocity divided by the time it takes that change to occur

Changing Direction

• A change in velocity can be either a change in how fast something is moving or a change in the direction of the movement

Calculating Acceleration

• When an object moves from one place to another, it might speed up, slow down, and change directions many times.

• Each change in velocity causes the acceleration of the object to change

Acceleration Equation

Calculation Positive Acceleration

• An airplane starts at rest and moves down the runway in a single direction. After accelerating for 20s it reaches a speed of 80m/s. What is the acceleration?

Calculating Negative Acceleration

• A car is moving in a straight line at a constant sped of 3m/s and comes to a complete stop in 2s. What is the acceleration?

Motion and ForcesSection 3-3

What is Force?

• Force is a push or pull that one object exerts on another

• Force can cause the motion of a object to change.

Baseball, Tennis, and Billiards

Balanced Forces

• Force does not always change velocity, two or more equal forces acting on an object in opposite directions can cancel each other out. The net force is zero.

Unbalanced Forces

• Unequal forces applied in opposite directions lead to a net force in the direction of the larger force and movement in that direction

Friction

• The force that opposes the sliding motion of two surfaces that are in contact

What Causes Friction?

• Frictional force depends on the materials that the surfaces are made of and the roughness of the two surfaces

• All surfaces have bumps and dips in them that can cause contact and thereby friction (microwelds)

Frictional Force

• FF increases when the force pushing the surfaces together increases.

Static Friction

• The frictional force that prevents two surfaces in contact from sliding past each other. Once the applied force is greater, the object will move.

Rolling Friction

• Rolling friction is usually much less that static friction, that is why we use wheels!

Air Resistance

• This force opposes the motion of objects that move through the air.

• Air resistance acts in the opposite direction of the objects movement

• The greater the surface area of the object the greater the power of air resistance on that object

More surface area means a slower fall

Terminal Velocity

• As an object falls it accelerates due to gravity, however air resistance eventually steps in to slow the object to a speed that no longer increases…that equaled out speed is called terminal velocity!