Honors- Motion notes

• Motion is an object’s change in position relative to a reference point.

• Displacement is the change in the position of an object.

• Distance measures the path taken.
• Displacement is the change of an object’s position. Displacement must always indicate direction.
• Speed is the distance traveled divided by the time interval during which the motion occurred. Speed describes how fast an object moves.

• Speed measurements involve distance and time.

• The SI units for speed are meters per second (m/s).
• When an object covers equal distances in equal amounts of time, it is moving at a constant speed.
• Speed can be studied with graphs and equations.

• Speed can be determined from a distance-time graph. When an object’s motion is graphed by plotting distance on the y-axis and time on the x-axis, the slope of the graph is speed.
• Average speed is calculated as distance divided by time.

Equation for Average Speed

Speed = distance /time (v = d/t)

• Instantaneous speed is the speed at a given point in time.

VelocityMetal stakes are sometimes placed in glaciers to help measure a glacier’s movement. For several days in 1936, Alaska’s Black Rapids glacier surged as swiftly as 89 meters per day down the valley. Find the glacier’s velocity in m/s. Remember to include direction.

1. List the given and the unknown values.

Given: time,t = 1 day

displacement,d = 89 m down the valley

Unknown: velocity, v = ? (m/s and direction)

Perform any necessary conversions.

To find the velocity in meters per second, the value for time must be in seconds.

t = 1 day = 24 hours so 24 times 60 minutes per hour times 60 seconds in each minute = 24 X 60 X60 = 86, 400 seconds in one day

= .001m/s downhill (best expressed as 1.0 x 10 -3)

Veolocity

• Velocity is the speed of an object in a particular direction.

• Velocity describes both the speed and the direction.

• Combine velocities to determine the resultant velocity.
• Acceleration can be a change in speed.

• Acceleration can be a change in direction.

• Uniform circular motion is constant acceleration.

• Acceleration is the rate at which velocity changes.

Acceleration Equation (for straight-line motion)

• In SI units, acceleration is measured in meters per second per second (m/s/s) or m/s2.

AccelerationA flowerpot falls off a second-story windowsill. The flowerpot starts from rest and hits the sidewalk 1.5 s later with a velocity of 14.7 m/s. Find the average acceleration of the flowerpot.

1. List the given and the unknown values.

Given: time,t = 1.5 s

initial velocity,vi = 0 m/s

final velocity,vf = 14.7 m/s down

Unknown: acceleration, a = ? (m/s2 and direction)

Insert the known values into the equation, and solve.

a = vf – vi = 14.7m/s -0 m/s = 9.8 m/s/s

t 1.5 s

The Force of Friction

• Friction is a force that opposes motion between two surfaces that are in contact.

• Friction opposes the applied force.
• Static friction resists the initiation of sliding motion between two surfaces that are in contact and at rest.

• Kinetic friction opposes the movement of two surfaces that are in contact and are sliding over each other.

• Static friction is greater than kinetic friction.
• There are many different types of kinetic friction, such as sliding friction and rolling friction.
• Fluid friction, such as air resistance, also opposes motion.
• Friction can be helpful or harmful.
• Friction is necessary to roll a vehicle or hold an object.
• However, friction can also cause excessive heating or wear of moving parts.

• Harmful friction can be reduced.

• Helpful friction can be increased.