1. Motion
Chapter 11

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

3. Distance vs. Displacement

4. Speed - distance traveled divided by the time interval during which the motion occurred. Speed describes how fast an object moves. S = d / t
Speed measurements involve distance and time.
The SI units for speed are meters per second (m/s).
constant speed -when an object covers equal distances in equal amounts of time, it is moving at a
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.

5. Average speed - calculated as distance divided by time.
Instantaneous speed - the speed at a given point in time.
Velocity is the speed of an object in a particular direction.
Velocity describes both the speed and the direction.
Resultant velocity - combined velocities

6. List the given and the unknown values.
Velocity Metal 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.
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)

7. 2. Perform any necessary conversions.
To find the velocity in meters per second, the value for time must be in seconds.

8. 3. Write the equation for speed.
4. Insert the known values into the equation, and solve.

9. Acceleration is the rate at which velocity changes over time.
An object accelerates if its speed, direction, or both change.
Acceleration can be a change in speed.
Acceleration can be a change in direction.
Uniform circular motion is constant acceleration

10. Acceleration is the rate at which velocity changes.
Acceleration Equation (for straight-line motion)
acceleration is measured in meters per second per second (m/s/s) or m/s2

11. 1. List the given and the unknown values.
Acceleration A 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)

12. 2. Write the equation for acceleration.
3. Insert the known values into the equation, and solve.

13. Acceleration can be determined from a velocity-time graph.

14. Force - an action exerted on a body in order to change the body’s state of rest or motion. Force has magnitude and direction.
The net force is the combination of all forces acting on an object.
balanced forces - objects either do not move or move at constant velocity.
unbalanced force - must be present to cause any change in an object’s state of motion or rest.

15. Friction is a force that opposes motion between two surfaces that are in contact.
Friction opposes the applied force.
Types of friction:
Static
Kinetic
Sliding
Rolling
fluid
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.

16. 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.

17. Frictional Forces & Acceleration

18. 1. A bicyclist traveling at 10 m/s applies her brakes, reducing her velocity to 5 m/s. If it takes 2 seconds to slow the bike, what is the acceleration during that period?
A. –5 m/s2
B. –2.5 m/s2
C. 0 m/s2
D. 2.5 m/s2
B m/s2

19. A. The object gradually slows and stops moving dueto friction.
What happens to a moving object when the forces acting on it are exactly balanced?
A. The object gradually slows and stops moving dueto friction.
B. The object moves at a constant speed, neither speeding up nor slowing down.
C. The object accelerates because there is no friction to oppose the forces acting on it.
D. The object accelerates because the force of gravity is stronger than friction.
B. The object moves at a constant speed, neither speeding up nor slowing down

20. 3. For which of the following is the velocity constant?
A. a baseball traveling away from a bat after a hit
B. a bicyclist on an oval track moving at a constant speed of 15 km/h
C. a helicopter hovering motionless above a fixed point on the ground
D. a canoe being carried down a winding river by a steady current
C. a helicopter hovering motionless above a fixed point on the ground

21. 4. When describing the motion of an object, why do you need a reference point?
5. A jet airplane must use a substantial amount of fuel to keep flying at a constant velocity. What two forces would change the velocity of the plane if the engines were shut off?
4. To describe the motion of an object, you need to compare the object’s position to a reference point to determine how the position is changing.
5. Air resistance and the force due to gravity would change the velocity of the plane.