Unit II Physical Science

Motion and Forces

Distance vs. Displacement

• What two changes help you identify that an object is in motion?– Change in Direction– Change in Speed

_______________ is necessary to determine if something is in motion.

Frame of Reference: background that appears to be stationary to a moving object

Earth is the most common frame of reference.

How can something appear to be motionless but actually be moving?

• If an object has the same frame of reference.

• If the object is moving at the same speed.

• Examples?

Dist. Vs. Disp.

• Distance– Actual path that an

object takes– Cannot be negative

• Displacement– Straight line from start

to finish– Can be negative or

zero

Think about your charts made to calculate distance and displacement on the grid.

Explain how an object can have a zero or negative displacement.

Determine the displacement for A and B below.

12 km 6 km

Calculating Distance and Displacement

• Pick a starting point on one side of the room, and an ending point on the other side.

• Have your partner travel to that point. • Draw a map on a separate sheet of paper. • Using a meter stick, measure the distance

traveled and either calculate or measure the displacement. (remember Pythagorean theorem)

• Show both distance and displacement on your map.

Speed and Velocity

• Speed: the rate at which an object changes position

Represented by v in a formula.

• m/s : SI unit for speed

Instantaneous vs. Average

• Instantaneous Speed:

speed at a particular point in time

Ex: Speedometer of a car

• Average Speed:

speed taken over a distance traveled

Ex: Calculating your speed over an entire trip

Speed = distance / timev = d/t

What is a good way to represent and describe motion?

A distance vs. time graph

What does the slope of the line in a distance vs. time graph tell you?

slope = rise/run

speed = rise (d)/ run (t)

How is velocity different than speed?

• Velocity is speed with a direction.

Ex: traveling 60 km/hr North

Two ways to change velocity:

1. Change speed (speed up/slow down)

2. Change direction (turn)

Example Problems

• A car travels 85 km from town A to town B, then 45 km from town B to town C. The total trip took 1.5 hrs. What was the average speed of the car?

Ex. Continued

• A bicyclist travels for 1.5 hrs at an average speed of 32 km/hr. How far does the bicyclist travel in that time?

What can you say about the motion of the car in the graph from 8-12 seconds?– The car is

stopped

If you were paddling down river at 16 mph and the river moved at 10 mph, how fast would you actually

be moving? What if you were paddling up river?

Acceleration

• Think of a situation where you were accelerating. What was happening in that situation?

Examples:

Acceleration

• Acceleration: is the rate of change in velocity

Acceleration = change in velocity

time

a = vf – vi

time

• SI unit of acceleration = m/s/s or m/s2

Free Fall

What happens to an object during free fall?

The object accelerates at a constant speed

Acceleration due to gravity = 9.8 m/s/s

For every second an object falls it will move 9.8 m/s faster

Ways to Change Acceleration

• Change Speed

• Change Direction

Can you have negative acceleration?

yes if the object is slowing down

(deceleration)

v vs. t Graph

• What is the difference between a linear speed vs. time graph and an non linear speed vs. time graph?

Linear = constant acceleration

Nonlinear = changing acceleration

Time

Speed

Example Problems

1. A car traveling at 10 m/s starts to decelerate steadily. It comes to a complete stop in 20 s. What is its acceleration?

Ex. Continued

2. An airplane travels down a runway for 4.0s with an acceleration of 9.0 m/s/s. What is its change in velocity during this time?

Ex. Continued

3. A boy throws a rock up into the air. It reaches the highest point of its flight after 2.5 s. How fast was the rock going when it left the boys hand?

4. What is the acceleration from 10 – 20 seconds in the graph below?

0 - 5 = 0.5 m

10 s2

Speed

• Graph the following table representing a car trip. Make sure to put the correct labels and units making it a complete graph.

1. What was the average speed for the entire trip

2. When was the car traveling at the greatest speed?

3. What happened between the 3rd and 4th hours

4. Give an example of a dependent variable and explain why it is a dependent variable.

Distance (km)

Time (hours)

40 1

100 2

170 3

170 4

210 5

1.Which graph shows periods of constant speed? Explain your answer.• Graph A shows periods of constant speed (0–8 s,

8–12 s, 12–20 s).

2.Describe the motion of the object in Graph A.• The object moves at constant speed for 8 seconds, is

at rest for the next 4 seconds, and then moves at constant speed for the next 8 seconds.

3.Which graph in Figure 11-2 shows acceleration? How do you know?• Graph B shows acceleration. The upward slope of

the line indicates that an increasing distance is covered each second.

4.Using Graph A in Figure 11-2, calculate the average speed of the object in motion from 12 s to 20 s. Explain your calculation.• The object moved a distance of 300 m in 8 s. The

object’s average speed is 37.5 m/s.

5.Compare Graphs A and B in Figure 11-2. At a time of 2 seconds, which graph shows a greater velocity? How do you know?• Graph A; the slope is steeper.