# III.2. Summary - Movement and Rest.

📚 **Summary - Movement and Rest.**

**A body is in a state of motion** when the body changes its position (or distance) from the chosen landmark.

**A body is at rest** when the body does not change its position (or distance) from the chosen landmark.

**The average speed (v _{m})** is the physical parameter equal to the ratio between the distance traveled (d) and the duration of the movement (Δt).

**Characterization of the average speed as physical parameter:**

**In uniform rectilinear motion:**

**- The mobile travels equal distances in equal time intervals.**

**- The current speed is equal to the average speed.**

**- Speed maintains its numerical value, direction and sense of movement.**

**The law of uniform rectilinear motion:**

**x = x _{0} + v (t - t_{0}) when the mobile moves away from the landmark**

**x = x _{0} - v (t - t_{0}) when the mobile is approaching the landmark.**

*Using the motion graph we can find more information about how a mobile moves:*

the graph of rectilinear and uniform motion is a straight line.

if the mobile is at rest, the motion graph is a horizontal line.

to draw the graph of the motion it is enough to represent at least two points that together, will determine the right of the graph.

the straight line representing the rectilinear and uniform motion is more inclined (forming a larger angle with the horizontal) when the speed is higher.

**The average acceleration (a _{m} sau a)** is the physical parameter equal to the ratio between the variation of the speed (its increase or decrease) and the duration in which this variation occurred.

**Characterization of average acceleration as physical parameter:**

**The uniformly varied rectilinear motion** is the motion of a mobile on a rectilinear trajectory with constant acceleration.

**In uniformly accelerated rectilinear motion:**

The mobile travels longer and longer distances at equal intervals.

The momentary acceleration is equal to the average acceleration, being constant.

Acceleration and speed have the same direction and sense as that of motion.

Remark:

The best known example of

uniformly accelerated rectilinear motionis the free fall of bodies to Earth.All bodies fall to Earth with a constant acceleration, called gravitational acceleration, g = 9,81 m/s^{2}.When the body falls free, its initial velocity is v_{0}= 0 m/s.

📋 **Self-assessment sheet solved - Movement and rest**

**1. What is the mechanical condition of a TV compared to the table. -1p**

**2. What can you say about the movement of a mobile whose speed increases and takes a curve? -1p**

**3. The movement data of the mobile 1 are entered in the below table.**

a) Determine the positions of the mobile M_{2} for the times passed in the table, using its law of motion: x_{M2} = 3 ∙ t. **-1p**

b) Make the motion graph of the mobile M_{1}. Use as standards: 1 cm : 10 m și 1 cm : 5 s. **-1,5p**

c) Determine the position of the mobile M_{1} at the time of 25 min, on the graph. **-0,5p**

d) Determine the average speed of the mobile M_{1}. **-1p**

**4. Transforms: v = 300 mm/min în m/s -1p**

**5. A body is let loose for 6s. Knowing the acceleration of 10 m/s ^{2}, find out the final speed of the body. -1p**

**Ex officio -2p**