Section 4.3 Uniform Motion

Section 4.3 Uniform motion

• Motion graphs for uniform motion

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4.3 Uniform motion (SB p. 14)

Uniform motion a body moves at constant speed along a straight line ⇒ uniform motion uniform motion

constant velocity

s Displacement = Velocity × Time taken s=vt

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4.3 Uniform motion (SB p. 14)

Motion graphs for uniform motion Displacement-time graph (s-t graph) and Velocity-time graph (v-t graph) s-t graph v-t graph

25

A car travels a displacement of 25 m in 1 s © Manhattan Press (H.K.) Ltd.

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4.3 Uniform motion (SB p. 14)

Displacement-time graph slope of s-t Change in displacement = Change in time graph ∆s = ∆t = Velocity(v) ∆s Velocity = ∆t = 75 − 25 3 −1 = 25 m s−1 © Manhattan Press (H.K.) Ltd.

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4.3 Uniform motion (SB p. 15)

Velocity-time graph slope of = velocity = constant s-t graph The corresponding v-t graph is: 25

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4.3 Uniform motion (SB p. 15)

Motion graphs for uniform motion At rest Body at distance (d) from a reference point

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4.3 Uniform motion (SB p. 15)

Motion graphs for uniform motion Two bodies A and B at different uniform velocities, vA > vB

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4.3 Uniform motion (SB p. 16)

Example 3: The displacement-time graph of a dog running along a straight road is shown.

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4.3 Uniform motion (SB p. 16)

Example 3: (Cont) (a) Find its velocities from 0 s to 4 s, and from 4 s to 6 s. Solut The velocity is the slope of the s-t graph. ion From 0 s to 4 s, 3 −1 v= = 0.5 m s -1 4 From 4 s to 6 s,

0−3 v= = −1.5 m s -1 6−4

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4.3 Uniform motion (SB p. 16)

Example 3: (Cont) Solut ion (i) At 0 s, the dog is 1 m from the reference position. From 0 s to 4 s, its displacement increases uniformly with time. The dog runs at a constant velocity of 0.5 m s−1 for 2 m.

(b) Explain the shape of the graph.

(ii) From 4 s to 6 s, its displacement decreases uniformly with time. The dog returns and runs at a constant velocity of 1.5 m s−1 for 3 m. (iii) As the slope of line BC is steeper than the slope of line AB, the dog runs at a higher speed. (iv) When the dog returns at B, its direction of motion reverses. Therefore, the slopes of lines AB and BC are of opposite signs. © Manhattan Press (H.K.) Ltd.

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4.3 Uniform motion (SB p. 16)

Example 3: (Cont) (c) Draw the corresponding velocity-time graph for the motion. Solut ion

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To section 4.4

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