7/29/2019 Cons Laws Unit Review

1/17

Conservation Laws Review

REMEMBER:

- hand in the HW before the exam

- review material on Student Data

- BONUS:

- example test (energy & p)

- conceptual test

- Practice Exam

THE KEY!!!!!

7/29/2019 Cons Laws Unit Review

2/17

Conservation Laws Review

1. Use the following number convention:

1 = Cons of ME 2 = Cons of p 3 = Both

Which perspective would you use for the following:

a) The explosion of a bombb) The collision between two protons

c) A child moving down a slide (no friction)

Your values of a, b, and c are

7/29/2019 Cons Laws Unit Review

3/17

1. Cons of ME - no loss in energy due to heat / sound

Cons of p - collisions / explosions

So, The explosion of a bomb

- Cons of p (explosion)

The collision between two protons

- Elastic: both cons laws

A child moving down a slide (no friction)

- Cons of ME

NR. 2 3 1

7/29/2019 Cons Laws Unit Review

4/17

Conservation Laws Review

2. A 0.15 kg ball is rolling off a bench at 2.4 m/s.

0.15 kg 2.4 m/s

0.85 m

The speed when it hits the floor 0.85 m below is

___________ m/s.

Your 2-digit answer is

7/29/2019 Cons Laws Unit Review

5/17

2. Ref: Ground below

METi = METf

PEgi + KEi = KEfmghi + 0.5mvi

2 = KEf

1.25 J + 0.432 J = KEf

KEf= 1.68 J

7/29/2019 Cons Laws Unit Review

6/17

2. Ref: Ground below

METi = METf

PEgi + KEi = KEfmghi + 0.5mvi

2 = KEf

1.25 J + 0.432 J = KEf

KEf= 1.68 J

KEf= 0.5mvf2

vf = 4.7 m/s

NR: 4 . 7

7/29/2019 Cons Laws Unit Review

7/17

3. A head-on collision:

470 kg 590 kg

26.0 m/s 13.0 m/s

After the collision, the 590 kg car is moving East at

11.50 m/s.

a) Find the velocity of the 470 kg car after the collision.

b) The magnitude of the impulse on the 590 kg car is

a.bc x 10d Ns. Your values of a, b, c, and d

are

7/29/2019 Cons Laws Unit Review

8/17

3. a) Ref direction: East is positive

Conservation of Momentum (1-D)

pT = pT

p1 + p2 = p1 + p2

m1v1 + m2v2 = m1v1 + m2v2

12,220 - 7670 = 470 v + 6785

v = -4.76 m/s

So, the 470 kg car is moving at 4.76 m/s West

7/29/2019 Cons Laws Unit Review

9/17

3. b) Ref: East is positive

Impulse (on 590 kg) = m2v

= m2 ( v2 - v2 )= (590 kg) [ (11.50 m/s) - (-13.0 m/s) ]

= 1.45 x 104 kg m/s

Thus, the impulse is 1.45 x 104 Ns towards the East.

NR. 1 4 5 4

7/29/2019 Cons Laws Unit Review

10/17

4. Explain why landing mats reduce injury.

e.g. For stunt persons falling from a building.

7/29/2019 Cons Laws Unit Review

11/17

4. F = m v

t

Impulse (m v) is not affected by the landing mat

- same m, same vi (landing speed), and same vf (0)

Fnet has an inverse relationship with time

Landing mats increase the time to change the momentum.

If time increases, then Fnet decreases.This results in less injuries.

7/29/2019 Cons Laws Unit Review

12/17

5. A bomb, moving at a horizontal momentum p, explodesinto two pieces, as shown.

51.8 kg m/s46.0

p

BOOM!

62.62 kg m/s

Find p and .

7/29/2019 Cons Laws Unit Review

13/17

Conservation Laws Review

4. Step 1: Find components

Before collision:

p

x = p

y = 0

7/29/2019 Cons Laws Unit Review

14/17

Conservation Laws Review

4. Step 1: Find components

After collision:

p1 = 51.8 kg m/s p1

y1 = 51.8 sin 46 46 y1

= 37.26 kg m/s x1

x1 = 35.98 kg m/s

p2 = 62.64 kg m/s x2

x2? y2

y2? p2

7/29/2019 Cons Laws Unit Review

15/17

4. Step 2:

37.26 kgm/s

yT = yT 37.26 kgm/s

0 = y1 + y2

0 = 37.26 kg m/s + y2

y2 = -37.26 kg m/s

7/29/2019 Cons Laws Unit Review

16/17

4. Step 2:

x2

62.64 kg m/s 37.26 kg m/s

(x2)2 + 37.262 = 62.642

x2 = 50.35 kg m/s

sin = 37.26 / 62.64

= 36.5

7/29/2019 Cons Laws Unit Review

17/17

4. Step 3:

35.98 kgm/s

p

50.35 kgm/s

xT = xT

p = x1 + x2

p = 35.98 kg m/s + 50.35 kg m/s

= 86.3 kg m/s