#### Transcript of LPH 105 W15 4:1-4

Now lets practice a bit.

how much tension must a cable withstand if it is used to accelerate a 1200 kg car vertically upwards at .70 m/s^2

12600 N

Which way is the car moving?

What is the acceleration?

If the tension is measured to be 10,000N for the same 1200 Kg car.

-1.47 m/s^2

don't know.

one 3.2 kg paint can is hanging by a massless cord from a 5.0 kg paint can, also hanging from the ceiling by a massless cord. What is the tension in each cord?

80 N

31 N

What is the tension in each cord if it is pulled up with an acceleration of 1.5 m/s^2

93 N

36 N

The engine of a 1000 kg car is able to create a horizontal force of 5000N on the ground. If this car is pushing a 2000 kg truck, what is the acceleration of the truck? and what is the force between the car and truck?

1.7 m/s^2

3333 N

What if this was reversed, same mass and force, but the truck pushing the car?

1.7 m/s^2 1667 N

A window washer pulls herself upwards using a bucket and pulley shown here. How hard mush she pull downward to raiser herself slowly at a constant speed, if she has a mass of 55 kg and the bucket is 15 kg?

What would her acceleration be if she pulled 15% harder?

343 N

1.47 m/s^2

If a 1000 kg mass is being pulled at a constant speed across a surface by two ropes as shown, and it is experiencing a 15000 N frictional force, what is the tension in each rope?

30

45

0

0

11000 N

7800 N

What would the acceleration be if the left guy pulled just as hard as the right guy?

3.22 m/s^2 towards left guy

"Could you explain inertial reference frames?"

"I don't exactly understand what inertia is, or how is mass the measure of inertia?"

"What average net force would be needed to accelerate a 1200 kg car (at a constant rate) up to a speed of 28 m/s, if you had a distance of 55 m to do it? (answer should look like '2500 N' that is two sig fig.) "

**2 lists: Concept**

1. Sum of all forces. F = ma

2. Gravity F = mg : always straight down.

G

3. Normal force: Always normal, and what it needs to be.

4. Newton's 3: Two FBD, substitution.

5. Friction: Links y direction to x direction, substitution.

6. Tension: Same everywhere, force in direction of rope.

7. Pulley: Turn force, count the ropes.

List: What to do

1. Draw picture

2. FBD (axis, force - all of them)

3. Components

a. On axis (list x and y, separately)

b. Off axis (put on both lists, x & y)

4. F = ma ( components, concepts)

5. Algebra

6. Answer problem (kinematics)

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