- Review the relationship of Newton’s 2nd Law (F = m ✕ a)

- Introduce the concept of energy

- Learn how the energy of an object is stored and transferred

TODAY’S OUTCOMES:FORCE, MOTION AND ENERGY

8. Suppose a barge carrying 100,000 Kg of coal (a bit more that 100 tons) is moving down the Ohio river at 10 m/sec when it is notice that there is a fishing boat in the channel, 100 meters away. The boat guiding the barge goes into reverse and stops the barge in 20 seconds. What force does the boat have to exert? (This question helps explain why the "tug" boats push from behind instead of pulling from the front).

Force = mass ✕ acceleration = 100,000 kg ✕ ?

acceleration = (final speed – initial speed)/time = (0 – 10 m/sec) / 20 sec = –0.5 m/sec2

Force = 100,000 kg ✕ –0.5 m/sec2

= –50,000 Newtons

9. Suppose you are a passenger in a car. The light turns green and the car starts down the road. What forces are acting on you? How large are they, and in what directions?

Sample answer: Let’s assume your body has a mass of 70 kg

Let’s assume (from the last homework) your acceleration is 2.0 m/sec2

Then in the forward direction, the force on your body is given by

F = m ✕ a = 70 kg ✕ 2.0 m/sec2 = 140 Newtons

There are also forces of your weight (downward) and seat pressing back upward that cancel in the vertical direction.

EXPERIMENTS WITH THE LAW OF FORCE AND ACCELERATION:

Changing direction of a cart

Force = mass × acceleration

Changing direction of a cart

More stretching = more

force

more mass ⇔ more force needed to make same acceleration

↑ ↑ constant

Racing balls down tracks

Force = mass × acceleration

Changing direction of a cart

mass did not matter

more mass ⇔ more force needed to make same acceleration

↑ ↑ constant

AGAIN:

Force of gravity on the ball (weight) is proportional to mass

EXPERIMENTS WITH THE LAW OF FORCE AND ACCELERATION:

Dropping clay

Force = mass × acceleration

Changing direction of a cart

↑ constant ↑

When distance increases:

What forces act on the ballat the moment of impact?

EXPERIMENTS WITH THE LAW OF FORCE AND ACCELERATION:

50 cm drop 200 cm drop

gravity

impact of floor

Net upward force → upward acceleration → ball slows to stop

Acceleration of an airborne ball:DISTANCE, VELOCITY AND ACCELERATION vs. TIME:

Velocity changesdirection

Acceleration is alwaysnonzero and downward

time

time

time

dis

tan

cevelo

city

acc

ele

rati

on

Force = weight - thisis always constantconstant force, constantmass means constantacceleration constant acceleration

means velocity changesat constant rate (straight line!)

0

- How to draw a velocity vs. time plot when acceleration is constant

- Understand conceptual problems usingNewton’s Laws of Motion

- Solve problems involving force, mass and acceleration using Force = mass × acceleration

WHAT YOU ARE EXPECTED TO KNOW:

NEWTON’S LAWS OF MOTION:

Observed in “steering a ball with plastic barriers” lab ;seeing a bubble level stay centered when velocity is constant

Observed in comparing straight line plots of force on a cart and acceleration of a rolling ball vs. steepness ; racing steel balls, dropping clay spheres, pulling carts with rubber bands

Observed in balanced forces on scales pulling cart and balancing a mass between 2 scales

- Review the relationship of Newton’s 2nd Law (F = m ✕ a)✓

- Introduce the concept of energy

- Learn how the energy of an object is stored and transferred

TODAY’S OUTCOMES:FORCE, MOTION AND ENERGY