Answer all questions on a piece of loose leaf and turn in at the end of the period for homework

credit.

Unit 4 Review

Mass vs Weight

• Mass- Amount of matter in an object– Units- kg– Unchanged by gravitational field

• Weight- Force of pull on an object caused by gravity– Units- N– Depends on gravitational field strength

Weight (Fg)

• Always directed towards Earth (down)

• Force of Gravity

• Fg = mg

• g = 9.8m/s2 or 9.8N/kg

FN

Fg

For an object resting on a level surface FN = -Fg

1) What is the weight of a 25kg object?

2) What is the normal force acting on this object if it rests on a level, horizontal surface?

3) If an object weighs 250N on Earth, what is the mass of the object?

Net Force

• FNet

• Sum or “Net” of all forces acting on an object

• FNet = F1 + F2 + F3…Not in reference table

Equilibrium

• A state where FNet is 0N and an object has constant velocity

• A force that causes equilibrium is called an equilibrant

• An equilibrant is equal in magnitude to, but opposite in direction of a resultant (or FNet)

Newton’s First Law

“An object at rest will stay at rest and an object in motion will stay in motion unless acted on by an unbalanced force.”

Inertia- The resistance of an object to a change in motion

Inertia is directly proportional to mass

35N10N

4)

5) An 80N wooden block is sliding down a wooden incline at constant velocity. The incline is angled at 25°. What is the net force acting on the block?

Newton’s Second Law

• The acceleration of an object is directly proportional to the net force acting on it and indirectly proportional to it’s mass.

Think of it this way:• Bigger forces cause bigger accelerations

• Bigger masses cause smaller accelerations

Newton’s Second Law

• You can express Newton’s 2nd Law as an equation:

6) What is the acceleration of this object?

10 N 5 N

m = 1kg

Newton’s Third Law

“For every action there is an equal and opposite reaction”

7) A football player kicks a football with a force of 15N. If the football is accelerated at a rate of 13 m/s2 at an angle of 30°, what force does the football apply to the player’s foot?

Fg

FNet =

a = + 2 m/s2

m = 70kg

Fg=FN= FN (What the scale

reads)

8) Find what the scale reads:

What do FII and F˔ mean?

FN

Fg FII

F˔θ

θ

What do FII and F˔ mean?

FN

Fg

FII

F˔θ

Calculating FII and F˔Sin(θ) = FII / Fg FII = Fg sin(θ)

Cos(θ) = F˔ / Fg F˔ = Fgcos(θ)