Ch. 4 Newton’s First Law of

Ch. 4 Newton’s First Law of

MotionMotion

Motion

• When an object begins moving or stops moving, naturally we look for a cause .

• What might be that cause???

Motion

• When an object begins moving or stops moving, naturally we look for a cause .

• That cause is a push or a pull.

Force (F)

• Any push or pull• Measured in Newtons (N)

• 1 Newton = 1 kg·m s2

Example wording: Mrs. Williams pushed the cart with a force of 5 newtons.

F = 5 N

Force is a vector quantity.

• What does this mean?

• It has magnitude (quantity) and direction

• Friction: a force that acts between materials as they move past each other.

• With out friction, velocity would be constant in the horizontal direction.

Newton’s 1st Law

Law of InertiaLaw of InertiaAn object at rest will remain at rest,

and an object in motion will remain in motion, unlessacted on by an outside force.

• Moving objects continue to move with constant velocity in the absence of a stopping force!!!!

Journal Review Question: What might cause an object to

stop its motion? What forces are acting against it?

• Consider a small lightweight car and a heavy truck travelling at 100 mph.

Think……. Think……. Journal: Which car will require more force to

stop its motion?

• Think about a soccer ball at rest and a bowling ball at rest…..

Give both a sharp kick.What will be your result????

The soccer ball will have greater acceleration than the bowling ball.

The two results differ because the masses differ!

Mass – A measure of Inertia

• Mass: the quantity of matter in an object. – measured in grams (g)

• ↑ mass an object has, ↑ Inertia

• mass inertia

Weight: measure of force of gravity on an object.

Weight = mass x gravity w = m·g

Gravity: 9.8 m/s2 = 9.8 N/kg

1 kg = 9.8 N

Problem Solving

• If an object has a mass of 40 kg, calculate the weight in newtons.

• w = m · g• w = 40 kg · 9.8 N

kg

w = 392 N

Net Force• The combination of ALL forces acting on

an object.

Net Force can be symbolized as the summation of all Forces acting on an object.

Summation symbol : ∑

∑ F = F1 + F2 + F3 + ……

Similar to the resultant. It is the result of all of the forces acting on an object!

• Forces in the same direction are added

• Forces in opposite direction are subtracted

• Forces at angles to each other can be resolved

• Forces at angles to each other can be resolved

Practice: What is the net force on each box???

A

DC

B

5 N

5 N

2 N 3 N

5 N

8 N

9 N 5 N

3 N

Practice: What is the net force on each box???

A

DC

B

5 N5 N

2 N 3 N

5 N

8 N

9 N 5 N

3 N

7 N

2 N

9.43 N

5 N

Equilibrium

• When net force equals zero. • Example: you standing around during lunch

– All forces acting on you are in equilibrium!!!

• Support force, also called normal force (Fn). – Perpendicular to surface of contact

Vectors not in the same plane…• Will balance to create a 10 N normal force• The closer the vectors are to being horizontal,

the more force is required at the point of tension to add up to the weight.

• This is why you may be able to swing from a rope that is vertical, but in the horizontal position it may snap.

In class Ch. 4 Concept Development Practice