Equilibrium Newtons 2 nd Law of Motion Do

Equilibrium & Newton’s 2 nd Law of Motion

Do Now: What is the weight of a 500 -g ball on earth? • 500 g x 1 kg/103 g = 0. 5 kg. • 0. 5 kg x 9. 81 m/s 2 = 4. 9 N.

Review: Law Of Inertia Newton's 1 st Law - If there is no net force acting on a body, then it will continue in it's state of constant velocity (which may be zero). Mathematically.

Add all the forces acting on a body. R = 0 - constant velocity or zero. It is at equilibrium – acceleration = 0

What if there is Fnet. • The object will accelerate in direction of Fnet. • We sketch free body diagrams to add up the forces and find Fnet.

A car speeding up to the right on a horizontal road. Friction is there too.

The net force acting on an object is the vector sum of all the forces acting on it. Find the net force on each – it is helpful to treat horizontal (x) & vertical (Y) separately. 10 N 3 N 3 N 6 N, up 7 N 3 N 4 N 4 N, right

What is the motion of each box? 10 N 3 N 3 N 6 N, up 7 N 3 N 4 N, right 4 N Accl up Accl right

Objects not in Equilibrium will accelerate in the direction of the net force. But how much?

“Law of Acceleration”

a = Fnet = m a m m - mass in kg a - m/s 2. F - Newtons 1 N = 1 kg m s 2 (N): Fnet means sum the forces in one direction.

A constant net force causes constant/uniform acceleration!

More massive objects require more force to accelerate!!

Finding acceleration • Sketch a free body diagram show all forces on one object. • Calculate magnitude & direction of Fnet. • Use a = Fnet/m to find a in any direction where Fnet ≠ 0.

1. A 2000 -kg car’s engine exerts a westward force of 12500 -N. Friction between the tires and the road is 500 -N. • Sketch the free body diagram. • Calculate Fnet • Calculate the acceleration. • a = (-12, 500 – 500) N/ 2000 kg • a = - 6 m/s 2 or west.

2. An 80 -kg man who weighs 800 -N fell out of an airplane. As he falls air resistance pushes him up with a force of 150 -N. • Sketch the free body diagram. • Calculate his acceleration as he falls to Earth. • Why is his actual acceleration different than g?

Fair. • • Fg = mg = (80 kg)(9. 81 m/s 2) = 785 N down Fnet = (785 – 150) N Fnet = 635 N (down) a = Fnet / m 635 N / 80 kg = 7. 9 m/s 2. F g.

3. Find the acceleration of the box below. Assume the box has mass 2. 5 kg. 3 N 4 N 7 N 3 N 8 N

X & Y forces separately. Vertically, we have +3 N(up) and – 3 N (down), so they add to zero. Vertically, a = 0. Horizontally, +8 N +4 N (right), -7 N (left), so Fnet is +12 – 7 = +5 N. a = F/m +5 N / 2. 5 kg = 2 m/s 2. Acceleration to the right.

example prb’s

Do Now: The forces on the man in the elevator are balanced. Which of the following are possible? He could be: • • Fn 1. Moving down. 2. At rest 3. Moving up 4. In freefall. Fg

Sketch the graph of: • Acceleration on the X axis • Force on the Y axis. • What is the slope?

Graphs of Newton’s 2 nd Law • Write the equation: • Fnet = ma • Slope = • mass.

Sketch the graph of: • Force on the X axis • Acceleration on the Y axis. • What is the slope?

Graphs of Newton’s 2 nd Law • a = Fnet/m. • Slope = • 1/mass

Ex: Sketch the graph of acceleration vs. mass. • a = Fnet/ m • How would you find Fnet? • Transform to linear, find slope.

• Acceleration vs. 1/mass. Slope = Fnet.

• At constant velocity, you feel normal. • Because a requires Fnet, • you feel acceleration!! • There is Fnet on you, you feel pushed or pulled. • Inertia makes it hard stop your state of motion!

Inertia is also noticed when one object has Fnet and accelerates the another doesn’t • Car goes forward. Coffee does not!

Hwk Handout

Review. • • • Newton 1 Newton 2 Weight Friction. Normal force.

Prac Set

If you are not accelerating toward the Earth, then some force is balancing your weight!

You feel acceleration!! If there is a net force on you, you feel pushed or pulled. If you are moving at a constant velocity, you feel normal.

Newton’s 3 Laws 4. 5 minutes. http: //www. youtube. com/watch? v=i. H 48 Lc 7 wq 0 U Newton 2 6 minutes. • http: //www. youtube. com/watch? v=Wzvhu Q 5 RWJE&feature=relmfu Or Weight with spring scale.

Equilibrium • When all forces are balanced they are in equilibrium. • How can we determine 3 force could be in equilibrium? • See Starreview pg 83.

Ted Elevator Forces • http: //ed. ted. com/lessons/would-you-weigh -less-in-an-elevator-carol-hedden