Using Newton’s Laws

Newton’s 2nd Law – Mass and Weight All objects, no matter what their weight, gain speed at the same rate, which means that they have the same downward acceleration. F = ma or F = mg g = acceleration due to gravity = 9.8 m/s2

Practice Two girls are holding a stuffed dog that weighs 0.30kg. If Girl A decides to pull it at a force of 10 N and girl B decides to pull it away at 11 N, what is the horizontal acceleration of the dog?

Practice You place a watermelon on a spring scale. If the mass of the watermelon is 4.0 kg, what is the reading on the scale? A girl wants her mother to pull her so that she has an acceleration of 0.80 m/s2. If her mass is 27.2 kg, with what force does her mother need to pull her?

Practice A spring scale is attached to a block that has a mass of 1.2 kg. The block is attached to another spring scale that is attached to a sphere that has a mass of 3.0 kg. What are the readings on the two scales? (they are hanging downward from a board)

Practice #s 1-5 on pg 138

Apparent Weight Weight is a force that is defined as Fg = mg Fg changes when g varies. G near Earth is constant, so it doesn’t change.

What happens if you stand on a scale in an elevator What happens if you stand on a scale in an elevator? As long as the elevator is in equilibrium, the scale will read your weight. If you are accelerating upwards, the scale will read more. If you are accelerating downwards, the scale will read less.

Apparent Weight The force exerted by the scale is called the apparent weight. An object’s apparent weight is the force an object experiences as a result of all the forces acting on it, giving the object an acceleration.

Practice Your mass is 75 kg. you stand on a bathroom scale in an elevator. Starting from rest, the elevator accelerates at 2 m/s2 for 2 s, then continues at a constant speed. What is the scale reading during the acceleration? Is it larger than, equal to, or less than the scale reading when the elevator is at rest?

Practice Fscale = ma + mg Known: m = 75kg a = +2.0 m/s2 T = 2.0s Unknown: Fscale Strategy: Must find the force due to gravity AND acceleration and of the scale. Fscale = ma + mg Fscale = m(a + g) = 75kg (11.8 m/s2) Fscale = 885 N

Practice Problems On Earth, a scale shows that you weigh 585 N. What is your mass? What would the scale read on the moon (g=1.6 m/s2) What force would the scale exert when it slows in an elevator at 2 m/s2 while moving upward? It moves downward at a constant speed?

Drag Force and Terminal Velocity When an object moves through any fluid, such as air or water, the fluid exerts a drag force on the object opposite its motion. A drag force is the force exerted by a fluid on the object moving through the fluid.

Drag Force and Terminal Velocity If you drop a tennis ball the downward force of gravity is much stronger than the upward drag force, so it accelerates downward. As the ball’s velocity increases, so does the drag force. Soon, the drag force equals the force of gravity and there is no net force and no acceleration.

Drag Force and Terminal Velocity The constant velocity that is reached when the drag force equals the force of gravity is called the terminal velocity.