TAKS Objective 5 Force and Motion Day 13

Forces and Motion Forces can create changes in motion. Acceleration Deceleration What happens if I put force on my gas peddle? What happens if I put force on my Breaks?

Definition of a Force A force is a push or a pull.

Balanced Force A force that produces no change in an object’s motion because it is balanced by an equal yet opposite force. If I were to add these two forces they would equal zero

Unbalanced Forces Are forces that result in an object’s motion being changed. Add together to equal greater force +

Motion can be described as: A change in an object’s position. Average velocity (speed) is the change of position of an object over time.

Newton’s 1 ST Law of Motion 1st Laws States that an object at rest will not move unless an outside force acts on it (such as friction). This law is also called the LAW OF INERTIA. Ex. This law explains why you fly forward in a car when someone slams on the brakes. Because of Inertia, your body wants to keep moving at the same speed as the car.

Newton’s 2 nd Law of Motion 2 nd Law States that a force on an object will move the object in the direction of the force. The relationship between force, mass and acceleration is summarized by the formula: f = ma Ex. This law explains why a golf ball will roll in the direction of a force applied to it.

(A) 0.2 N (B) 0.8 N (C) 1.5 N (D) 6.0 N Q: The frog leaps from its resting position at the lake’s bank onto a lily pad. If the frog has a mass of 0.5 kg and the acceleration of the leap is 3 m/s 2, what is the force the frog exerts on the lake’s bank when leaping? Formula chart says F=ma, m is mass in kg, a is acceleration in m/s 2. So,.5 kg x 3 m/s 2 = 1.5 N

Newton’s 3 rd Law of Motion 3 rd Law States that for every action there is an equal but opposite action. Ex. A skater pushes back on the skates but the skater moves forward. THESE LAWS EXPLAIN ALL MOTION

Q: The hands of a swimmer pushing backward against water represent an action force. What is the reaction force? A. The swimmer’s body moving forward? B. The water pushing against the swimmer’s hands C. The swimmer’s body pushing against the water. D. The water moving backward from the swimmer.

Momentum The product of an object’s mass and its speed. A force applied to an object causes a change in its momentum. p(momentum)= m(mass) x v(velocity) p = mv common unit for momentum (kg x m/s)

Q: A ball moving at 30 m/s has a momentum of 15 kg·m/s. The mass of the ball is — A. 45 kg B. 15 kg C. 2.0 kg D. 0.5 kg Formula Page says that Momentum = Mass x Velocity So, 15 kg. m/s = M x 30 m/s solving for M it is:

Velocity Graphs V = distance time Velocity (v) is the slope (rise over run) of a position (d) vs. time (t) graph

Q: The diagram represents the total travel of a teacher on a Saturday. Which part of the trip is made at the greatest average speed? A. Q B. R C. S D. T How do we work this one? Calculate v = d/t for each segment.

Q: The picture shows the position of a ball every 0.25 second on a photogram. Using a ruler, determine the velocity of the ball. A. 3.5 cm/s B cm/s C cm/s D cm/s

Use the ruler on the side of the chart and the equation for velocity. The answer was H. Measure from the center of ball 1 to the center of ball 2 and multiply by 4.

Acceleration When an object’s speed changes over time it is accelerating (or decelerating) A = v final – v initial / time Units for acceleration m/s/s or m/s 2

Acceleration Graphs Acceleration (a) is the slope of a velocity (v) vs. time (t) graph Velocity (m/s) Time (s) Velocity (m/s) Positive Acceleration Negative Acceleration NO Acceleration

Teresa runs in one direction at 1.5 meters per second (m/s). She hen turns around and runs in the opposite direction at 2.0 m/s. The entire trip takes 5.0 seconds (s). What is Teresa’s average acceleration, in meters per second squared (m/s2)? A m/s 2 B m/s 2 C m/s 2 D m/s 2

Work Work: application of a force to an object that results in the movement of the object over a certain distance. W = F x d The work done by forces on an object = changes in energy for that object. Work and Energy are measured in Joules 1 Joule = 1 Newton meter

Q: How much work is performed when a 50 kg crate is pushed 15 m with a force of 20 N? A. 300 J B. 750 J C. 1,000 J D. 15,000 J Use the formula Work = Force x distance Force of 20 N x 15 meters = 300 Joules Answer:

A. accelerated rapidly B. remained motionless C. decreased its velocity D. gained momentum Q: If a force of 100 newtons was exerted on an object and no work was done, the object must have — Work = Force x Distance Work = 0 Force = 100 N so 0 J = 100 N x d distance must be 0 It did not move!

Work Example: 1. The teacher pushes on the wall until she is exhausted. 2. A book falls off the table and hits the floor. 3. The waiter carries a tray of food. 4. A rocket accelerates through space. Is Work Being Done? 1. No. The wall did not move. 2. Yes, gravity applied a force and moved the book in the direction of the floor. 3. No. The force to hold the tray is not applied in the direction of the motion. 4. Yes. The force of the rocket thrust is causing the rocket to move.

Friction A force that opposes, or works against, motion of two objects that are touching.

Friction Friction causes an object to slow down and stop. Since the amount of energy stays constant, the energy becomes heat.

Why Use a Machine? In an ideal (perfect) machine the work put into the machine (W in ) = the work put out by that machine (W out )

Machines Make Work Easier The ideal mechanical advantage of a machine (IMA) of a machine is the number of times the output force is larger than the input force IMA = F out /F in A machine can only make this happen by moving the input force through a farther distance than the output force F in d in = F out d out

Q: The diagram shows an electric motor lifting a 6 N block a distance of 3 m. The total amount of electrical energy used by the motor is 30 J. How much energy does the motor convert to heat? A. 9 J B. 12 J C. 18 J D. 21 J

Work Output = Resistance Force x Resistance Distance Work out = 18J = 6N x 3m Work Input = 30J done by the motor The difference is lost as heat due to friction, which is 30J – 18J = 12J Answer B

Real Machines use Energy No real machine is 100 % efficient. i.e. none put out more work than is put in Efficiency of a machine is work output/work input X 100 % %Efficiency = W output / W input X 100%

Machines use Power Power: the rate at which energy is used (work is done) P=Work/time Power is measured in H.P. or watts 1 watt = 1 Joule 1 sec

Q: Shelby does 150 J of work to move a cart 3 meters in 30 seconds. How much power did Shelby use to do this work? A W B. 450 W C. 50 W D. 5 W

6 Types of Simple Machines Inclined planes Screws Pulleys Wheel and axle Levers Wedge

Universal Law of Gravity All objects in the universe attract each other by the force of gravity.

Universal Law of Gravity Gravity varies depending on two factors: 1) the mass of the object doing the pulling, and 2) the distance from the center of that object

On Earth gravity = 9.8 m/s/s For every second that an object falls its speed increases by 9.8 m/s

Weight= Mass (m) X gravity (g) Unit of mass = kg Unit of acceleration = m/s/s Unit of weight = Newton 1 Newton= about ¼ pound

USE THE FORMULA PAGE Some of the problems require you to grid in an answer. Make sure you pay attention to the decimal point in the square in the middle

Discussion Question Lamont wants to move a 4,800 gram box from the floor to a shelf directly above the box. It takes Lamont 8 seconds to move the box to a shelf that is 0.4 meters from the ground. It takes 12 seconds to move the box to a shelf that is 1.2 meters off the ground. How much more work in joules is required to put the box on the higher shelf?