Chapter 11 Forces

1. Chapter 11Forces

2. Laws of Motion • Force and motion are connected. • An object will have greater acceleration if a greater force is applied to it. • The mass of an object and the force applied to it affect acceleration. • Newton’s second law of motion connects force, mass, and acceleration in the equation acceleration equals net force divided by mass

3. Laws of Motion • Friction—force that opposes motion between two surfaces that are touching each other • Microwelds, areas where surface bumpers stick together, are the source of friction. • Friction between two surfaces that are not moving past each other is called static friction. • Sliding friction—force that opposes the motion of two surfaces sliding past each other • Friction between a rolling object and the surface it rolls on is called rolling friction.

4. Laws of Motion • Air resistance that opposes the force of gravity. • The amount of air resistance depends on an object’s shape, size, and speed. • Terminalvelocity—forces on a falling object are balanced and the object falls with constant speed

5. Air Resistance • Air Resistance • a.k.a. “fluid friction” or “drag” • force that air exerts on a moving object to oppose its motion • depends on: • speed • surface area • shape • density of fluid

6. Fair Fgrav Air Resistance • Terminal Velocity • maximum velocity reached by a falling object • reached when…Fgrav = Fair • no net force •  no acceleration •  constant velocity

7. Air Resistance • Falling with air resistance • heavier objects fall faster because they accelerate to higher speeds before reaching terminal velocity Fgrav = Fair • larger Fgrav •  need larger Fair •  need higher speed Animation from “Multimedia Physics Studios.”

8. Gravity • Weight • the force of gravity on an object W = mg W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) MASS always the same (kg) WEIGHT depends on gravity (N)

9. elephant feather Gravity • Accel. due to gravity (g) • In the absence of air resistance, all falling objects have the same acceleration! • On Earth: g = 9.8 m/s2 Animation from “Multimedia Physics Studios.”

10. Gravity • Law of gravitation—any two masses exert an attractive force on each other • Gravity is one of the four basic forces that also include the electromagnetic force, the strong nuclear force, and the weak nuclear force. • Gravity is a long-range force that gives the universe its structure. • Due to inertia, all objects fall with the same acceleration regardless of mass.

11. Gravity • Weight—gravitational force exerted on an object • Weight decreases as an object moves away from Earth. • Weight results from a force; mass is a measure of how much matter an object contains. • Objects in the space shuttle float because they have no force supporting them. • Projectiles have horizontal and vertical velocities due to gravity, and follow a curved path. • Acceleration toward the center of a curved path is called centripetal acceleration; it is caused by centripetal force, an unbalanced force.

12. Gravity • Gravity • force of attraction between any two objects in the universe • increases as... • mass increases • distance decreases

13. Gravity • Would you weigh more on Earth or Jupiter? • Jupiter because... greater mass greater gravity greater weight

14. Newton’s Third Law • Newton’s third law of motion—to every action force there is an equal and opposite reaction force • Action-reaction forces act on different objects and differ from balanced forces. • Rocket propulsion is based on Newton’s third law of motion. • Before it was discovered, the existence of the planet Neptune was predicted based on gravitational forces and Newton’s laws.

15. Newton’s Third Law • Momentum—related to how much force is needed to change an object’s motion; momentum equals mass times velocity. • Law of conservation of momentum—momentum can be transferred between objects; momentum is not lost or gained in the transfer.

16. p v m Momentum • Momentum • quantity of motion p = mv p: momentum (kg ·m/s) m: mass (kg) v: velocity (m/s)

17. p v m Momentum • Find the momentum of a bumper car if it has a total mass of 280 kg and a velocity of 3.2 m/s. GIVEN: p = ? m = 280 kg v = 3.2 m/s WORK: p = mv p = (280 kg)(3.2 m/s) p = 896 kg·m/s

18. p v m Momentum • The momentum of a second bumper car is 675 kg·m/s. What is its velocity if its total mass is 300 kg? GIVEN: p = 675 kg·m/s m = 300 kg v = ? WORK: v = p ÷ m v = (675 kg·m/s)÷(300 kg) v = 2.25 m/s

19. Conservation of Momentum • Law of Conservation of Momentum • The total momentum in a group of objects doesn’t change unless outside forces act on the objects. pbefore = pafter

20. Conservation of Momentum • Elastic Collision • KE is conserved • Inelastic Collision • KE is not conserved

21. p v m Conservation of Momentum • A 5-kg cart traveling at 1.2 m/s strikes a stationary 2-kg cart and they connect. Find their speed after the collision. BEFORE Cart 1: m = 5 kg v = 4.2 m/s Cart 2 : m = 2 kg v = 0 m/s AFTER Cart 1 + 2: m = 7 kg v = ? p = 21 kg·m/s p = 0 v = p ÷ m v = (21 kg·m/s) ÷ (7 kg) v = 3 m/s pbefore = 21 kg·m/s pafter = 21 kg·m/s

22. Conservation of Momentum • A clown is shot out of a 250 kg cannon at 20 m/s. If the cannon moves backwards at 2 m/s, how much does the clown weigh? Given: Clown: m = ? v = 20 m/s Cannon: m = 250 kg v = -2 m/s

23. p v m Conservation of Momentum • So…now we can solve for velocity. GIVEN: p = -1000 kg·m/s m = 250 kg v = ? WORK: v = p ÷ m v = (-1000kg·m/s)÷(250kg) v = - 4 m/s (4 m/s backwards)