Forces

1. Forces Chapter 10

2. The nature of Force Force – a push or a pull All forces have both magnitude and direction SI unit for Force = Newton (N) All forces are exerted by one object on another object: One is the Giver, the other is the Receiver

3. Forces in Combination Occurs when more than one force is exerted on an object at same time. Net Force – the final force you get from combining all the forces exerted on an object

4. Forces in same Direction Forces in same direction can be added together to determine the net force ← 25N + 20N← = 45N net force Pulls Pushes

5. Forces in different Direction Each force is exerted in opposite directions. Net force is determined by subtracting the smaller force from the larger force. ←12N - 10N→ = 2N

7. Balanced vs Unbalanced Forces If you know the Net Force of an object, you can predict its motion. Balanced Force – a force that produces no change in motion Net Force = Zero Unbalanced Force – a force that produces a change in motion Net Force ≠ Zero

9. Friction A force that opposes motion between two surfaces that are touching. Why does this occur? The surfaces of any smooth object is rough REALLY!!! Smooth polished aluminum alloy

10. Causes of Friction Strength of the force of friction depends on the types of surfaces involved and on how hard the surfaces push together…… In other words: 1 – Rougher the surface the greater the friction 2 – Greater the force the greater the friction

11. 4 Types of Friction • Static – when a force is applied to an object, but does not cause object to move. • Sliding – two objects sliding across each other. • Rolling – when an object rolls across a surface (less force than sliding) • Fluid – when a solid object moves thru a fluid.

12. Gravity and Motion Earth revolves around the sun and the moon around the Earth. What keeps them in orbit? GRAVITY A force (push or pull) that attracts all objects toward each other. (units = newtons) Universal Law of Gravitation states that every object in the universe attracts every other object. Recall that the gravity here on Earth is 6X that of the gravity on the moon.

14. Gravity and Motion Late 1500’s – Galileo discovered that all objects will land at the same time when dropped at the same time from the same height. All objects accelerate downward at the same rate due to gravity at the rate of : 9.8m/s²

15. What causes a feather to flutter down to the ground while an elephant drops steadily? Air Resistance – a type of fluid friction that exerts an upward force on falling objects. Objects with greater surface area or less mass (feather) experience greater air resistance. Slows the objects down.

16. Free Fall – gravity is pulling the object down with no other forces acting upon it. Projectile motion – when an object is thrown.

17. Elastic Forces Compression – force that squeezes or pushes matter together. Tension – force that stretches or pulls matter.

18. Newton’s First Law of Motion An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force. The clothes on your floor of your room will stay there unless you pick them up (you provide the force)

19. 1st Law • Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction– it would never stop!

20. 1st Law • Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

21. Inertia Tendency of an object to resist a change in motion. It does depend on the objects mass. The greater the mass, the greater the inertia. Think about you in a car moving forward when the brakes are suddenly pressed. That is why we wear seatbelts.

22. Newtons’s 1st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

23. Newton’s Second Law of Motion Acceleration depends on the net force acting on the object and on the object’s mass. Acceleration = Forceor Force = Mass X Acceleration Mass Which wagon would require a greater force to move and why?

24. Newton’s 2nd Law Acceleration depends on the net force acting on the object and on the object’s mass. F = M x A

25. 2nd Law (F=MxA) • How much force is needed to accelerate a 1400 Kilogram car 2 meters per second/per second? • Write the formula • F = M x A • Fill in given numbers and units • F = 1400K x 2 meters per second/second • Solve for the unknown • 2800 K-meters/second/second or 2800 N

26. Newton’s 2nd Lawproves that different masses accelerate to the earth at the same rate, but with different forces. • We know that objects with different masses accelerate to the ground at the same rate. • However, because of the 2nd Law we know that they don’t hit the ground with the same force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s

28. Check Your Understanding • 1. What acceleration will result when a 12-N net force applied to a 3-kg object? A 6-kg object? • 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass. •  3. How much force is needed to accelerate a 66-kg skier 1 m/sec/sec? • 4. What is the force on a 1000-kg elevator that is falling freely at 9.8 m/sec/sec.?

29. Newton’s Third Law of Motion For every action, there is an equal but opposite reaction. All forces act in PAIRS – Action-Reaction Pairs Ball bumps head Bar pushes downward

30. Other examples of Newton’s Third Law • The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

31. Momentum A characteristic of a moving object that depends on both the mass and the velocity of the object. Momentum = Mass(kg) x Velocity(m/s) (P) = (m) x (v) Momentum units are kg m/s The momentum of an object is the same direction as its velocity.

32. The more momentum a moving object has, the harder it is to change its velocity. Easier to change the velocity of a baseball moving at 20m/s than to change the velocity of a car moving at 20m/s. Why? The car has more momentum due to its larger mass.

33. Law of Conservation of Momentum The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects. (Friction is an example of an outside force) (500)(5) + (400)(2) = 3300kgm/s Momentum before Momentum after (500)(3) + (400)(4.5) = 3300kgm/s