1. Chapter 10 Forces

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 deteremined 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 1 st Law of Motion An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force Inertia-the tendency of an object to resist change in motion Inertia and mass have a direct relationship ↑ mass= ↑ inertia & ↓ mass= ↓ inertia

19. 1 st Law Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on by an unbalanced force.

20. 1 st Law The moon will keep revolving around the earth forever, unless acted on by an unbalanced force.

21. Moon in orbit around earth

22. 1 st Law Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction) it would never stop!

23. 1 st Law Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

24. Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? It’s a force we sometimes cannot see – friction. Objects on earth, unlike the frictionless space the moon travels through, are under the influence of friction.

25. Newtons’s 1 st 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.

26. Newton’s 2 nd Law Acceleration depends on the net force acting on the object and on the object’s mass. Force = mass x acceleration Newton = kilogram x m/s/s

27. 2 nd 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 = 1400kg x 2 meters per second/second Solve for the unknown 2800 kg-meters/second/second or 2800 N

28. Newton ’ s 2 nd Law proves 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 2 nd 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

30. 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 15 N causes a mass to accelerate at a rate of 5 m/s 2. 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.? 1. 4m/s/s 2m/s/s2. 3Kg3. 66N4. 9,800N

32. Newton’s 3 rd Law If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object (For every action, there is an equal and opposite reaction.)

33. 3 rd Law According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

34. 3 rd Law There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

35. 3 rd Law The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

36. Momentum Momentum = Mass x Velocity Kg m/s = kgx m/s 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 object