Forces

1. Forces J. McLeod

2. Force What is a force? • Force (N) = a push or a pull • Forces can cause • a resting object to move • a change in velocity • a change in direction

3. Force • Net Force = total force acting on an object. • Balanced Forces = No movement • When the forces on an object are balanced, the net force is zero. • Unbalanced Forces = Movement in the direction of the larger force • When an unbalanced force acts on an object, the object accelerates.

4. Force Frictional Force (friction) • A force that opposes the motion of objects. Magnified view of a smooth surface.

5. Force • There are four main types of friction: • Static friction: force acting on stationary objects. • One must overcome this force to move an object • Sliding friction: ice skating • Rolling friction: bowling • Fluid friction (air or liquid): air or water resistance

6. Force Gravity • An attractive force between any two masses. • Force of gravity increases as objects get closer to one another and the more mass and object has.

7. more mass Force • Who experiences more gravity - the flag or the astronaut? • Which exerts more gravity - the Earth or the moon? less distance

8. Force • Is the following statement true or false • An astronaut has less mass on the moon since the moon exerts a weaker gravitational force. • False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

9. Force What is the difference between weight and mass?? • Weight is dependent upon gravity. • Because gravity is a force and weight is dependent upon gravity, then weight is also a force. • Fg = mg • Fg = • m = • g = Fg = mg Fg Look on your reference tables and write what each symbol stands for and the value for g in the blank provided. ÷ g m

10. Force • What is the force on a 1-kg ball that is falling freely due to the pull of gravity? • A barbell weighs 850 N on Earth. What is its mass in kg? • What is the weight of a 65 kg object? • What is the mass of an object that weighs 500 N? • If you drop a 20 kg object, what is its weight? • What is the mass of an object that weighs 120 N? • What is the force needed to accelerate a 4.8kg object?

11. Force Falling without air resistance • In the absence of air resistance, all falling objects have the same acceleration! 9.8m/s2 • Will the elephant or feather hit the ground first???

12. Force Air Resistance • If an object falls for a long period, the upward force of air resistance becomes equal to the downward force of gravity. • When force of air resistance = force of gravity, this is called terminal velocity. • Fair = Fgravity

13. Force Falling with Air Resistance • Heavier objects fall faster because they accelerate to higher speeds before reaching terminal velocityFgrav = Fair • Now… will the elephant or feather hit the ground first???

14. Forces Quiz 1 • With what force will a ball hit the ground if it has a mass of 350kg? 3430 N • What is the mass of a falling rock if it hits the ground with a force of 147 Newtons? 15 kg

15. Newton’s First Law “The Law of Inertia”

16. Newton’s 1st Law • The tendency of an object to resist a change in motion. • If an object is stationary, it will remain stationary (zero velocity). • If an object is moving at constant velocity, it will keep moving at a constant velocity. • All objects have inertia. • The more mass an object has, the more inertia it has (and the harder it is to change its motion).

17. Newton’s 1st Law • Which is harder to change it’s motion…. F150 or a mini-cooper? WHY??? These pumpkins will not move unless an unbalanced force acts on them.

18. Newton’s 1st Law Because of inertia, objects (including you) resist changes in their motion. When the car going 60mph is stopped by the brick wall, your body keeps moving at 60 mph. Don’t let this be you. Wear seat belts!!!!

19. Newton’s 1st Law • When you stop suddenly, like in a collision, (think football hit) the skull stops, but the brain keeps going – crashing into the skull. • Causing a bruise to the brain (a concussion).

20. Newton’s 1st Law

21. Newton’s Second Law

22. Newton’s 2nd Law • The net force of an object is equal to mass x acceleration. • Mass (kg) • Acceleration (m/s2) • Force = Newtons (N) = kg·m/s2 F = ma F ÷ a m

23. Newton’s 2nd Law • Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.

25. Newton’s 2nd Law • Very massive objects that accelerate at a low rate, like a glacier, have great force. • Objects with low masses that accelerate at a high rate, like a bullet, have a great force.

26. Newton’s 2nd Law • Example 1: How much force is needed to accelerate a 1400kg car 2 m/s2? • F= ? • m = 1400 kg • a = 2m/s2 F = ma = (1400kg x 2m/s2) = 2800kgm/s2

27. Newton’s 2nd Law Example 2: A 4.0 kg shot-put is thrown with 30 N of force. What is its acceleration? F = 30 N m = 4.0 kg a = ? Example 3: What force would be required to accelerate a 40 kg mass by 4 m/s2? F = ? m = 40 kg a = 4 m/s2 7.5 m/s2 160 N

28. Newton’s 2nd Law Practice • What is the acceleration of a 1,500 kg truck if acted upon by a net force of 7,500 N? • A 60.0 kg runner accelerates at 2.2 m/s2. What is the net force on the runner? • Find the mass of a flying disc that has a net force of 1.05 N and accelerates at 3.5 m/s2. • While playing baseball, Jim swung at a 150 kg baseball and accelerated it at a rate of 30,000 m/s2. What was the net force applied to the baseball? • What is the net force required to accelerate a 0.030 kg arrow at 1720 m/s2? • What is the mass of Butch, the football quarterback, if a net force of 1440 N causes him to accelerate at 20.0 m/s2? • Claudia stubs her toe on the coffee table experiencing a net force of 100 N. What is the acceleration of her 1.80 kg foot? 5m/s2 132 N 0.3kg 4,500,000 N 51.6 N 72 kg 55.56m/s2

29. 30 20 Force (N) 10 0 5 10 15 20 25 30 acceleration (m/s2) What is the mass of the object being accelerated?

30. Newton’s Third Law

31. Newton’s 3rd Law • For every action there is an equal & opposite reaction. Sliding friction Rolling friction Fluid friction

32. Newton’s 3rd Law • What happens if you are standing on a skateboard or a slippery floor and push against a wall? • You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.

33. Newton’s 3rd Law • Why does it hurt so much when you stub your toe? • When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

34. Newton’s 3rd Law • Action: tire pushes on road • Reaction: road pushes on tire

35. Newton’s 3rd Law • Action: rocket pushes on gas • Reaction: gas pushes on rocket

36. Newton’s 3rd Law

37. Newton’s 3rd Law

38. Momentum • Momentum = mass x velocity • Mass = kg • Velocity = m/s • Momentum = kg·m/s • A 1 kg object with a speed of 100m/s & a 100kg object with a speed of 1m/s have the same momentum. p p=mv ÷ v m

39. Momentum • Example 1: Find the momentum of a bumper car if it has a total mass of 280 kg and a velocity of 3.2 m/s. • p = ? • m = 280 kg • v = 3.2 m/s • Example 2: The momentum of a second bumper car is 675 kg·m/s. What is its velocity if its total mass is 300 kg? • p = 675 kgm/s • m = 300 kg • v = ?

40. Momentum Practice • What is the momentum of a 700 kg bird flying at a velocity of 2.5 m/s • What is the momentum of a 1000 kg car traveling at 5 m/s • What is the momentum of a 40 kg shopping cart rolling along at 0.5 m/s • What is the momentum of a 300 kg roller coaster car traveling at 625 m/s • A marble rolling at a velocity 10m/sec has a momentum of a 5,000 kgm/sec. What is its mass? • An object whose mass is 3 kg is fired from a cannon, giving it a forward momentum of 1050 kg-m/sec. What is its velocity?

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

42. Warm-Up • Of which type of friction is the following an example? A dolphin swimming through the ocean. • What is the value for acceleration due to gravity?

43. Warm-Up – you must write the questions, with the answers to turn in!!! • 2 examples of a physical change • 2 examples of a chemical change • On the periodic table where are the metals located? • On the periodic table where are the nonmetals located? • On the periodic table where are the following located? alkali metals, alkaline earth metals, transition metals, halogens, noble gases

44. Warm-Up • How much does a 1,000kg weight weigh? Use your reference sheets to determine which formula you should use. • If a cylinder rod moves at 3m/s and has a mass of 50kg, what is its momentum? • What type of friction exists in liquid water? Is this friction higher in water or syrup?

45. Warm-Up • What is the weight of someone who has a mass of 300kg? • What is the momentum if a 34kg object is moving at a velocity of 3.5m/s? • What is the acceleration of object that has a mass of 8kg and is thrown with a force of 55kgm/s2? • What is the velocity of an object that has an acceleration of 5m/s2 in a time of 23.4seconds?

46. Warm-Up • Which type of friction is exerted between the tires of a car and the road? • What is the distance covered by a car traveling at a velocity of 50mph north for 5 hours? • A truck was traveling at 55km/hr and slows down to 37 km/hr in 0.2hr. What is the truck’s acceleration? • Identify which law applies to the following situation. While running down the hall, you miss the corner, and hit the wall and get a concussion. • What is the mass of a rock that has an acceleration of 2.3 m/s2 after a force of 2300 Newtons has hit it?

47. Warm-Up • What force is necessary to accelerate a 6 kg bowling ball at 2.1 m/s2? • A force of 250 N is applied to a 1 kg softball when struck with a bat. What is the acceleration?