Forces Types of Forces Balanced Unbalanced 1 Different

Forces Types of Forces Balanced / Unbalanced 1

Different forces A force, (a push or a pull) occurs when 2 objects interact with each other. Forces always act in PAIRS, whenever one object exerts a force on another it always experiences an EQUAL and OPPOSITE force in return. Forces are VECTOR quantities, they have a Magnitude (SIZE) and a DIRECTION. 2

Forces. • GRAVITY (weight) – two masses are attracted to each other • AIR RESISTANCE (drag) – the air tries to slow down a skydiver by pushing upwards against him/her. The skydiver pushes the air out of the way with an equal and opposite force. • Rockets and jet engines – the engine pushes gas backwards (action) and the gas pushes the rocket forwards (reaction) 3

Measuring magnetic forces 4

Object falling freely 5

Object resting on a table 6

Reaction of surfaces The bag squeezes the foam until the upward force of the springy foam on the bag exactly balances the downward gravity force on the bag 7

Balanced/Unbalanced forces 8

Alternative version A B Advantage: reduced friction so use a small difference between the two masses 9

Method 1 1. Use a level runway or bench. Connect the light gates to the data logger to measure velocity at each light gate. 2. Push the trolley from each end to check if the runway or bench is level; make adjustments as necessary. 3. Fix the bench pulleys to the runway or the bench and connect string to the trolley and both the mass hangers. 4. With no slotted masses on the hangers release the trolley and the mass hangers, first when trolley is not moving, then given a push in either direction. Comments? 10

Method 1 1. Use a level runway or bench. Connect the light gates to the data logger to measure velocity at each light gate. 2. Push the trolley from each end to check if the runway or bench is level; make adjustments as necessary. 3. Fix the bench pulleys to the runway or the bench and connect string to the trolley and both the mass hangers. 4. With no slotted masses on the hangers release the trolley and the mass hangers, first when trolley is not moving, then given a push in either direction. No movement if trolley at rest 11

Method 2 1. With no slotted masses on the hangers release the trolley and the mass hangers, first when trolley is not moving, then given a push in either direction. 2. Repeat step 4 with 100 g on each mass hanger and then 200 g. What conclusion can be made from these results? 12

Method 2 1. With no slotted masses on the hangers release the trolley and the mass hangers, first when trolley is not moving, then given a push in either direction. No movement if trolley at rest Same speed or velocity at each light gate if given a push. 2. Repeat step 4 with 100 g on each mass hanger and then 200 g. What conclusion can be made from these results? With equal masses hanging at each end; if the trolley is initially stationary it remains so, if it is given a push then its velocity remains the same. 13

Action of gravity and the pulley • The force of gravity is pulling the masses downwards and the pulleys change the direction of these forces to be horizontal and parallel to the runway or bench. • Strings can ONLY PULL so the two forces acting on the trolley are in opposite directions. 14

Conclusion • The force of gravity is pulling the masses downwards and the pulleys change the direction of these forces to be horizontal and parallel to the runway or bench. • Strings can ONLY PULL so the two forces acting on the trolley are in opposite directions. With equal forces acting in opposite directions on the trolley; if the trolley is initially stationary it remains so, and if it is given a push then its velocity remains the same. 15

Method 3 3. Now add 100 g to one of the mass hangers, with no extra masses on the other, i. e. the total mass is 200 g one end and 100 g the other. Release the trolley from the end with the smaller moving mass, measuring its velocity as it passes the light gates. 4. Add one 100 g mass to each mass hanger, making the hanging masses 300 g and 200 g. Release the trolley from the end with the smaller moving mass, again measuring its velocity as it passes the light gates. 5. Repeat with total masses of 400 g and 300 g. Conclusions ? 16

Final Conclusion • The trolley speeds up in the same direction as that of the larger falling mass, i. e. in the direction of the larger force, the driving force acting on the trolley. • The larger force is the Driving force • The smaller force is the Counter force • The difference is the Resultant force 17

Effects of resultant force Comment on the motion of the object while the R. F. acts. a. ? b. ? c. ? d. ? 18

Effects of resultant force Comment on the motion of the object while the R. F. acts. a. Initially object stationary, velocity of object increases, same direction as the force. Car at traffic lights b. Initially object is moving in same direction as the R. F, velocity of object increases, same direction as the R. F Car preparing to overtake c. Initially object is moving in opposite direction to R. F, object accelerates in direction of R. F. Car preparing to stop d. Initially object turning, object accelerates in direction of R. F, while still turning 19

Forces on a cycle Tim is cycling along on his bicycle. The horizontal forces are: • a driving force caused by Tim pedalling • a counter-force caused by air resistance and friction • On each of the following diagrams: • draw arrows to show the direction of these two forces and where they act • the length of the arrow should show the size of each force • label each arrow to show which force it is

Forces on a cycle – 1 and 2 1. Tim has just started off. He is pedalling to make the bike move faster. 2. Tim is now cycling along at a steady speed.

Forces on a cycle – 3, 4, and 5 3. He wants to go a bit faster to get to his destination sooner. So he pedals harder to speed up. 4. Tim is now going as fast as he can. His speed is steady again, but faster than it was before. 5. Tim finds it too tiring pedalling so hard, so he eases off. He is slowing down to a more comfortable speed.

Muddy problem-2 23

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Who wins – Red or Blue? 25

Who wins – Red or Blue? = 950 N R = 1000 N 26

Force diagrams Size ? Direction ? The size of the arrow represents the size of force. The direction of the arrow shows the direction of the force. 27

Force diagram-Truck stationary Size? Direction? 28

Force diagram-Truck stationary Size? Direction? The truck is moving to the right when the man starts pulling. Does the truck speed up, slow down or move at the same speed? 29

Force diagram-Truck stationary Size? Direction? The truck is moving to the left when the man starts pulling. Does the truck speed up, slow down or move at the same speed? 30

Forces acting when an object is at rest 31

A falling apple 32

Size of force on an apple 33

Gravity as seen by Matt in the Telegraph 34

Streamlines 35

Forces acting on a parachute 36

Terminal velocity 37

Motion of parachute 38

Forces during flight In order for an object to fly in a stable manner, it needs to balance 4 forces: Lift Drag Thrust Weight 39

Forces during flight In order for an object to fly in a stable manner, it needs to balance 4 forces: Lift Drag Thrust Weight comes from gravity pulling down on the object. Thrust is a force that pushes the forward. It can be generated by a propeller, a rocket, a catapult- anything that makes the object move. 40

Forces during flight In order for an object to fly in a stable manner, it needs to balance 4 forces: Lift Drag Thrust Weight But Lift and Drag can only arise as air moves past an object. Lift pushes the object upward, and Drag, a type of air resistance, slows it down. What causes these forces? To find out look at the cross section of an aircraft wing-a shape called an aerofoil. 41

The causes of Lift At least 2 forces combine to cause LIFTThe Bernoulli Effect and an example of Newton’s Third Law of Motion. 42

Lift: The Bernoulli Effect A Difference in Pressure As an aerofoil moves, its shape and angle force oncoming air to curve as it passes the aerofoil’s top side. 43

Lift: The Bernoulli Effect A Difference in Pressure Because of this curve, the air above the aerofoil moves farther and faster than air flowing underneath. 44

Lift: The Bernoulli Effect A Difference in Pressure As the speed of air increases, its pressure drops. This is called the Bernoulli Effect, after Daniel Bernoulli, the Swiss mathematician. 45

Lift: The Bernoulli Effect A Difference in Pressure The curved airflow generates more pressure below the aerofoil than above, and the aerofoil is pushed upward. 46

Lift: Newton’s Third Law of Motion Opposing forces An aerofoil also creates Lift by “bending” or redirecting airflow 47

Lift: Newton’s Third Law of Motion Opposing forces Oncoming air follows the curved shape of the aerofoil, shifting downward as it moves past 48

Lift: Newton’s Third Law of Motion Opposing forces This downward motion causes an opposing force that pushes the aerofoil up. 49

Lift: Newton’s Third Law of Motion Opposing forces This is Newton’s Third Law of Motion at work: Every action has an equal and opposite reaction. 50

Drag As an object moves through air, it encounters a form of resistance called drag. 51

Drag The amount of drag depends on an object’s size and shape. A thick, boxy object will cause more drag than a streamlined one. 52

Drag is also influenced by other factors, on of which is the quality of the object’s surface. A rough surface causes more drag than a smooth polished one. 53