Forces and Pressure (PDF)

Ch 13.Forces In this lesson you will learn about forces , types of forces , effects of balanced and un balanced forces , turning forces, mass , volume, density, weight, pressure and all calculations.  What is a force? A force is a push or pull that causes...

Ch 13.Forces In this lesson you will learn about forces , types of forces , effects of balanced and un balanced forces , turning forces, mass , volume, density, weight, pressure and all calculations.  What is a force? A force is a push or pull that causes an object to move, stop, or change direction In physics a force is anything that makes an object accelerate The strength of a force is measured in the SI unit called the newton (N) and measured by using spring balance. Mass is the amount of matter in an object. Mass is measured in grams (g) or kg. We use a balance/scale to measure mass. Weight is the pull of gravity on an object and it is a type of force. It is measured in Newton. (N) We use a spring balance to measure it. 1.If mass of an object is 74 kg. Calculate its weight. W=mg =74x10 =740N 2.If mass of an object is 800 grams, Calculate its weight. =800/1000=0.8kg W=mg =0.8x10=8N If the weight of an object is 560 N. Calculate its mass in grams. W=m x g 560=mx10 M=560/10=56Kg =56x1000=56000g Air resistance, which is also known as drag, is a kind of friction (a force that opposes motion) which occurs between air and another object. It is the force that the object experiences as it passes through the air. Air resistance and gravity are the two fixed forces of nature which move on any object on Earth.  What’s up thrust? Up thrust is a force that only exists in liquids, such as lakes, oceans, swimming pools and even cups of tea! It is a push that always goes straight up, against the direction of gravity. It pushes UP! That’s why it’s called UP thrust!  Upthrust in action This boat has gravity pushing it down (weight), and the up thrust of the water pushing up. Because the forces are balanced, the boat floats. What would happen if the forces weren’t balanced? More examples of upthrust If you let go of your drinking straw, it floats up and sometimes tries to bob out of the glass!  What forces are acting on this powerboat? weight Water Resistance Thrust Upthrust Force Forces come in pairs Forces have a magnitude and a direction Magnitude: 5N 5N, north (up) Direction: north (up) Balanced and Unbalanced Forces Forces occur in pairs and they can be either balanced or unbalanced Balanced Forces Balanced forces do not cause change in motion. They are equal in size and opposite in direction. Unbalanced Forces An unbalanced force always causes a change in motion. When unbalanced forces act in opposite directions you can find the net force. Unbalanced forces lead to a change in speed or direction.  Net force or resultant force Magnitude  The difference between the two forces Direction  Direction of the largest force Unbalanced Forces 6 N, -3 N, = 3N, left Unbalanced Forces 4 N, left , 10 N, right = 10-4= 6N, Unbalanced Forces 5 N, right + 10 N, right =15N, right The balanced forces on an object moving at constant speed. Thrust=air resistance or drag or (drag + friction) 30-10=20N 45-30=15N 10-10=0N no motion 30-25=5N  Types Of Moments There are 2 types of moments: Clockwise moment Anticlockwise moment Moment=F x d Unit is(Nm) When a force causes an object to turn in a clockwise direction, it is called a CLOCKWISE MOMENT. To determine if the force causes a clockwise or anticlockwise moment, we have to ignore all other forces acting on the object. When the girl sits on the see-saw, her weight is the force exerted on the see-saw. When a force causes an object to turn in an anti-clockwise direction, it is called an ANTI- CLOCKWISE MOMENT. Find the value of d. Solution: Clockwise moment = Force x distance = 30 x d = 30d Nm Anticlockwise moment = Force x distance = 10 x 6 = 60 Nm Using the principle of moments, Clockwise moment = Anti-clockwise moment 30d = 60 d = 60  30 d =2m Clockwise moment=F x d = 5 x 0.50 = 2.5 Nm Anticlockwise moment = F x d = F X 0.25 = 0.25F Nm CLOCKWISE MOMENT= ANTICLOCKWISE MOMENT 2.5 = 0.25F F = 2.5/0.25 = 10 N 3.What is the moment if force applied is 10N and distance is 125cm. D= 125/100 = 1.25m Moment= F X d = 10 x 1.25 = 12.5 Nm Density= mass/volume units are g/ or kg/ Mass= Density x volume Volume= Mass /density An object is made from steel. It has a volume of 25 meter cube and a mass of 425 kg. Calculate the density of the object. Density= m/v = 425/ 25 = 17 kg/ An object has volume of 0.004 meter cube and density of 980kg/meter cube. Calculate the mass. Mass= density x volume =0.004 x 980=3.92 Kg A bottle of water has a density of 1000kg/ and a mass of 0.5 Kg. Calculate the volume of water bottle. Volume = Mass/ density = 0.5/1000 = 0.0005 If an object has a mass of 5 kg and density of 2g/.Calculate its volume. Mass=5 x 1000 = 5000g Volume = mass/density =5000/2 =2500 Ch 14 Pressure textbook pages 149-156 Objectives : To study about pressure in liquids and gases and how to calculate pressure. Pressure is defined as force per unit area. Pressure=Force/Area Area= Force/ Pressure Force= Pressure x Area The S.I units are N/ or N/ or Pascal (Pa) A force of 800 N acts on an area of 200 meter square. Calculate the pressure. P=F/A=800/200=4 N/ A crate exerts a force of 120 N on a table. The pressure on the table is 15 N/.Calculate the area of a crate that is in contact with surface of the table. A= F/P=120/15=8.  Pressure depends on 2 factors Forces acting – Increasing the force will increase the Pressure. Area of surface – Decreasing the area of the force will increase the Pressure  Pressure in Liquids Pressure is transmitted throughout the liquid. Pressure acts in all direction. Pressure increases with depth. Pressure in liquids does not depend on shape or size of the container.