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How can forces be represented? The forces acting on any object can be shown using a force diagram. A force diagram uses labelled arrows to show all the forces acting on the object. air l The direction of each arrow shows resistance the direction of each force. l The length of each arrow is proportional to the size of the force. What is the force diagram for this ball when it first starts to fall? weight Forces are measured in newtons (N) 3 of 42 © Boardworks Ltd 2007

What forces act on a swimming fish? What forces are acting on this fish as it swims at a constant speed? upthrust friction thrust weight Upthrust is the upwards force on the fish caused by the water around the fish. This is sometimes called buoyancy. Thrust is the forwards force created by the fish. 4 of 42 © Boardworks Ltd 2007

What forces act on a moving boat? What forces are acting on this accelerating boat? air resistance friction upthrust weight Air resistance is friction caused by movement through air. 5 of 42 © Boardworks Ltd 2007

Identifying forces 6 of 42 © Boardworks Ltd 2007

What are resultant forces? There are usually several different forces acting on an object. The overall motion of the object will depend on the size and direction of all the forces. The motion of the object will depend on the resultant force. This is calculated by adding all the forces together, taking their direction into account. 50 N 30 N Resultant force on the crate = 50 N – 30 N = 20 N to the left 7 of 42 © Boardworks Ltd 2007

Calculating resultant forces 8 of 42 © Boardworks Ltd 2007

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What is friction? What is friction and why is it an important force? 10 of 42 © Boardworks Ltd 2007

What is friction? Friction is a resistive force that slows things down and tries to stop objects sliding past each other. friction pulling force Friction always acts in the opposite direction to which an object is moving or trying to move. What would happen if friction didn’t exist? 11 of 42 © Boardworks Ltd 2007

Friction – useful or a problem? 12 of 42 © Boardworks Ltd 2007

What causes friction? Friction occurs between two objects because the surfaces of those objects are rough, and contain bumps and hollows. This roughness means that a force is needed to move the two objects over each other. Even objects that appear very smooth, such as polished metal or ice, have a rough surface if viewed under a microscope. 13 of 42 © Boardworks Ltd 2007

Different types of friction Friction is most obvious when it acts between two solid objects, but it also acts between solid objects and gases, and between solid objects and liquids. l Friction caused by an object moving through air is called air resistance. l Friction caused by an object moving through a liquid, such as water, is called drag. 14 of 42 © Boardworks Ltd 2007

How does friction change? 15 of 42 © Boardworks Ltd 2007

Reducing the effects of friction Friction always tries to stop things moving or slow things down. What effect does this have? Friction creates heat and eventually wears down surfaces, which can be very damaging. Lubricants are used to reduce friction in machinery and so help protect surfaces. In car engines, oil is used as a lubricant. Bearings and rollers can also be used to reduce friction, for example, in wheels 16 of 42 © Boardworks Ltd 2007

Reducing the impact of air resistance Friction caused by air resistance affects the speed and fuel consumption of vehicles such as cars, bikes and aeroplanes. Vehicles are specially shaped, or streamlined, to enable air to flow past them as easily as possible, therefore reducing air resistance. Which of these cars is the most streamlined? In a similar way, ships and boats have streamlined hulls to reduce the drag effects of water. 17 of 42 © Boardworks Ltd 2007

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Introducing balanced forces 19 of 42 © Boardworks Ltd 2007

What is Newton’s first law? If the resultant force acting on an object is zero, all the forces are said to be balanced. This forms the basis of Newton’s first law of motion, which states: If the forces on an object are balanced, the object will continue to do what it is already doing: l if the object is stationary, it will remain stationary l if the object is moving, it will continue to move at the same speed and in the same direction. 20 of 42 © Boardworks Ltd 2007

Terminal velocity of a skydiver 21 of 42 © Boardworks Ltd 2007

Velocity–time graph of skydiver 22 of 42 © Boardworks Ltd 2007

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Introducing unbalanced forces 24 of 42 © Boardworks Ltd 2007

What is Newton’s second law? If the resultant force acting on an object is not zero, all the forces are said to be unbalanced. This forms the basis of Newton’s second law of motion, which states: If the forces on an object are unbalanced, two things about the object can change: l the speed of the object may change – it may either increase or decrease l the direction of motion may change. 25 of 42 © Boardworks Ltd 2007

How is movement calculated from force? The resultant force acting on an object is related to the object’s mass and acceleration. These three factors are linked by the following equation: force = mass x acceleration l Resultant force is measured in newtons (N). l Mass is measured in kilograms (kg). l Acceleration is measured in metres per second (m/s 2). 26 of 42 © Boardworks Ltd 2007

How do we use Newton’s second law? A car has a mass of 1, 000 kg. What force must the car’s engine supply to cause an acceleration of 2 m/s 2? force = mass x acceleration = 1, 000 x 2 = 2, 000 N 27 of 42 © Boardworks Ltd 2007

Using a formula triangle A formula triangle helps you to rearrange a formula. The formula triangle force (f), mass (m) and acceleration (a) is shown below. Cover the quantity that you are trying to work out, which gives the rearranged formula needed for the calculation. So to find force (f), cover up f… …which gives the formula… f = mxa x 28 of 42 © Boardworks Ltd 2007

How do we use Newton’s second law? A lorry has a mass of 12, 000 kg. What acceleration is caused by a force of 10, 000 N? force = mass x acceleration = force mass = 10, 000 12, 000 = 0. 83 m/s 2 29 of 42 © Boardworks Ltd 2007

F = ma calculations 30 of 42 © Boardworks Ltd 2007

What forces support objects? What forces are acting between Mel’s computer and the table it is resting on? The computer pushes down on the table because it is attracted by the Earth’s gravity. The table exerts an equal and opposite force pushing upwards on the computer. This is called a reaction force. 32 of 42 contact force reaction force © Boardworks Ltd 2007

What is Newton’s third law? A force cannot exist on its own – there is always a second force present. This forms the basis of Newton’s third law of motion states, which states: If object A exerts a force on object B, then object B exerts an equal but opposite force on object A. These pairs of forces that act between two objects are sometimes called action–reaction pairs. 33 of 42 © Boardworks Ltd 2007

Action–reaction pairs 34 of 42 © Boardworks Ltd 2007

Balanced and unbalanced forces How many pairs of balanced, unbalanced and action–reaction forces can you spot? 35 of 42 © Boardworks Ltd 2007

Glossary (1/2) l air resistance – A frictional force that acts against an object moving through air. l balanced – A pair of opposing forces that are the same size and which have no effect on an object’s speed or direction. l drag – A frictional force that acts against an object moving through liquid. l friction – A force between two touching objects that tries to oppose movement. l reaction force – The opposing force created by an object in response to another force. 37 of 42 © Boardworks Ltd 2007

Glossary (2/2) l resultant force – A single force that has the same effect as all the forces acting on an object added together. l terminal velocity – The velocity of a falling object reached when its weight and air resistance are balanced and it no longer accelerates. l unbalanced – A pair of opposing forces that are different in size and which cause an object to change its speed or direction. l upthrust – An upwards force acting on an object in a liquid or a gas. l weight – The force created by the gravitational attraction on a mass, measured in newtons (N). 38 of 42 © Boardworks Ltd 2007