Newton's Laws - Science Lesson Notes PDF

Newton's laws Resultant forces will cause acceleration, which can be described and calculated using Newton's laws of motion. Weight is caused by the gravitational effect of a planet attracting an object’s mass. Newton's first law According to Newton's first law of motion, an object remains in the s...

Newton's laws Resultant forces will cause acceleration, which can be described and calculated using Newton's laws of motion. Weight is caused by the gravitational effect of a planet attracting an object’s mass. Newton's first law According to Newton's first law of motion, an object remains in the same state of motion unless a resultant force acts on it. If the resultant force on an object is zero, this means:  a stationary object stays stationary  a moving object continues to move at the same velocity (at the same speed and in the same direction) Examples of objects with uniform motion Newton's first law can be used to explain the movement of objects travelling with uniform motion (constant velocity). For example, when a car travels at a constant velocity, the driving force from the engine is balanced by the resistive forces such as air resistance and frictional forces in the car's moving parts. The resultant force on the car is zero. Other examples include:  a runner at their top speed experiences the same air resistance as their thrust  an object falling at terminal velocity experiences the same air resistance as its weight If the forces acting on an object are balanced, the resultant force is zero If the resultant or net force is zero, the object cannot accelerate. It will remain in its current state: either at rest, or in motion at constant velocity. Examples of objects with non-uniform motion Newton's first law can also be used to explain the movement of objects travelling with non-uniform motion. This includes situations when the speed changes, the direction changes, or both change. For example, when a car accelerates, the driving force from the engine is greater than the resistive forces. The resultant force is not zero. 1 Other examples include:  at the start of their run, a runner experiences less air resistance than their thrust, so they accelerate  an object that begins to fall experiences less air resistance than its weight, so it accelerates If the forces acting on an object are not balanced, the resultant force is not zero, and the object accelerates or changes velocity. Forces on a submarine The submarine above has both vertical forces and horizontal forces acting on it. The horizontal forces will not affect its vertical movement and the vertical forces will not affect its horizontal movement. The horizontal forces are equal in size and opposite in direction. They are balanced, so the horizontal resultant force is zero. This means that there is no horizontal acceleration. The vertical forces are equal in size and opposite in direction. They are balanced, so the vertical resultant force is also zero. This means that there is no resultant vertical acceleration. The submarine will continue with the same motion, either remaining stationary or moving at a constant speed. If the submarine is moving, it is impossible to tell which direction it is moving from the forces alone, only that it will continue in the same direction at the same speed. 2 Newton's second law Force, mass and acceleration Newton's second law of motion can be described by this equation: resultant force = mass × acceleration This is when:  force (F) is measured in newtons (N)  mass (m) is measured in kilograms (kg)  acceleration (α) is measured in metres per second squared (m/s2) The equation shows that the acceleration of an object is:  proportional to the resultant force on the object  inversely proportional to the mass of the object In other words, the acceleration of an object increases if the resultant force on it increases, and decreases if the mass of the object increases. Inertial mass The ratio of force over acceleration is called inertial mass. Inertial mass is a measure of how difficult it is to change the velocity of an object, including moving it from rest.. Example Calculate the force needed to accelerate a 22 kg cheetah at 15 m/s2. Question Calculate the force needed to accelerate a 15 kg gazelle at 10 m/s 2. 3 Estimations It is important to be able to estimate speeds, accelerations and forces involved in road vehicles. The symbol ~ is used to indicate that a value or answer is an approximate one. The table gives some examples. Maximum legal speed on a single Mass in Acceleration in Vehicle carriageway in m/s kg m/s family ~27 ~1,600 ~3 car lorry ~22 ~36,000 ~0.4 Example Estimate the force needed to accelerate a family car to its top speed on a single carriageway. Using values of ~1,600 kg and ~3 m/s2, and F = m a: 1,600 × 3 = ~4,800 N Question Estimate the force needed to accelerate a lorry to its top speed on a single carriageway. Newton's third law According to Newton's third law of motion, whenever two objects interact, they exert equal and opposite forces on each other. This is often worded as 'every action has an equal and opposite reaction'. However, it is important to remember that the two forces:  act on two different objects  are of the same type (eg both contact forces) Examples of force pairs Newton's third law can be applied to examples of equilibrium situations. A cat sits on the ground There are contact gravitational forces between Earth and the cat:  the cat pulls the Earth up 4   the Earth pulls the cat down These forces are equal in size and opposite in direction. Pushing a pram There are contact forces between the person and the pram:  the person pushes the pram forwards  the pram pushes the person backwards These forces are equal in size and opposite in direction. Car tyre on a road There are contact forces between the tyre and the road:  the tyre pushes the road backwards  the road pushes the tyre forwards These forces are equal in size and opposite in direction. A satellite in Earth orbit There are non-contact gravitational forces between Earth and the satellite:  the Earth pulls the satellite  the satellite pulls Earth These forces are equal in size and opposite in direction. Explaining Newton's third law 5 Weight, mass and gravitational field strength The weight of an object may be thought of as acting at a single point called its centre of mass. Depending on the object's shape, its centre of mass can be inside or outside it. The weight of an object and its mass are directly proportional. For a given gravitational field strength, the greater the mass of the object - the greater its weight is. Weight can be calculated using the equation: weight = mass × gravitational field strength This is when:  weight (W) is measured in newtons (N)  mass (m) is measured in kilograms (kg)  gravitational field strength (g) is measured in newtons per kilogram (N/kg) Example An apple has a mass of 100 g. Calculate its weight on Earth (g = 10 N/kg). 100 g = 100 ÷ 1000 = 0.1 kg Question Calculate the weight of a 30 kg dog (g = 10 N/kg) on Earth and on the moon (the moon’s gravitational pull at its surface is about 1/6 that of the Earth).. 6 Measuring weight To make a measurement of weight, we have to measure the force pulling the object towards the centre of the Earth. We do this by balancing it with a known force. If the object is stationary, Newton's third law then tells us that the known force is the same as the weight. A spring balance will stretch until the force from the spring is enough to balance the weight of the object, and the distance the spring in it has stretched indicates the force it is exerting to hold up the object. 7