3 IGCSE-CAM -CHP-3-FORCES AND MOTION.ppt
- 1. 3. Forces and Motion 3.1 Balanced and unbalanced forces 3.2 Friction 3.3 Circular motion
- 2. Learning outcomes • Candidates should be able to: • (a) state Newton’s third law. • (b) describe the effect of balanced and unbalanced forces on a body. • (c) describe the ways in which a force may change the motion of a body. • (d) recall and use the equation force = mass × acceleration. • (e) explain that friction is a force that impedes motion and produces heating. • (f) discuss the effect of friction on the motion of a vehicle in the context of tyre surface, road conditions • (including skidding), braking force, braking distance, thinking distance and stopping distance. • (g) describe qualitatively motion in a circular path due to a constant perpendicular force, including • electrostatic forces on an electron in an atom and gravitational forces on a satellite. (F = mv 2 / r is not • required.) • (h) discuss how ideas of circular motion are related to the motion of planets in the solar system.
- 3. Learning outcomes • Candidates should be able to: • (i) state that a force may produce a change in size and shape of a body. • (j) *plot, draw and interpret extension-load graphs for an elastic solid and describe the associated • experimental procedure. • (k) *recognise the significance of the term “limit of proportionality” for an elastic solid (an understanding of • the elastic limit is not required). • (l) calculate extensions for an elastic solid using proportionality.
- 4. Force A force is a push or a pull of one object on another. Force can not be seen but its effect can be seen A force can cause an object to: – speed up – slow down – change direction – change shape Force is measured in newtons (N). Force is measured with a newtonmeter. newtonmeters
- 5. Some types of force 1. Gravitational Force This is the attractive force exerted between bodies because of their masses. This force increases if either or both of the masses is increased and decreases if they are moved further apart. Weight is the gravitational force of the Earth on an object. Bathroom scales measure weight. A mass of 1kg weighs about 10N 1 stone is about 63N.
- 6. 2. Normal reaction or contact This is the repulsive force that stops two touching bodies moving into each other. The word ’normal’ means that this force acts at 90° to the surfaces of the bodies. It is caused by repulsive molecular forces. normal reaction forces weight The two upward reaction forces on the tyres balance the downward weight of the car
- 7. 3. Friction This is the force that opposes motion. The kinetic energy of the moving object is converted to heat energy by the force of friction. Friction is needed for racing cars to grip the road Friction is needed for walking!
- 8. 4. Air resistance or drag This is the force that opposes the movement of objects through air. Drag is a more general term used for the opposition force in any gas or liquid. Objects are often streamlined to reduce this force. streamlined car a parachute maximises drag force
- 9. 5. Upthrust This is the force experienced by objects when they are placed into a fluid (liquid or gas). An object will float on a liquid if the upthrust force equals its weight. A hot air balloon rises when the upthrust from the surrounding air is greater than the balloon’s weight.
- 10. 6. Magnetic Between magnets but also the force that allows electric motors to work. 7. Electrostatic Attractive and repulsive forces due to bodies being charged. Electrostatic force causes the girls’ hair to rise when they touch the Van der Graaff generator.
- 11. Choose appropriate words to fill in the gaps below: A _____ is a push or a pull. A force can cause an object to ___________ or change shape. Force is measured in _______ (N) with a newtonmeter. There are many types of force. ________ force occurs when two bodies touch each other. Friction is a force that _______ the _______ of one body relative to another. It is caused by the _________ forces between ___________. attractive motion contact newtons opposes force accelerate WORD SELECTION: molecules attractive motion contact newtons opposes force accelerate molecules
- 12. Force mass and acceleration The force, mass and acceleration of an object are related by the equation: force = mass × acceleration F = m x a force is measured in N mass is measured in kg acceleration is measured in m/s2
- 13. also: acceleration = and: mass = m a F force mass force acceleration
- 16. Question 1 Calculate the force required to cause a car of mass 1200 kg to accelerate by 5 m/s2 . F = m x a = 1200 kg x 5 m/s2 Force = 6000 N
- 17. Question 2 Calculate the acceleration produced by a force of 200N on a mass of 4kg. F = m x a becomes: a = F ÷ m = 200N ÷ 4kg acceleration = 50 m/s2
- 18. Question 3 Calculate the force that accelerates a mass of 300kg from rest to 6 m/s over a time of 3 seconds. acceleration = change in velocity ÷ time = (6 – 0)m/s ÷ 3s acceleration = 2 m/s2 F = m x a = 300kg x 2 m/s2 force = 600N
- 19. Answers F m a 24 N 4 kg 6 m/s2 200 N 40 kg 5 m/s2 600 N 30 kg 20 m/s2 2 N 5 g 400 m/s2 5 N 100 g 50 cm/s2 24 N 40 kg 20 2 N 5000 Complete:
- 20. Car forces When a vehicle travels at a steady speed the frictional forces balance the driving force. To slow the car the engine force is reduced by releasing the throttle and the frictional force is increased by applying the brakes.
- 21. Stopping a car The total distance required to stop a car, the stopping distance, is equal to the thinking distance plus the braking distance.
- 22. Factors affecting stopping distance 1. The reaction time of the driver This will increase if the driver is tired, distracted or has consumed alcohol or drugs. Increasing reaction time increases the thinking distance. 2. The speed of the car The greater the speed the greater will be both the thinking and braking distances. Doubling the speed increases the overall stopping distance by about four times.
- 23. 3. The mass of the car and its contents The greater the mass the greater will be the braking distance. 4. The condition of the road Wet and icy roads will cause the braking distance to increase. 5. The condition of the vehicle Worn brakes or worn tyres will both increase the braking distance.
- 24. Choose appropriate words to fill in the gaps below: When a car is moving at a _______ speed the engine force is equal to the resistive force. The __________ distance of a car is equal to the thinking distance plus the _________ distance. Tiredness, ________ and drugs are all likely to increase the __________ distance. A car travelling at 60 m.p.h. will require roughly ______ times stopping __________ of a car travelling at 30 m.p.h.. alcohol four distance steady stopping thinking WORD SELECTION: braking alcohol four distance steady stopping thinking braking
- 25. Mass and weight Mass is the amount of matter in an object. Mass is measured in kilograms. The mass of an object is the same on the Moon as on the Earth. Weight is the force of gravity on an object. Weight is measured in newtons. The weight of an object on the Moon is about one sixth that on the Earth. A newtonmeter is used to determine the weight of the parcel
- 26. The acceleration due to gravity (g) The acceleration due to gravity (g) varies with planet, moon and star and depends on the height of an object. Some examples of the acceleration due to gravity: Location m/s2 Location m/s2 Earth 10 Jupiter 24 Moon 1.6 Pluto 0.7 Mars 3.7 The Sun 270
- 27. Gravitational field strength (g) This is an alternative way of measuring the strength of gravity. The gravitational field strength is equal to the gravitational force exerted per kilogram. Near the Earth’s surface, g = 10 N/kg In most cases gravitational field strength in N/kg is numerically equal to the acceleration due to gravity in m/s2 , hence they both use the same symbol ‘g’.
- 28. Calculating weight weight = mass × gravitational acceleration W = m x g weight is measured in N mass is measured in kg gravitational acceleration is measured in m/s2 On the Earth’s surface a mass of 1kg has a weight of 10N.
- 29. Falling objects When an object falls through air or some other fluid initially the only significant force acting on it is the downward pull of gravity. On Earth, it will initially accelerate downwards at 10 m/s2 .
- 30. As the object speeds up frictional forces such as air resistance become greater the faster the object moves. Eventually the resultant force on the object will be zero when the frictional forces equal the weight of the object. The object then moves at a constant speed called terminal velocity.
- 31. Velocity-time graphs for a falling object
- 32. Parachuting A parachutist will have two different terminal velocities. Before opening the parachute it is about 60 m/s (140 m.p.h..). Afterwards, due the much greater drag force, the terminal velocity is about 5 m/s (12 m.p.h.)
- 33. velocity time first terminal velocity initial acceleration = 10 m/s2 Velocity-time graph of a parachutist second terminal velocity parachute opened ground reached
- 34. Vectors and Scalars All physical quantities (e.g. speed and force) are described by a magnitude and a unit. VECTORS – also need to have their direction specified examples: displacement, velocity, acceleration, force, Moment, Momentum SCALARS – do not have a direction examples: distance, speed, mass, work, energy.
- 35. Addition of vectors The original vectors are called COMPONENT vectors. The final overall vector is called the RESULTANT vector. 4N 6N object 4N 6N object resultant = 10N object 4N 6N object 4N 6N object resultant = 2N object
- 36. Resultant force A number of forces acting on a body may be replaced by a single force which has the same effect on the body as the original forces all acting together. This overall force is called resultant force. In the example opposite, 5N is the resultant force of the 3N and 2N forces. 3N 2N 5N
- 37. Determine the resultant force in the cases below: 4N 6N 1. 4N 3N 2. 7N 3N 3. 2N 6N 4. 4N 4N 5. 10N 1N 4N 4N There is no resultant force in this case
- 38. Resultant force and motion Resultant force Effect on the motion of an object Zero Object’s velocity stays the same including staying stationary In the direction the object is moving Object accelerates In the opposite direction in which the object is moving Object decelerates
- 39. Examples 1 & 2 The box will move when the man’s push force is greater than the friction force. The plane will accelerate provided that the engine force is greater than the drag force.
- 40. Examples 3 & 4 The brakes exert a resultant force in the opposite direction to the car’s motion causing the car to decelerate. Once released, the glider moves at a near constant velocity as it experiences a nearly zero horizontal resultant force.