SP9: Forces And Their Effects Flashcards
SP9a
1) Describe the effect of a gravitational field on objects.
2) Describe the effects of magnetic fields on objects.
3) Explain contact and non-contact forces.
1) The gravitational forces between two objects with mass can be represented as vectors (arrows that show both magnitude and direction). These two forces are action-reaction forces: pairs of forces acting on different objects, in opposite directions. The force from the moon on the Earth is the same size but in the opposite direction.
2) The space around a magnet where it can affect other materials is called the magnetic field. A magnet can attract objects made from magnetic materials. A magnet can attract or repel another magnet.
3) A contact force is a force that acts between objects that are touching. Examples of contact forces include friction, normal contact force and air resistance. A non-contact force is a force that acts between objects that are not touching. Examples of non-contact forces include gravitational force, magnetic force and electrostatic force.
SP9a
1) Describe the effects of electrostatic fields on objects.
2) Describe how pairs of forces occur when objects affect each other (Newton’s Third Law).
3) Use examples to explain the difference between vector and scalar quantities.
1) An object charged with static electricity has an electric (electrostatic field) around it. The electric field can affect objects within it. Two objects with the same charge that are close to each other produce a pair of forces that are equal in size and acting in opposite directions.
2) According to Newton’s third law of motion, whenever two objects interact, they exert equal and opposite forces on each other.
3) Force is a vector quantity because it has direction as size (magnitude). Mass is a scalar quantity because it has a magnitude but no direction.
SP9c
1) Describe situations where forces can cause rotation (in a door)
2) What is a moment, and what is the equation for a moment?
3) What is the principal of moments?
1) An example is when you push a door, the door will turn on its hinges. The turning effect of the force causes the object to rotate.
2) A moment is the turning effect of a force. The equation for the moment of a force: moment of a force (newton metre, N m) = force (newton, N) × distance normal to the direction of the force (metre, m).
3) When a system involving rotational forces is in equilibrium: the sum of clockwise moments = the sum of the anti-clockwise moments
SP9c
1) Explain how levers transmit the rotational effects of forces.
2) Explain how gears transmit the rotational effects of forces.
3) What is the moment of a force dependent on?
1) As effort is applied to rotate one end about the pivot, the opposite end is also rotated about the pivot of the lever in the opposite direction. This is how levers transmit the rotational effects of forces.
2) As one gear turns, the other gear must also turn. When the gears meet, the teeth must both move in the same direction. This means that the gears rotate in opposite directions. This is how gears transmit the rotational effects of forces.
3) The moment of a force depends on the size of the force and where the force is applied. The greater the force, and the further it is applied from the pivot, the greater the moment. The distance between the force and the pivot is measured normal (at right angles) to the direction of the force.
SP9b
1) Describe how to resolve forces.
2) Use scale drawings to work out the net force on an object.
1) Two forces can be added together to find a resultant force. A single force can be resolved (broken down) into two component forces at right angles to each other.
2) Draw the horizontal and vertical forces. Draw the horizontal and vertical forces in a vector diagram. When you draw the components, use a vector – i.e. the magnitude of the vector will be the size of the force and the direction of the vector will be the direction of the force. For example, the 3N vertical force would be 6cm according to the scale given in the question.
Work out the resultant force. The resultant force is the hypotenuse between the vertical and horizontal forces. However, from the diagram we can see the direction will be in between the horizontal and vertical forces, so we have to re-arrange our triangle, as shown in the diagram below.
Use a ruler to measure the resultant force. Measure the length of the vertical force.
SP9b
1) Draw free body diagrams to represent the forces on an object.
2) Explain what happens in situations where several forces are acting on an object.
1) To draw free body diagrams, first draw arrows at the correct angles to represent the forces. The length of each arrow should represent the size of the force.
Then, draw lines to make a parallelogram.
The resultant force is the diagonal of the parallelogram. Measure this arrow to work out the size of the resultant force.
2) When multiple forces act on an object, you can replace them with a single force. The single force can be represented by one arrow on a diagram. The single force will be called the resultant force. The resultant force is the combined effect of all the other forces added together.
SP9b
Where should an arrow be drawn to show the force applied on an object?
The arrow should be pointing towards where the force is applied.
