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# Forces & Motion

### Physics (GCSE) Revision (Force&Motion)

Statement | Response | Comment |
---|---|---|

For an object moving at steady speed in a straight line, SPEED = | DISTANCE / TIME | Units are metres per second ( m/s ) |

If an object moves in a straight line, its distance from a certain point can be represented by . . . | a distance-time graph | The steeper the slope the faster the speed. |

When a body is at rest (stationary) its distance-time graph is a . . . | horizontal straight line. | It hasn't gone anywhere! |

When a body is moving with constant speed its distance-time graph is a . . . | diagonal straight line. | It travels equal distances in equal times. |

The VELOCITY of an object is . . . | its SPEED in a given DIRECTION | Velocity is a VECTOR quantity. |

The motion of a body can be represented by a . . . | VELOCITY-TIME graph | Not the same as a distance-time graph. |

On a velocity-time graph, a horizontal straight line represents a body which is . . . | moving with constant speed in a straight line. | We also need to know its direction e.g. left to right. |

On a velocity-time graph, a diagonal straight line represents a body moving with . . . | constant acceleration. | Its velocity is increasing if the slope is positive. |

If a velocity-time graph shows a curved line then the body is . . . | changing its acceleration. | It could be increasing or decreasing the rate at which its velocity is changing. |

The steeper the slope of a velocity-time graph . . . | the greater the acceleration. | Think what a rocket does. |

For an object moving in a straight line with a steady acceleration, ACCELERATION= | change in velocity / time taken for change | Units are metres per second squared ( m/s^2) |

The gradient of a distance-time graph represents the . . . | speed of a body. | Steeper gradient = higher speed. |

The gradient of a velocity-time graph represents the . . . | acceleration of a body. | Steeper gradient = greater acceleration. |

The distance travelled by a body can be found by working out the . . . | AREA underneath a velocity-time graph. | This is a very useful way to find the distance travelled, especially if the velocity is not constant. |

When the forces acting on an object cancel each other out (balance) the object is in . . . | EQUILIBRIUM | It cannot speed up or slow down. |

When an object rests on a surface its weight exerts a downwards force on the surface. The surface will . . . | exert an equal and opposite force on the object. | The size of the two forces is the same - they balance. |

Whenever two bodies interact, the forces they exert on each other are . . . | EQUAL and OPPOSITE | This is Newton's 3rd law. |

Balanced forces have no effect on the movement of an object. It will either . . . | remain stationary or if it is already moving, it will continue to move at constant speed in a straight line in the same direction. | This is Newton's 1st law. |

If the forces acting on an object do not cancel out, an unbalanced force will act. This will cause the object to . . . | ACCELERATE | This is Newton's 2nd law. |

Force, mass and acceleration are related by the equation, FORCE = | mass x acceleration | This is the usual form of Newton's 2nd law. |

The unit of FORCE (appropriately) is the . . . | NEWTON | 1N gives 1kg an acceleration of 1m/s^2 |

If the unbalanced force which acts on a body is kept constant but the mass of the body is increased, the acceleration will be . . . | decreased. | acceleration is inversely proportional to mass |

If the mass of a body is constant but the resultant external force increases, the acceleration will . . . | increase. | Acceleration is directly proportional to resultant force. |

If a force of friction acts on a body, the external force needed to give it a particular acceleration will be . . . | greater. | It is necessary to apply a larger force to overcome the effect of the frictional force. |

The direction in which a frictional force acts is always . . . | in the opposite direction to the motion of a body. | Friction acts to reduce the speed of a body. |

A force of friction is needed when a car accelerates because . . . | otherwise the tyres would not grip the road. | The tyre pushes against the road, which pushes back by Newton's 3rd law. |

Air resistance is a form of friction (drag) which limits the top speed of a racing car because . . . | when the driving force is equal to the friction force, the two forces are balanced. | The car has reached TERMINAL VELOCITY |

A sky-diver experiences a frictional drag which increases as his speed increases. The result of this is . . . | he reaches a terminal velocity and no longer accelerates. | Newton's 1st law again |

Terminal velocity occurs when . . . | there is no resultant force acting on a body. | If forces are balanced, acceleration is zero. |

A force of friction acts when an object moves . . . | through air or water. | Also called DRAG |

Friction causes objects to . . . | heat up and wear away. | Can be reduced by lubricating with oil or graphite. |

Friction is used to slow down and stop moving vehicles by applying . . . | the BRAKES! | Friction between brake pads and wheel drum ( or disks) creates a lot of excess heat energy. |

In order to stop a vehicle moving at high speed we need . . . | a larger braking force. | To stop it within a given distance. |

For a given braking force, if a vehicle moves at a higher speed it . . . | travels further before stopping. | Stopping distance increases dramatically with increasing speed. |

If too great a braking force is applied, the vehicle may . . . | SKID | Because the wheels will lock and the tyres will slide against the road surface. |

Stopping distance also depends on . . . | the driver's reaction time. | If he has taken drugs or alcohol, his reactions will be slower. |

The braking distance depends on . . . | the speed and road conditions. | If there is water or ice on the road, the car may skid. |

The faster an object moves through a gas or liquid (fluid) the greater the . . . | friction or drag acting on it. | Put your hand out of a moving car window (take care!) and feel the force. |

When a body falls, it accelerates initially due to the force of . . . | GRAVITY | All bodies accelerate at approximately 10 m/s^2 on Earth. |

When a car has a steady speed, the frictional forces balance the . . . | driving force produced by the engine. | A bigger engine gives a bigger top speed. |

When a sky diver pulls the ripcord he slows down because the parachute exerts a large . . . | upward force. | Initially - but as he slows down this force decreases until it equals his weight. |