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Forces & Motion

Physics (GCSE) Revision (Force&Motion)

StatementResponseComment
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.
Created by: J Thomson
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