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18 Cards in this Set

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What is friction?

Whenever an object moves against another object, it feels frictional forces.

These forces act in the opposite direction to the movement.

Friction makes it harder for things to move.

Friction can be useful:

State 3 types of friction that can be useful

Friction between our shoes and the floor stop us from slipping

Friction between tyres and the road stop cars from skidding

Friction between the brakes and wheel help bikes and cars slow down

When is friction not useful?

When there is a lot of friction between moving parts, energy is lost to the surroundings as heat.

Explain air resistance.

Air resistance is caused by the frictional forces of the air against an object.

The faster the object moves, the greater the air resistance becomes.

What is drag?

Drag is the result of friction through a fluid.

A fluid can be a gas or liquid - so air resistance is a form of drag.

Explain streamlining?

Streamlining is the use of smooth shapes that allow the air to flow over an object more easily.

These smooth shapes make the air resistance smaller which allows the object to travel faster.

A streamlined object allows a fluid to flow over it easily.

Why does streamlining have a greater effect at higher speeds?

The faster an object moves, the greater the air resistance (drag).

Explain the difference between mass and weight?

Weight is not the same as mass.

Mass is a measure of how much stuff is in an object.

Weight is a force acting on that stuff caused by the gravitational force between the object and the Earth, measured in newtons.

What two forces affect a falling object?

The weight of the object. This is a force acting downwards, caused by the object’s mass the Earth’s gravitational field.

Air resistance. This is a frictional force acting in the opposite direction to the movement of the object.

There are three stages of falling when an object is dropped.

Describe them.

At the start, the object accelerates downwards because of its weight. There is no air resistance. There is a resultant force acting downwards.

As it gains speed, the object’s weight stays the same, but the air resistance on it increases. There is a resultant force acting downwards.

Eventually, the object’s weight is balanced by the air resistance. There is no resultant force and the object reaches a steady speed.

What is the 'steady speed' when the object’s weight is balanced by the air resistance called?

Terminal velocity.

Whatis the weight in newtons of a 5.5 kg mass?

An object has a weight of 2.3 N. What is itsmass in kg?

What is the gravitational field strength onthe Moon if an object of mass 10 kg weighs 16 N?

Approximately 55 N.

0.23 kg.

1.6 N/kg.

A teacher challenged students to get identical lumps of clay to fall fastest through a liquid. One student decided to cheat by putting small lumps of metal inside the clay.

His friend said that this would not work as it would make the object bigger and this would slow it down.

Explain which student is right and why.

Putting the metal in will increase the size of the object slightly but this will be more than compensated for by the increased weight, so adding the metal will make it fall faster.

Usethe data in the table  to draw a graph of a pencil falling out of an office windowseveral floors up. 

Explain why the last few values for velocity are the same.
Use the data in the table to draw a graph of a pencil falling out of an office window several floors up.

Explain why the last few values for velocity are the same.

Graph of data provided;

smooth curve from 0 to 1 s, then horizontal line.

The last few values for velocity are the same because it has reached its terminal velocity.

A skydiver falls from a plane and reaches a top speed of 40 m/s after 20 seconds.

She then falls for 30 seconds until she opens her parachute.

After opening the parachute her downward speed is 6 m/s and she falls for a further 40 seconds.

Draw a velocity–time graph for this descent.

Velocity–time graph;

smooth curve upwards from 0–20 s, 0–40 m/s; horizontal line at 40 m/s from 20–50 s;

vertical drop from 40 m/s to 6 m/s at 50 s; horizontal line at 6 m/s from 50–90 s;

vertical drop from 6 m/s to 0 at 90 s.

A falcon folds its wings to dive at high speed onto its prey below. Explain how folding the wings helps the falcon to increase its speed. Include comments about weight and drag.

The falcon has reduced its wind resistance so it will reach a higher terminal velocity as its weight is the same, but it will fall faster before the drag will balance the weight.

When a parachutist falls out of a plane, he quickly gains speed and reaches his terminal velocity. While falling, he takes out of his pocket a small lead sphere and a wooden ball of about the same size. The lead ball falls faster than the parachutist, while the wooden ball seems to move upwards compared with the parachutist. The lead ball is heavier than the wooden ball.

Describe the forces on the two objects as each changes speed and reaches its own terminal velocity.

Thelead ball has more weight than wind resistance when released by the parachutistso it accelerates to a higher speed. As it gets faster, the wind resistanceincreases until it exactly balances the weight of the lead ball. After this the lead ball falls at a constant speed. The wooden ball has less weight than air resistance so it slows down. It does not actually move upwards; it falls less quickly than the parachutist and so seems to move upwards. When the wind resistance has reduced to the same as the weight of the wooden ball, it will have reached its terminal velocity and so will carry on falling at a constantspeed.

What are the key points to remember?

Forces resisting motion depend on a range of factors including size, speed, surface,shape and the fluid the object is moving through.

Terminal velocity is reached when weight is balanced by drag.

Weight is the force due to a gravitational field acting on a mass, W= m × g. On the earth this is about 10 N/kg.