P2.2- Forces
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- Created by: EllaBella888
- Created on: 19-12-16 21:40
Name three things that forces can do to an object.
1) Change its shape 2) Change its state of rest 3) Change its motion
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When two objects push or pull on each other, what can you say about the forces acting?
They exert equal and opposite forces on one another.
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What is the unit of force?
The newton (N)
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If a kick-boxer kicks his opponent with a force of 60 N, what force will he experience?
A reverse force of 60 N
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When a tractor is used to pull a car out of the mud, which force must be greater than the force of the mud on the car?
The force of the ground on the tractor
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Why aren’t these forces necessarily equal to one another?
Because the tractor and the car are not the same
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For a moving car, where does friction act?
Where the tyres are in contact with the ground
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What is the resultant force on an object?
What is the resultant force on an object?
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What happens to a stationary object if the resultant force on the object is zero?
The object remains at rest.
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What would you observe if the object was in motion (still with a resultant force of zero)?
The object would continue to move at the same speed, and in the same direction.
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When a heavy box is pushed across the floor at constant velocity, what can be said about the resultant force on the box?
The resultant force is zero.
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How can you find the resultant force on a jet during take-off?
By finding the difference between the thrust force and the force of air resistance on it
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How could the take-off speed be increased?
By increasing the resultant force (through increasing the thrust force or minimising the drag force)
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When the driver of a race car applies the brakes, how large is the braking force compared to the engine force?
The size of the breaking force is greater than the engine force.
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How would you calculate the resultant force acting on the car?
By finding the difference between the engine force and the braking force
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How would you calculate the resultant force on an object when two forces are acting in the same direction?
By finding the sum of the two forces
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If the resultant force on an object is increased, what will happen to its acceleration?
Its acceleration will also increase.
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Write down the word equation that links the resultant force, the mass and the acceleration.
Resultant force = mass × acceleration
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Resultant force = mass × acceleration
F=ma
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Using the equation, calculate the resultant force acting on an object of mass 2 kg and an acceleration of 3 m/s2.
F = ma ∴ 2 kg × 3 m/s = 6 N
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Write down the rearranged version of the equation used to find acceleration
a = F/m
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True or false: the acceleration of an object is in the opposite direction to the resultant force acting on it?
False, it is in the same direction.
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What happens to the speed of an object if the resultant force acts in the opposite direction to its velocity?
It decreases.
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Give two examples of resistive forces acting on a car in motion.
1) Friction 2) Air resistance
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Write down an equation which can be used to find the braking force needed to stop a vehicle.
Resultant force (braking force) = mass × acceleration
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How does the braking force needed to stop a vehicle depend on the speed of the vehicle?
The greater the speed, the greater the braking force needed.
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What other factor affects the braking force needed?
The mass of the vehicle
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What is the stopping distance of a car?
The shortest distance that the car can safely stop in
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Write down the word equation for finding the stopping distance.
Stopping distance = thinking distance + braking distance
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What does the thinking distance depend on?
The driver’s reaction time
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How can the thinking distance be calculated?
By multiplying the car’s speed by the driver’s reaction time
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How can the braking distance be calculated?
By multiplying the average speed of the car during braking by the braking time
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Name two factors which increase both the thinking and braking distances.
1) Driving in poor weather conditions 2) Driving a vehicle more quickly
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Where on the Earth can you go if you want to reduce your weight?
The equator
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What will happen to your mass?
It will stay the same.
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What is the difference between weight and mass?
The weight of an object is the force of gravity on an object, whereas the mass is the quantity of matter in it.
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Give the units of weight and mass.
Weight: newtons (N) Mass: kilograms (kg)
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What is the unit of gravitational field strength?
newton per kilogram (N/kg)
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What is the value of the Earth’s gravitational field strength at the Earth’s surface?
About 10 N/kg
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Write down a word equation you could use to find the weight of an object.
Weight = mass × gravitational field strength
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Rewrite the equation using symbols.
W = mg
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Calculate the weight in newtons of a person of mass 60 kg standing on the Earth’s surface.
Weight = mass × gravitational field strength ∴ 60 kg × 10 N/kg = 600 N
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What is the acceleration of a falling object acted on only by gravity?
10 m/s²
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What is this acceleration known as?
The acceleration due to gravity
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How would you find the resultant force on an object falling through a fluid?
By subtracting the drag force from its weight
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What is the terminal velocity of a falling object?
The constant velocity the object reaches when the drag force is equal and opposite to its weight
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What is the value of the resultant force on the object when it has reached terminal velocity?
Zero
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What is the drag force called for an object moving through air?
Air resistance
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What do we mean when we say that an object is elastic?
It regains its original shape when the forces deforming it are removed.
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Give two examples of elastic objects.
1) A rubber band (or spring) 2) A squash ball
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A weight is hung from a spring. Write down a word equation to calculate the extension of the spring.
Extension of the spring = length of the spring – original length
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A graph is drawn of the extension of the spring against the weight added. The graph line is straight and goes through the origin. What does this tell us?
That the extension of the spring is directly proportional to the weight hung on it
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What kind of energy is stored in a stretched spring?
Elastic potential energy
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What happens to this energy when the spring is released?
The energy is released and transferred into kinetic energy and heat.
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What is Hooke’s law?
The extension of a spring is directly proportional to the force applied to it, provided its limit of proportionality is not exceeded.
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What happens when a spring is stretched beyond its limit or proportionality?
The spring stretches more than predicted, as the extension is no longer directly proportional to the force applied to it.
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Write down Hooke’s law as an equation.
Force applied = spring constant × extension
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What is the spring constant?
The force per unit extension needed to stretch an object
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Why do vehicles use more fuel when travelling at greater speeds?
Because air resistance at high speeds is much greater than at low speed, and so more power is needed to maintain a higher speed
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Apart from reducing speed, how can the fuel used by lorries be reduced?
By fitting wind deflectors to the lorries, which decrease their air resistance and therefore reduce the fuel used
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Give two reasons why driving at high speeds is dangerous to pedestrians.
1) The driver is much less likely to be able to stop safely. 2) The force on a person struck by a car is much greater if the car is speeding.
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Why might a speeding motorist be discouraged by speed cameras?
Speed cameras catch motorists who are driving faster than the speed limit. They are then fined and can even lose their licence.
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What are average speed cameras?
Speed cameras that are linked to measure the average speed of a vehicle
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Why are anti-skid surfaces designed to be rougher than normal road surfaces?
The rough surface increases the friction between the tyres and the road, thus reducing the chance of skidding when a car brakes.
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When does skidding usually happen?
When the brakes are applied too harshly
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Other cards in this set
Card 2
Front
When two objects push or pull on each other, what can you say about the forces acting?
Back
They exert equal and opposite forces on one another.
Card 3
Front
What is the unit of force?
Back
Card 4
Front
If a kick-boxer kicks his opponent with a force of 60 N, what force will he experience?
Back
Card 5
Front
When a tractor is used to pull a car out of the mud, which force must be greater than the force of the mud on the car?
Back
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