Download A Newton pair of forces

5 Forces
Revision summary
Forces are vector quantities.
Forces are measured in Newtons (N).
Types of forces
• forces acting at a distance – gravitational, electrostatic, electromagnetic
• contact forces – between two solids, friction, viscous forces in fluids
• tension – a stretched solid
Gravitational force (weight)
The gravitational field of the Earth acts on every object with a force proportional
to the mass of the object (m) and the gravitational field strength (g). This is
known as the weight of the object.
At the surface of the Earth g has a value of 9.81 N kg−1.
Gravitational force or weight (W) = mg
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5 Forces
Revision summary
Newton’s laws of motion
Newton’s first law of motion
A body remains at rest or in a state of uniform motion unless acted on by a
resultant force.
The resultant force on a body is the vector sum of all the forces acting on that
body.
Resultant force on a body = F
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5 Forces
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Newton’s second law of motion
Large force small mass gives large acceleration
Force = mass × acceleration
F = ma
One Newton (N) is the force that will give a mass of one kilogram an
acceleration of 1 m s−2.
Newton actually stated his second law of motion as:
The rate of change of momentum of a body is directly proportional to the
applied force and in the direction of that force.
See chapter 4 of the student book for a full explanation of
momentum.
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5 Forces
Revision summary
Newton’s third law of motion
If a force acts on one body an equal and opposite force acts on another body
or
Action and reaction are equal and opposite.
A Newton pair of forces
A Newton pair of forces must:
Force of caravan
on car
•
•
•
•
•
act on two separate bodies
be of the same type
act along the same line
be equal and opposite
act in opposite directions.
Force of car on
caravan
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5 Forces
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Review questions
1. The weight of an object is dependent on the gravitational field acting on the
object. Discuss and explain the variation in the mass and weight of an object at
different places on the surface of the Earth.
2. ‘Driving at high speed on the roads is always dangerous’. Discuss this
statement. Your answer should include a consideration of the driver, other
road users, road conditions, acceleration and type of road.
3. A heavy load of shopping may be supported in a plastic bag. However if the
bag is lifted upwards suddenly the bag may split. Explain why this happens.
4. A skydiver jumps from a plane and opens his parachute after falling for 3 s.
This causes him to decelerate. At this moment, is the force on him greater
than, equal to or less than his own weight.
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5 Forces
Revision summary
Review questions
5. The apparatus in the diagram is designed to verify Newton’s second law of
motion. Give a careful account of how this experiment should be carried out.
trolley
Mask size 1 cm
light gates
washers
Accelerating washers
The mass of each trolley is 60 g and that of each washer 10 g.
© Hodder & Stoughton 2015
5 Forces
Revision summary
Review questions
6. The diagram shows a uniform box at rest on a slope. The weight of the box
has been shown. Copy the diagram adding any additional forces acting on the
box. Explain your additions.
mg
q
7. Write a short, but well explained, account of the forces between colliding
objects in sport. You could include a cricket ball and bat, a tennis ball and
tennis racquet, a high jumper’s foot and the ground, two stock cars and a
football and a boot.
8. When training in a plane that is moving in ‘free fall’ an astronaut is said to be
weightless. Comment on this statement. Explain your answer.
© Hodder & Stoughton 2015