Transcript Year 12 11th - Hinchingbrooke

For today’s lesson all questions will assume
that the acceleration due to gravity, g, will be
10ms-2
Quick Starter
Fred Lemming (mass 80 kg) goes on holiday to Blackpool so that
he can throw himself from the top of Blackpool tower (158m tall).
How long will he take him to hit the floor if we consider air
resistance to be negligible?
Lesson Objective:
1) To use Newton’s first law to find missing forces when
an object is in equilibrium
2) To use Newton’s first law to find missing forces when
two or more objects are connected in equilibrium
Bodies and Connected Bodies in Equilibrium
Newton’s first law:
Every body continues in a state of rest or in uniform
motion in a straight line unless acted on by a resultant force
In short:
A particle/body is in equilibrium if it is at rest or
travelling at constant velocity in a straight line
Eg A 60kg crate rests on a rough horizontal surface. Fred
Lemming pulls it using a rope that is parallel to the
floor with a force of 15N and Gertrude Lemming
pushes it from behind using a force of 18N, but it does
not move. Find:
a) The Normal contact force between the crate and the
floor
b) The Resistance force between the crate and the floor
Eg A 50kg crate rests on a rough horizontal surface. Fred
Lemming pushes it along at a constant speed using a
force of 20N. If air resistance is 1N, find:
a) The Normal contact force
b) The resistance force between the crate and the floor
Connected Bodies in equilibrium
YOU CAN EITHER:
Consider the bodies separately
(This is usual for working out internal forces between the bodies
inside the system)
OR
Ignore the internal forces and consider the system as a whole
Look at the system of masses below:
1 kg
2
kg
6
kg
If the system is in equilibrium, what can you say about the
table top?
Calculate the tension in each string
Calculate the magnitude of the friction force
Fred Lemming parachutes from a plane. Fred Lemming has a
parachute and air resistance on his body is 20N. He is supported by
a harness that connects to the parachute with 4 equal light
inextensible ropes. The parachute weighs 10kg. If he is travelling
at constant speed:
a)Find the Tension in each rope
b)Find the Resistance Force on the parachute
The diagram shows a train, consisting of an engine of mass
50,000kg pulling two trucks, A and B, each of mass 10,000kg..
The force of resistance on the engine is 2000N and that on each
of the trucks 200N. The train is travelling at a constant speed.
a) By considering the train as a single system
find the driving force it produces.
b) By considering the forces on each truck find
the Tension forces in each coupling.
Eg Look at the diagram
4 kg
a) What is the friction force on the 4kg
block?
b) What is the tension in the string
between the 6kg and 3kg block?
6
kg
3
kg
Sheet
Key points when solving problems involving forces:
1) Draw a clear diagram
2) Mark on all the forces
3) Write down equations in two perpendicular
directions
4) Solve the equations to find the missing forces
Probably need a calculator tomorrow!