Transcript Year-11-solutions-to-test-on-Newton`s

Year 11 set 3 topic test on
Newton’s laws
solutions
1a
• Explain the difference between mass and
weight
• Mass is the amount of substance in a body
and does not change if the object is taken
into space
[1]
• Weight is the pull of gravity on that mass
and will be different on the moon [1]
1b
• What is the weight of a 150g mass?
• You have to change the mass into kg and
then multiply by 10
• 0.15 kg x 10 = 1.5N
[2]
2a
• What does it mean when we say two
forces are balanced?
• They are the same size and are acting on
the object in opposite directions [2]
5N
5N
2b
• Explain what Newton’s law says when the
forces acting on an object are balanced
• When forces are balanced there is no
resultant force and so there is no change
in velocity (remain at rest or at same
speed in same direction)
[2]
2c
• A parachutist of weight 700N has a
constant velocity, called terminal velocity.
Draw a simple diagram to show the
direction and size of the forces acting on
the person.
700N from air friction on
parachute
[4]
700N from weight
3a
• Write Newton’s Second law as an equation
•
•
•
•
F = ma
Where F = resultant force
m is the mass in kg
a is the acceleration in m/s2
[3]
3b
• What does ‘resultant force’ mean
• The force left over after you have
subtracted forces pulling in opposite
directions, or the single force that could
replace two forces pulling at an angle to
each other
[2]
3c
• A box of mass 20kg, pulled along a floor
with a force of 75N, accelerates at 2m/s2.
Calculate the friction force
• We use F = ma, but we need to know F
• Resultant force F = (pull – friction)
• So (75 – friction) = 10 x 2
• Changing sides, 75 - 20 = friction
• Friction = 55N
[3]
3d
• A car is moving from left to right when the
breaks are applied. Draw a diagram to
show the direction of motion, the 2
horizontal forces, and the direction of
acceleration.
acceleration
Force from
breaks
Direction of motion,
but the car is
slowing down
Driving force
[4]
4
• Two forces are acting on a small object.
One is 5N acting to the right, and the other
is 6N acting upwards on the page. Use
scale drawing or calculation to find the
resultant force
You could use
Pythagoras
Use 1cm = 1N for your scale
and then find the length of the
red line to the nearest mm.
6N
It should be about 7.8cm so
answer is 7.8N
[4]
5N
5a A car is turning a wide corner at
a constant speed
• Explain how it has a changing velocity
• Velocity changes if speed or direction
changes, and in this case the car is
changing its direction each second so it
has an acceleration
[2]
5b
• What is the name of the force causing this
change in velocity? What direction does
this force have?
• The force is called centripetal force and it
acts towards the centre of the circle or the
curve
[2]
5c
• For the example with the car, what is
causing this force?
• The turning force is from the sideways
friction of the tyres on the road, and
without it the car would slide or skid
straight ahead
[2]
6a
• How do we calculate the moment of a
force?
• The moment, or the turning effect of a
force is the force x distance between the
force and the pivot. The distance has to
be the perpendicular distance.
[2]
Distance to
force from pivot
6b
• A uniform rod of length 80cm is balanced
on a pivot with a mass of 200g attached
5cm from the left end. Draw a diagram
clearly showing these details and the
position of the pivot.
40 cm
5cm
a
Weight of
200g mass
b
We do not know what
distances a and b are
yet
Weight of the rod at the
centre of mass
[5]
6c
• Calculate the mass of the rod in grams
• We cannot work this out because the
question has not given us the distance b
• moment clockwise from rod’s mass is
mass of rod x distance b
• Moment anticlockwise is 200 x distance a
[this question will not be counted]
6d
• The 200g mass is now removed and replaced by
100g. Calculate the new position for the pivot in
order to balance the rod.
• Once more, we cannot calculate this because
the question has not given us enough
information. The pivot would need to me moved
away from the 100g mass to increase the
distance a
•
[this question will not be counted]
7
• Using three drawings, show examples of
stable, unstable, and neutral equilibrium.
Stable
Unstable – the
slightest
movement will
make it fall over
neutral
[6]