Download 3_Newton_s_Laws_1_2

What happens to an object when there is no resultant force acting?
Newton stated: “Every body continues in a state of rest or uniform motion
in a straight line unless acted upon by a resultant force”
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What happens to an object when there is a
resultant force?
A pupil decides to do an experiment to see how the acceleration of a toy
car changed for different resultant forces.
This is what the pupil did:
1. Used a long stretched rubber band to accelerate a toy car along a
table top.
2. Used a set of light gates to calculate the acceleration of the toy car.
3. Repeated the experiment for 2, 3, 4 and 5 stretched rubber bands.
Long Rubber band
Toy Car
The pupil obtained the following results:
5Resultant
Force (N)
Acceleration
(m/s2)
1
0.25
2
0.5
4-
3-
3
0.75
4
1.0
5
1.25
2-
1-
00
0.25
0.5
0.75
Acceleration m/s2
1.0
1.25
Can you plot the results and make a conclusion about the pupil’s findings.
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Newton’s second law can be written in word equation form as follows:
Resultant Force (N) = Mass (kg) x acceleration (m/s2)
Using symbols:
F=mxa
Note: Only the value for the resultant force can be used in this equation, if the
resultant force is zero then the acceleration will also be zero. (Back to Newton’s
first law again!)
F
1. What resultant force produces an
acceleration of 5 m/s2 on a car of mass 0.2 kg?
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2. What would be the mass of a spacecraft that developed
an acceleration of 20 m/s2 when acted upon by a resultant
force of 60,000 N?
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3. A sports car of mass 500 kg accelerates steadily
due to a 2000N resultant force. Calculate the
acceleration of the car.
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4. (a) A mass of 8 kg is accelerated at 5 m/s2 . What resultant force is acting on
the mass?
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(b) If the mass is doubled what resultant force is required to produce the same
acceleration?
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5. (a) What is the acceleration of a mass of 24 kg when a force of 6 N acts on
it?
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(b) What would be the acceleration if the twice the force acted on an object
with half the mass?
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6N
1. When a force of 6 N is applied to a
block of mass 2 kg, it moves with a
constant velocity along a table.
(a) What is the size of the force of friction acting on the block?
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(b) Explain how you worked this out? _________________________________
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(c) The driving force is suddenly increased to 10 N, what is the resultant force
acting at this instant?
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(d) Calculate the acceleration of the block when the new force is suddenly
applied.
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2. A rocket has a mass of 50,000 kg at take
off. When the engines first ignite the thrust
produced is 1,000,000 N.
(a) Calculate the weight of the rocket.
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(b) Draw the two forces acting on the rocket
at take-off.
(c) Calculate the resultant force acting on the rocket.
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(c) Calculate the initial acceleration of the rocket at take-off.
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(d) As the rocket speeds up another force acts on the rocket. Name this force
and explain how this force is produced.
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3. A speed boat has a mass of 3000kg. The engines produce a thrust of 5000N
and the water resistance is 1000N.
3000kg mass of speed boat
5000N
1000N
(a) Calculate the resultant force acting on the boat.
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(b) Calculate the acceleration of the boat.
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(c) Explain what happens to the water resistive force as the boat speeds up.
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1. In the following examples decide whether the forces on the moving object
balanced or unbalanced?
(a) A car maintaining a constant speed along a straight road.
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(b) A train braking as it approaches a station.
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(c) A rocket during take-off. _______________________________________ [1]
(d) A pupil sitting on a stool. ______________________________________ [1]
2. Calculate the resultant force on each car in the following diagrams.
100N
20N
950N
35N
150N
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250N
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3. A bike and its rider have a total mass of 95kg.
(a)Calculate the acceleration if the rider can
produce a resultant force of 250N.
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(b) Why do cyclists crouch down over the handlebars when they want to go
faster?
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4. A family car has a total
mass of 1500kg.
3500N
250N
It produces a driving force of
3500N in order to start moving.
The friction force is 250N.
(a) Calculate the resultant force acting on the car.
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(b) Calculate the acceleration as it starts to move.
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5. For each of the following objects calculate:
(a) Its weight
(b) The resultant force acting
(c) The acceleration
10N
50N
20N
4kg
10kg
2kg
Weight
Weight
Weight
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6. A rocket has a mass of 10,000kg.
The thrusters produce a thrust force of 400,000N and
air resistance just after take-off is 500N.
(a) Calculate the weight of the rocket.
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(b) Find the resultant force on the rocket.
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(c) Calculate the acceleration of the rocket.
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7. A sailing boat of mass 5000kg experiences a water resistive force of 800N.
The wind, helping to push the boat along, provides a force of 2000N. The
engines, turning the propeller, also provide a forwards force of 1000N.
(a) Draw a diagram of a sailing boat
and show the action of these forces.
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(b) Calculate the resultant force acting on the boat.
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(c) Calculate the acceleration of the boat.
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