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SPH3U: Forces
Name: _____________________________
Newton’s Laws of Motion
1. Draw the free body diagram in the following cases using the forces indicated.
a) a sailboat is experiencing a force of thrust (an applied force) forward due to the wind, the force of
gravity downward, a buoyancy force upwards (provided by the water – type of normal force), and a
force of water resistance (type of friction) backwards.
b) a rocket flying upwards is experiencing a force of thrust upwards, the force of gravity downwards, the
force of air resistance (in the opposite direction of the motion), and a force to the right due to a
crosswind.
2. In each of the following cases, identify whether the forces acting on the object will be balanced or
unbalanced and also state whether the net force is zero or non-zero.
a) a puck bounces off a goalie’s pads
b) a volcano shoots lava straight up at a constant speed
c) a yoyo drops straight down without speeding up
d) a car drives along a winding mountain road at a constant speed
e) a car comes to a stop while traveling on a straight road
f) a jellyfish pushes water backwards so that it can begin to move forward
3. Use Newton’s first law to explain
a) why people might fall backwards when a bus starts moving
b) why seatbelts in a car are a good idea
4. Find the NET force in each of the following situations.
a) a hot air balloon experiences a gravitational force of 5000 N[down] and a lifting force (buoyancy) of
6000 N[up] .
b) a submarine experiences the following forces; a force of gravity of 2.5 107 N, a buoyancy force of 2.5
107 N [up], a thrust force of 300,000 N[forward], and a force of water resistance of 300,000
N[backward]
c) a jet taking off from a runway experiences a thrust force from the engines of 75,000 N [forwards], and
a force of air resistance of 55,000 N
5. In which of the situations in question 4 will the object be accelerating?
6. If a 200 g baseball is accelerating at 7.5 m/s2 [East] what is the net force acting on it?
(1.5 N[E] )
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7. If an electron (mass 9.10 1031 kg) experiences a force of 5.00 1027 N[forwards] in a TV tube, what will
its acceleration be?
(5490 m/s2[f] )
8. If a push of 25 N [left] produces an acceleration of 5.0 m/s2 [left], what is the mass of the object?
(5.0 kg)
9. If a 200 kg rocket sled experiences a forward thrust of 25 000 N and an air resistance force of 10 000 N,
what will its acceleration be? (Assume the force of gravity and the normal force will cancel each other.)
(75 m/s2[f])
10. A piano mover is pushing a 300 kg piano across a floor. If the piano is accelerating at a rate of 0.10 m/s 2
[forward] and experiencing a frictional force of 140 N[back], what force is the piano mover exerting?
(170 N[f] )
11. A 1600 kg car is being pushed and pulled along a road. If the person pushing is exerting a force of 1200
N[forward], the person pulling is exerting a force of 700 N[forward], and the car is experiencing a force of
friction of 2300 N backwards, what is the acceleration of the car? Is it speeding up or slowing down?
(0.25 m/s2[f] )
12. A 285 kg rowboat is being pushed by a forward force of 112 N from the paddles. If it is accelerating at
0.120 m/s2, how much water resistance is it experiencing? (assume gravity and buoyancy cancel each
other)
(77.8 N[b] )
THE FOLLOWING QUESTIONS REQUIRE YOU TO KNOW NEWTON’S THIRD LAW
13. In the following situations, what are the relevant action-reaction pairs of forces?
a)
b)
c)
d)
e)
f)
a stunt-person is fired forwards from a cannon
the earth's pulls on an apple attached to a tree
a paddle makes a boat go forward
a propeller makes an airplane move forward
the sailboat described in question 1a)
the hot-air balloon described in question 4a)
14. An astronaut (mass 70 kg) is repairing a satellite (mass 700 kg) and is attached to the satellite by a wire. If
the astronaut pulls on the wire with a force of 45 N,
a) what acceleration will the astronaut have?
(0.64 m/s2 [toward sat.] )
b) What will the force on the satellite be?
(45 N [toward astro.] )
c) what acceleration will the satellite have?
(0.064 m/s2 [toward astro.] )
d) If the pull lasts for 1.5 seconds, and both objects start at rest, what will their speeds be after the
astronaut finishes pulling?
(0.96 m/s and 0.096 m/s)
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